UBC Theses and Dissertations

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UBC Theses and Dissertations

Pulmonary receptors and their role in the control of breathing in turtles Milsom, William Kenneth 1978

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PULMONARY RECEPTORS AND THEIR ROLE IN THE CONTROL OF BREATHING IN TURTLES by W i l l i a m Kenneth M i l s o m B.Sc. H., U n i v e r s i t y o f A l b e r t a , 1969 M.S., U n i v e r s i t y o f Wash i n g t o n , 1974 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY' OF GRADUATE STUDIES Department >of Z o o l o g y We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA J u l y , 1978 ^ W i l l i a m Kenneth M i l s o m , 1978 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of t h e r e q u i r e m e n t s f o r an advanced degree a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e l i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r ' a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my Department o r by h i s r e p r e s e n t a -t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f Z o o l o g y The U n i v e r s i t y o f B r i t i s h C o l u m b i a Vancouver, B. C , Canada D a t e / ^ ^ / g ? / i i : ABSTRACT The n o r m a l b r e a t h i n g p a t t e r n r e c o r d e d i n u n a n a e s t h e t i z e d , l i g h t l y re--s t r a i n e d t u r t l e s , Chrysemys p i c t a , c o n s i s t e d o f p e r i o d s o f c o n t i n u o u s b r e a t h -i n g i n t e r s p e r s e d w i t h p e r i o d s o f b r e a t h h o l d i n g . D u r i n g each v e n t i l a t o r y p e r i o d , r e s p i r a t o r y f r e q u e n c y and t i d a l volume were c o n t r o l l e d s e p a r a t e l y and i n d e p e n d e n t l y o f b r e a t h l e n g t h , t h e t o t a l i n s p i r a t o r y i n t e r v a l , t h e a c t i v e i n s p i r a t o r y i n t e r v a l and t h e e x p i r a t o r y i n t e r v a l . I n c r e a s e s i n pulmonary m i n u t e v e n t i l a t i o n d u r i n g h y p e r c a p n i a were caused by i n c r e a s e s i n r e s p i r a t o r y f r e q u e n c y due s o l e l y t o s h o r t e n i n g o f t h e i n t e r v a l s o f breath;'.holding. The f r e q u e n c y o f b r e a t h i n g w i t h i n each v e n t i l a t o r y p e r i o d remained c o n s t a n t . There was a l a r g e v a r i a b i l i t y i n i n s p i r a t o r y and e x p i r a t o r y gas f l o w r a t e s y e t t i d a l volume was m a i n t a i n e d w i t h i n n a rrow l i m i t s by a d j u s t m e n t o f t h e l e n g t h s o f t h e a c t i v e i n s p i r a t o r y and e x p i r a t o r y i n t e r v a l s . T h i s mechanism was dependent upon l u n g volume i n f o r m a t i o n c a r r i e d w i t h i n t h e vagus n e r v e . F o l l o w i n g vagotomy, changes i n m i n u t e v e n t i l a t i o n due t o h y p e r c a p n i a stemmed p r i m a r i l y f r o m changes i n t i d a l volume w h i l e changes i n r e s p i r a t o r y f r e q u e n c y were m a r k e d l y r e d u c e d . Lung volume i n f o r m a t i o n c a r r i e d w i t h i n t h e vagus n e r v e a r o s e from s l o w l y a d a p t i n g pulmonary s t r e t c h r e c e p t o r s . S i n g l e f i b r e n e r v e a c t i v i t y f r o m pulmonary r e c e p t o r s was r e c o r d e d f r o m v a g a l s l i p s i n s i n g l e - p i t h e d t i d a l l y v e n t i l a t e d t u r t l e s . The major s t i m u l u s o f t h e s e r e c e p t o r s was t h e change i n l u n g volume t h r o u g h o u t each b r e a t h i n g c y c l e . The r a t e and d e g r e e o f change i n t r a n s p u l m o n a r y p r e s s u r e were w i t h o u t d i r e c t e f f e c t on r e c e p t o r d i s c h a r g e . The f u n c t i o n a l c h a r a c t e r i s t i c s o f t h e s e r e c e p t o r s d i f f e r e d o n l y q u a n t i t a t i v e l y from t h o s e r e c o r d e d i n pulmonary s t r e t c h r e c e p t o r s o f mammals i i i and t h e s e d i f f e r e n c e s p r o b a b l y stem from t h e l o w e r body t e m p e r a t u r e of t h e t u r t l e and t h e l o c a t i o n o f t h e r e c e p t o r s i n t h e t u r t l e l u n g . Most r e c e p t o r s were s e n s i t i v e t o CO^, s e v e r a l s u f f i c i e n t l y s e n s i t i v e t h a t b o t h t o n i c and p h a s i c r e c e p t o r d i s c h a r g e were t o t a l l y i n h i b i t e d t h r o u g h -out t h e v e n t i l a t o r y c y c l e by 5 t o 10% CC^ i n t h e i n s p i r e d gas. Pulmonary m e c h a n o r e c e p t o r s i n t h e f r o g were a l s o shown t o be s e n s i t i v e t o . The a c u t e s e n s i t i v i t y t o o f a few r e c e p t o r s i n t u r t l e s and f r o g s p a r a l l e l s t h a t o f t h e i n t r a p u l m o n a r y r e c e p t o r s d e s c r i b e d - i n b i r d s and s u g g e s t s t h a t a pulmonary r e c e p t o r w i t h d i s t i n c t mechano- and c h e m o s e n s i t i v e p r o p e r t i e s may r e p r e s e n t t h e f u n c t i o n a l p r e c u r s o r o f t h e v a r i e t y o f pulmonary r e c e p t o r t y p e s w h i c h appear i n modern day v e r t e b r a t e s . To examine t h e r o l e o f s e n s i t i v i t y o f pulmonary r e c e p t o r s i n t h e o v e r a l l r e s p o n s e of t u r t l e s t o i n h a l e d CC^, v e n t i l a t o r y r e s p o n s e s of unanaes-t h e t i z e d t u r t l e s t o changes i n t h e i n t r a p u l m o n a r y CC>2 c o n t e n t o f a v a s c u l a r l y ' i s o l a t e d l u n g ( c o n s t a n t Pa ) and an i n t a c t l u n g were measured d u r i n g spontaneous b r e a t h i n g . The i s o c a p n i c h yperpnea a s s o c i a t e d w i t h i n h a l a t i o n o f C O 2 by t h e v a s c u l a r l y i s o l a t e d l u n g was s m a l l and a b o l i s h e d by vagotomy. I t i s c o n c l u d e d t h a t b o t h i n h i b i t i o n o f pulmonary s t r e t c h r e c e p t o r d i s c h a r g e w i t h i n c r e a s i n g l e v e l s o f F J ^ Q and a f u n c t i o n a l i n c r e a s e i n c e n t r a l i n s p i r a t o r y volume t h r e s h o l d c o n t r i b u t e d s i g n i f i c a n t l y t o t i d a l volume i n c r e a s e s d u r i n g h y p e r c a p n i a . The p r i m a r y v e n t i l a t o r y r e s p o n s e o f i n t a c t t u r t l e s t o i n c r e a s i n g l e v e l s of F T _ ^ was an i n c r e a s e i n r e s p i r a t o r y f r e q u e n c y and t h i s r e s p o n s e 1CO2 was g r e a t l y r e d u c e d when C O 2 was i n s p i r e d o n l y by t h e v a s c u l a r l y i s o l a t e d l u n g . Thus t h e v e n t i l a t o r y r e s p o n s e o f t u r t l e s t o i n c r e a s i n g l e v e l s o f F T . i s i v p r i m a r i l y dependent upon i n c r e a s e d l e v e l s o f a r t e r i a l CC^. The e f f e c t o f vagotomy i n p r o d u c i n g a p n e u s i s i n t u r t l e s s u p p o r t s sugges-t i o n s they l a c k a pneumotaxic c e n t r e . The a r r h y t h m i c b r e a t h i n g p a t t e r n i n t u r t l e s w i t h i n t a c t v a g a l n e r v e s , however, b e a r s no s i m i l a r i t y t o t h e p a t t e r n o f b r e a t h i n g i n mammals w i t h o n l y t h e pneumotaxic c e n t r e a b l a t e d . I t i s c o n c l u d e d t h a t t h e v a g a l i n p u t f r o m pulmonary r e c e p t o r s t o t h e r e s p i r a t o r y c e n t r e s i n t u r t l e s i s q u a l i t a t i v e l y s i m i l a r t o t h a t i n mammals y e t t h e d i f f e r e n c e s i n c e n t r a l i n t e g r a t i o n o f l u n g volume i n f o r m a t i o n i n t u r t l e s and mammals a r e n o t due s o l e l y t o t h e absence o f a pneumotaxic c e n t r e i n t h e t u r t l e . Many o f t h e r e m a i n i n g d i f f e r e n c e s may a r i s e from t h e l o w e r m e t a b o l i c demand o f t u r t l e s b u t how t h i s a f f e c t s c e n t r a l i n t e g r a t i o n and r e s p i r a t o r y p a t t e r n g e n e r a t i o n i s unknown. V TABLE OF CONTENTS G e n e r a l I n t r o d u c t i o n S e c t i o n I . An A n a l y s i s o f S l o w l y A d a p t i n g Pulmonary S t r e t c h R e c e p t o r s i n t h e T u r t l e I n t r o d u c t i o n Methods R e s u l t s D i s c u s s i o n S e c t i o n I I . The R o l e o f Pulmonary A f f e r e n t I n f o r m a t i o n and H y p e r c a p n i a i n C o n t r o l o f t h e B r e a t h i n g P a t t e r n i n t h e T u r t l e I n t r o d u c t i o n Methods R e s u l t s D i s c u s s i o n S e c t i o n I I I . The R o l e o f Pulmonary R e c e p t o r C h e m o s e n s i t i v i t y i n t h e V e n t i l a t o r y Response t o I n h a l e d CO2 I n t r o d u c t i o n Methods R e s u l t s D i s c u s s i o n S e c t i o n IV. Carbon D i o x i d e S e n s i t i v i t y d f Pulmonary R e c e p t o r s i n t h e F r o g I n t r o d u c t i o n Methods R e s u l t s and D i s c u s s i o n G e n e r a l D i s c u s s i o n Summary L i t e r a t u r e C i t e d 12 14 22 45 51 53 56 82 89 91 94 99 102 103 104 112 115 118 v i L I ST OF TABLES T a b l e 1. E f f e c t o f CO2 on dynamic l u n g c o m p l i a n c e . 35 T a b l e 2. V e n t i l a t o r y v a r i a b l e s o f t u r t l e s b r e a t h i n g 61 a i r and CO2 g a s e s . T a b l e 3. Comparison o f v e n t i l a t o r y c o n t r o l i n mammals 84 and t u r t l e s . T a b l e 4. E f f e c t o f r a t e and degree o f l u n g i n f l a t i o n 110 and o f CO2 on f r o g pulmonary r e c e p t o r d i s -c h a r g e . v i i L I ST OF FIGURES F i g u r e 1. S c h e m a t i c d i a g r a m i l l u s t r a t i n g e x p e r i m e n t a l a r r a n g e - 16 ment d u r i n g a c u t e e x p e r i m e n t s . F i g u r e 2. Lung o f t h e t u r t l e showing l o c a t i o n o f pulmonary 19 s t r e t c h r e c e p t o r s . F i g u r e 3. E f f e c t o f i n t r a t r a c h e a l p r e s s u r e and CO2 on pulmonary 24 r e c e p t o r d i s c h a r g e d u r i n g s t a t i c l u n g i n f l a t i o n s . F i g u r e 4. E f f e c t o f l u n g i n f l a t i o n r a t e on pulmonary r e c e p t o r 26 d i s c h a r g e . F i g u r e 5. Pulmonary r e c e p t o r d i s c h a r g e d u r i n g i n t r o d u c t i o n and 29 r e m o v a l o f CO^ i n t h e v e n t i l a t i n g gas. F i g u r e 6. E f f e c t o f CO2 on t h e t i m e i n t e r v a l h i s t o g r a m s and 31 p o s t s t i m u l u s t i m e h i s t o g r a m s o f pulmonary r e c e p t o r d i s c h a r g e . F i g u r e 7. E f f e c t o f CO2 on pulmonary r e c e p t o r d i s c h a r g e d u r i n g 34 pump v e n t i l a t i o n . F i g u r e 8. E f f e c t o f CO2 on dynamic l u n g c o m p l i a n c e and pulmonary 37 r e c e p t o r d i s c h a r g e . F i g u r e 9. E f f e c t s o f t r a n s p u l m o n a r y p r e s s u r e a t c o n s t a n t i n f l a - 39 t i o n volume on pulmonary r e c e p t o r d i s c h a r g e . F i g u r e 10. E f f e c t o f changes i n i n f l a t i o n volume w i t h i n f l a t i o n 42 t o c o n s t a n t p e a k ' t r a n s p u l m o n a r y p r e s s u r e on pulmonary r e c e p t o r d i s c h a r g e . F i g u r e 11. XY p l o t s o f t h e r e l a t i o n s h i p s between t r a n s p u l m o n a r y 44 p r e s s u r e , i n f l a t i o n volume and pulmonary r e c e p t o r d i s c h a r g e d u r i n g a s i n g l e b r e a t h . v i i i L i s t o f F i g u r e s ( C o n t i n u e d ) F i g u r e 12. E f f e c t s o f CC^ and vagotomy on t h e b r e a t h i n g ' 58 p a t t e r n o f t h e t u r t l e . F i g u r e 13. S c h e m a t i c d i a g r a m o f t h e b r e a t h i n g p a t t e r n i n 60 t h e t u r t l e - i l l u s t r a t i n g t h e r e s p i r a t o r y i n t e r v a l s . F i g u r e 14. E f f e c t s o f CO2 and vagotomy on t h e r e l a t i o n s h i p s 65 between V and V_, V _ x a n d f , and V_, and 1/f. Ji 1 J_ 1 F i g u r e 15. E f f e c t s o f CO2 and vagotomy on t h e r e l a t i o n s h i p s 69 F i g u r e 21. Response o f V_, V and f t o changes i n F i n an , co 2 i n t a c t o r a v a s c u l a r l y i s o l a t e d l u n g , b e f o r e and a f t e r vagotomy. 72 between T t and i t s component i n t e r v a l s . F i g u r e 16. E f f e c t s o f CO2 and vagotomy on t h e r e l a t i o n s h i p s between T„ and T T, and T_ and T ' . Ji 1 Ji 1 F i g u r e 17. E f f e c t s o f CO2 and vagotomy on t h e r e l a t i o n s h i p s 75 between V-, and T T, T ' T„ and T 1 1 1 Ji t o t F i g u r e 18. E f f e c t s o f CO2 and vagotomy on t h e r e l a t i o n s h i p s 77 between i n s p i r a t o r y i n t e r v a l and i n s p i r a t o r y f l o w r a t e , and e x p i r a t o r y i n t e r v a l and e x p i r a t o r y f l o w r a t e . F i g u r e 19. E f f e c t s o f CO2 and vagotomy on t h e r e l a t i o n s h i p s 80 between T ^ p and V_, f and V-,. F i g u r e 20. E f f e c t s o f CO2 and i n f l a t i o n volume on pulmonary 96 r e c e p t o r d i s c h a r g e . 98 i x L i s t o f F i g u r e s ( C o n t i n u e d ) E f f e c t o f CC>2 on t h e d i s c h a r g e o f p r o p o r t i o n a l s e n s i t i v e r e c e p t o r s and r a t e s e n s i t i v e r e c e p t o r s i n t h e l u n g o f t h e f r o g . E f f e c t o f CC>2 on t h e d i s c h a r g e o f r a t e and p r o p o r -t i o n a l s e n s i t i v e r e c e p t o r s i n t h e l u n g o f t h e f r o g ACKNOWLEDGEMENTS I w i s h t o e x p r e s s my s i n c e r e a p p r e c i a t i o n t o Dr. D a v i d R. Jones f o r h i s encouragement, a d v i c e and generous a s s i s t a n c e t h r o u g h o u t t h i s s t u d y . I a l s o w i s h t o t h a n k D r s . Owen S. Bamford, L o w e l l L. L a n g i l l e and N i g e l H. West f o r t h e i r h e l p f u l s u g g e s t i o n s and t e c h n i c a l a s s i s t a n c e . The a s s i s t a n c e o f the t e c h n i c a l s t a f f o f t h e department i s a l s o g r a t e f u l l y acknowledged. F i n a n c i a l s u p p o r t was p r o v i d e d by T e a c h i n g A s s i s t a n t s h i p s from t h e Department o f Z o o l o g y , t h e H. R. M a c M i l l a n F a m i l y F e l l o w s h i p , and Graduate T r a v e l Awards from t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a . 1 . GENERAL INTRODUCTION A l t h o u g h modern r e p t i l e s e x h i b i t a v a s t d i v e r s i t y o f r e s p i r a t o r y mechan-i c s , l u n g m o r p h o l o g y and gas t r a n s p o r t mechanisms, t h e b r e a t h i n g p a t t e r n o f a l m o s t a l l r e p t i l e s i s s i m i l a r ; an a r r h y t h m i c b r e a t h i n g p a t t e r n c o n s i s t i n g o f s e r i e s o f one t o s e v e r a l b r e a t h s s e p a r a t e d by a h i g h l y v a r i a b l e , r e s p i r a t o r y pause ( f r o m a few s econds t o s e v e r a l h o u r s ) commencing a t e n d - i n s p i r a t i o n ( t u r t l e s , M cCutcheon, '43; Gans & Hughes, '67; c r o c o d i l e s , N a i f eh ' e j : ' a l . , '70; Gans & C l a r k , '76; s n a k e s , G l a s s & J o h a n s e n , ' 7 6 ) . A l t h o u g h t h e r e s p i r a t o r y p a t t e r n i s v e r y s i m i l a r i n t h e s e a n i m a l s , t h e t e r m i n o l o g y used t o d e s c r i b e t h e e v e n t s d u r i n g v e n t i l a t i o n i s n o t . A c o n t i n u o u s s e r i e s o f b r e a t h s has been d e s c r i b e d as a v e n t i l a t o r y p e r i o d ( N a i f e h '_et_ _ a l . , '70; G l a s s & J o h a n s e n , ' 7 6 ) , a r e s p i r a t o r y sequence (Gaunt & Gans, '70; L u c e y & House, ' 7 7 ) , a r e s p i r a t o r y s e r i e s (McCutcheon, '43; S h e l t o n & B u r g g r e n , ' 7 6 ) , a b r e a t h i n g p e r i o d ( L e n f a n t e t a l . , ' 7 0 ) , a b r e a t h i n g group (Huggins e t a l . , ' 7 0 ) , o r an a c t i v e p e r i o d ( L u c e y & House, '77; F r a n k e l e t a l . , ' 6 9 ) . The p e r i o d between c o n s e c u t i v e s e r i e s o f b r e a t h s has been l a b e l l e d t h e n o n v e n t i l a t o r y p e r i o d ( G l a s s & J o h a n s e n , '76; N a i f e h e t a l . , ' 7 0 ) , b r e a t h h o l d i n g p e r i o d ( L e n f a n t e t a l . , ' 7 0 ) , i n t e r v a l between s e r i e s (McCutcheon, '43; S h e l t o n & B u r g g r e n , '76) o r t h e p a s s i v e o r a p n e i c p e r i o d ( F r a n k e l e t a l . , '69; L u c e y & House, '77). To add f u r t h e r c o n f u s i o n , F r a n k e l e t a l . ('69) use t h e t e r m r e s p i r a t o r y sequence t o d e s c r i b e j o i n t l y t h e p e r i o d d u r i n g w h i c h t h e r e i s a c o n t i n u o u s • s e r i e s o f b r e a t h s and t h e s u b s e q u e n t i n t e r v a l b e f o r e t h e n e x t such s e r i e s . Many o f t h e s e terms a r e c o n f u s i n g , m i s l e a d i n g o r i n c o r r e c t . The s i m p l e s t 2 y e t a d e q u a t e l y d e s c r i p t i v e terms f o r t h e s e e v e n t s , v i z v e n t i l a t o r y p e r i o d and n o n v e n t i l a t o r y p e r i o d w i l l be used t h r o u g h o u t t h i s s t u d y . T h i s a r r h y t h m i c b r e a t h i n g p a t t e r n b e a r s many s i m i l a r i t i e s t o abnormal b r e a t h i n g p a t t e r n s i n mammals w i t h d e f e c t i v e c e n t r a l r e s p i r a t o r y c o n t r o l s . As a consequence, t h e p a t t e r n has been v a r i o u s l y termed a p n e i c b r e a t h i n g ( B o y e r , '63; F r a n k e l e t a l . , '69; J a c k s o n e t a l . , '74; S h e l t o n & B u r g g r e n , '76; G l a s s & J o h a n s e n , ' 7 6 ) , a p n e u s t i c b r e a t h i n g (Lumsden, '23a; R a n d a l l e_t a l . , '44; Lucey and House, ' 7 7 ) , Cheyne-Stokes r e s p i r a t i o n ( H o f f & B r e c k e n -r i d g e , ' 5 4 ) , and B i o t ' s r e s p i r a t i o n ( H o f f and B r e c k e n r i d g e , '54; F r a n k e l et^ a l . , ' 69). I n k e e p i n g w i t h t h i s image o f poor v e n t i l a t o r y c o n t r o l , r e p o r t s o f w i d e s c a l e v a r i a t i o n s i n t h e v e n t i l a t o r y r e s p o n s e t o h y p o x i a , h y p e r c a p n i a and t e m p e r a t u r e , a c r o s s r e p t i l i a n g r o u p s , a r e p r e v a l e n t i n t h e l i t e r a t u r e ( c f . Wood & L e n f a n t , '76, f o r r e v i e w ) . Many o f t h e s e s t u d i e s a s s e s s o n l y t h e e f f e c t s o f t h e r e s p i r a t o r y s t i m u l i on m e t a b o l i s m , v e n t i l a t i o n r a t e o r d i v e l e n g t h i n a q u a t i c s p e c i e s . T h i s l a c k o f complete a n a l y s i s has made m e a n i n g f u l c o m p a r i s o n s i m p o s s i b l e . Many r e p t i l e s show l a r g e t r a n s i e n t a l v e o l a r - a r t e r i a l d i f f e r e n c e s i n P and P n when t h e l e v e l s o f i n s p i r e d g ases a r e a l t e r e d due t o v a r i a b l e degrees o f i n t r a c a r d i a c o r c e n t r a l v a s c u l a r s h u n t i n g ( r i g h t t o . l e f t ) ( F r a n k e l e t a l . , '69; L e n f a n t ert a l . , '70). These t r a n s i e n t s may l a s t o v e r l o n g p e r i o d s ( % - l h r ) b u t have n o t always been a c c o u n t e d f o r when t h e r e s p i r a t o r y s e n s i t i v i t y t o CO2 and O2 o f t h e s e a n i m a l s was measured. F u r t h e r , c o m p a r i s o n s o f t h e m i n u t e v e n t i l a t i o n (V_) 1-r e s p o n s e t o i n s p i r e d CO2 i n e c t o t h e r m s a t v a r i o u s t e m p e r a t u r e s have now been shown t o be v a l i d o n l y i f oxygen c o n s u m p t i o n (V_ ) d i f f e r e n c e s a r e t a k e n 2 . . i n t o a c c o u n t , i . e . , i t i s more m e a n i n g f u l t o measure V_,/Vn as a f u n c t i o n o f P r n t h a n s i m p l y m e a s u r i n g V/F ( D e j o u r s e t a l . , '70). I n v i e w o f a l l t h i s , i t i s n o t s u r p r i s i n g t h a t t h e r e i s no c o h e r e n t c o n c e p t of r e s p i r a t o r y c o n t r o l i n r e p t i l e s . I t has become c l e a r , however, t h a t t h e c o n c e p t s o f r e s p i r a t o r y c o n t r o l i n mammals e m p h a s i z i n g b l o o d P and pH h o m e o s t a s i s , do n o t s t r i c t l y a p p l y t o e c t o t h e r m s . I n e c t o t h e r m s , b l o o d pH changes w i t h t e m p e r a t u r e i n s u c h a way t h a t t h e OH /H + r a t i o r e m a i n s c o n s t a n t , and, t h e r e f o r e , t h e d i f f e r e n c e between b l o o d pH and pN ( n e u t r a l i t y o f w a t e r ) remains c o n s t a n t . S i n c e t h e pH o f t h e b l o o d i s always g r e a t e r t h a n pN, i t f o l l o w s t h a t t h e b l o o d i s a lways a l k a l i n e w i t h r e s p e c t t o w a t e r a t t h e same t e m p e r a t u r e . As l o n g as t h i s r e l a t i v e a l k a l i n i t y (pH-pN) rema i n s c o n s t a n t , t h e OH /H + r a t i o o f t h e a n i m a l i s c o n s t a n t and a c i d - b a s e b a l a n c e i s m a i n t a i n e d ( H o w e l l & Rahn, '76). The n e t consequence o f t h i s i s t h a t as t e m p e r a t u r e i n c r e a s e s , changes i n r e s p i r a t o r y f r e q u e n c y , t i d a l volume and m i n u t e v e n t i l a t i o n w i l l be s m a l l e r t h a n a n t i c i p a t e d on t h e b a s i s o f measured i n c r e a s e s i n oxygen c o n s u m p t i o n . T h i s l e a d s t o a r e l a t i v e " h y p o v e n t i l a t i o n " (a d e c r e a s e i n Vg/V^ ) , an i n c r e a s e i n a r t e r i a l P and pH, and m a i n t e n a n c e o f a c o n s t a n t OH /H r a t i o 2 ( J a c k s o n , '71; Rahn, '73). These o b s e r v a t i o n s s u g g e s t t h a t r e s p i r a t i o n i s c o n t r o l l e d t o m a i n t a i n a c o n s t a n t r e l a t i v e a l k a l i n i t y o f t h e b l o o d (temper-a t u r e dependent pH and ) (Reeves, '72) and i t i s from t h i s h o m e o s t a t i c s t a t e t h a t v e n t i l a t i o n i s a l t e r e d i n r e s p o n s e t o m e t a b o l i c l o a d as has now been d e m o n s t r a t e d f o r t h e t u r t l e ( J a c k s o n , '71). Thus, i t b e g i n s t o appear t h a t r e p t i l e s a c c u r a t e l y c o n t r o l v e n t i l a t i o n , d e s p i t e an a r r h y t h m i c b r e a t h i n g p a t t e r n . F u r t h e r , as a consequence o f t h e a r r h y t h m i c p a t t e r n , - t h e r e a r e now t h r e e major v a r i a b l e s i n r e s p i r a t o r y c o n t r o l ; t i d a l volume, r e s p i r a t o r y r a t e and t h e d u r a t i o n o f t h e n o n v e n t i l a -4 t o r y p e r i o d . To d a t e v i r t u a l l y n o t h i n g i s known o f t h e way t h e b r e a t h i n g p a t t e r n i s changed, e i t h e r t o m a i n t a i n r e l a t i v e a l k a l i n i t y o r i n r e s p o n s e t o c h a n g i n g m e t a b o l i c l o a d s , o r o f t h e means by w h i c h t h i s r e g u l a t i o n of b r e a t h i n g i s a c h i e v e d . A H e r i n g - B r e u e r ( v a g a l , i n f l a t i o n volume r e l a t e d , i n s p i r a t o r y i n h i b i t o r y r e f l e x ) o r s i m i l a r r e f l e x has now been d e m o n s t r a t e d f o r f i s h (see S h e l t o n , ' 7 0 ) , f r o g s ( T a g l i e t t i & C a s e l l a , '66,'68), r e p t i l e s (Huggins e t a l . , ' 7 1 ) , b i r d s (see E a t o n et_ a l . , ' 7 1 ; Osborne & B u r g e r , '74) and mammals (see Guz e t a l . , ' 6 9 ) and s e v e r a l o t h e r v e n t i l a t o r y c o n t r o l mechanisms r e f l e x l y i n d u c e d by pulmonary m e c h a n o r e c e p t o r s have been d e s c r i b e d . The f u n c t i o n o f pulmonary m e c h a n o r e c e p t o r s appears t o be i n m a i n t e n a n c e o f a b a l a n c e between t i d a l volume (V-,) and r e s p i r a t o r y f r e q u e n c y ( f r e S p ) i - n s u c h a way t h a t adequate v e n t i l a t i o n i s a c h i e v e d w i t h minimum work o r minimum ave r a g e f o r c e d e v e l o p e d by t h e r e s p i r a t o r y m u s c l e s ( O t i s e t a l . , ' 5 0 ; Mead, '60; E u l e r e t a l . , ' 7 0 ) . As l u n g volume i s i n c r e a s e d above f u n c t i o n a l r e s i d u a l c a p a c i t y (FRC) i n mammals, i n h i b i t i o n o f i n s p i r a t i o n and p r o m o t i o n o f e x p i r a t i o n a r e r e f l e x l y e l i c i t e d by pulmonary s t r e t c h r e c e p t o r s . The r e s p o n s e o f mammalian pulmonary m e c h a n o r e c e p t o r s t o i n f l a t i o n shows b o t h a p h a s i c and a t o n i c component. The p h a s i c component o f t h i s r e s p o n s e i s r e s p o n s i b l e f o r t h e t r u e H e r i n g - B r e u e r r e f l e x . The i n f l a t i o n accompany-i n g i n s p i r a t i o n s t i m u l a t e s pulmonary m e c h a n o r e c e p t o r s i n c r e a s i n g t h e i r d i s c h a r g e r a t e and r e f l e x l y i n h i b i t i n g , t h e o n g o i n g i n s p i r a t i o n . The r a t e o f decay o f t h e p h a s i c a c t i v i t y ( w hich i s f a s t e r f o r s m a l l e r i n f l a t i o n volumes) and t h e l e v e l o f c o n t r o l e x c i t a t o r y a c t i v i t y i n t h e r e s p i r a t o r y c e n t r e s w i l l d e t e r m i n e t h e o n s e t o f t h e n e x t b r e a t h . T h i s p h a s i c a c t i v i t y , t h e n , a f f e c t s t h e t i m e c o u r s e o f i n s p i r a t i o n (T_) and t h e r e s p i r a t o r y f r e q u e n c y ( f )' (Guz e t a l . , '69; B y s t r z y c k a & H u s z c z u k , '73; Younes r e s p . e_t a l . , ' 7 4 ) . T h i s p a r t i c u l a r r e f l e x may f u r t h e r c o n t r i b u t e t o v e n t i l a t o r y s t a b i l i t y b y b a l a n c i n g v a g a l i n f o r m a t i o n on r a t e o f i n f l a t i o n w i t h c e n t r a l i n s p i r a t o r y d r i v e and r e f l e x l y i n c r e a s e o r d e c r e a s e r e s p i r a t o r y muscle r e c r u i t m e n t d u r i n g t h e t i m e c o u r s e o f an i n s p i r a t i o n d e p e n d i n g on the l o a d ( r e s i s t a n c e t o a i r f l o w ) e n c o u n t e r e d ( M c C l e l l a n d j i t a l . , ' 7 2 ) . The w i d e s p r e a d a p p e a r a n c e o f t h i s r e s p o n s e i n a m p h i b i a n s t o mammals ( l u n g -ed f i s h e s r e m a i n u n i n v e s t i g a t e d ) i s a n . i n d i c a t i o n o f i t s i m p o r t a n c e . The c o n v e r g e n t a p p e a r a n c e o f s i m i l a r m e c h a n o r e c e p t o r s on t h e b r a n c h i a l p r o c e s s e s o f t h e g i l l s o f f i s h w h i c h when s t i m u l a t e d by d i s p l a c e m e n t r e a c t t o t h e m e c h a n o s t i m u l a t i o n by r e f l e x l y d e c r e a s i n g f r e S p v l a p r o l o n g i n g T^, i s n o t , t h e n , s u r p r i s i n g ( S a t c h e l l & Way, '62). A d j u s t i n g and o p t i m i z i n g r e s p i r a -t o r y work a c c o r d i n g t o l o a d a p p e a r s t o have l o n g b e e n a f u n c t i o n o f p e r i p h -e r a l ( g i l l o r p u l m o n a r y ) m e c h a n o r e c e p t o r s . The t o n i c component o f t h e i n f l a t i o n r e s p o n s e has o n l y r e c e n t l y been t h e s u b j e c t o f much a t t e n t i o n . R e s e a r c h i n d i c a t e s t h a t a s e n s i t i v e v a g a l c o n t r o l o f c e n t r a l r e s p i r a t o r y f r e q u e n c y by changes i n t o n i c , non-modulated v a g a l a c t i v i t y e x i s t s w h i c h a f f e c t s t h e d u r a t i o n o f e x p i r a t o r y a c t i v i t y (Tg) (Knox, '73; B y s t r z y c k a & H u s z c z u k , '73; Younes ejt a l _ . , '74; M i s e r o c c h i & M i l i c - E m i l i , '75; D'Angelo & A g o s t o n i , ' 7 5 ) . A t e n d - i n s p i r a t i o n , p h a s i c r e s p i r a t o r y a c t i v i t y d e c a y s . The t o n i c , v a g a l , i n s p i r a t o r y - i n h i b i t o r y a c t i v i t y w h i c h r e m a i n s may e s t a b l i s h a l e v e l o f c e n t r a l i n h i b i t i o n w h i c h i n s p i r a t o r y d r i v e must overcome t o i n i t i a t e t h e n e x t i n s p i r a t i o n . I t i s t h i s component o f t h e r e s p o n s e w h i c h d e t e r m i n e s t h e l e n g t h o f apnoea f o l l o w i n g f o r c e d i n f l a t i o n (Younes e t a i . , ' 7 4 ) . T o n i c a c t i v i t y i s a f u n c t i o n o f l u n g volume. When e n d - i n s p i r a t o r y v o lume i s l o w and t h e r e f o r e t o n i c a c t i v i t y i s s m a l l , o r i n s p i r a t o r y d r i v e i s h i g h , t h e r a t e o f decay 6 o f p h a s i c v a g a l a c t i v i t y w i l l d e t e r m i n e f _ . e S p • I f > however, t o n i c v a g a l a c t i v i t y i s h i g h accompanying a l a r g e i n f l a t i o n volume, o r i n s p i r a t o r y d r i v e i s l o w , t h e t o n i c v a g a l a c t i v i t y w i l l d e t e r m i n e f by a f f e c t i n g T_. The p e r i p h e r a l c o n t r o l o f b r e a t h i n g i n b i r d s has a l s o been t h e s u b j e c t o f much r e c e n t i n v e s t i g a t i o n . S i n c e b i r d l u n g s a r e r e l a t i v e l y i n e x p a n s i b l e , t h e r e a r e s h o r t c o m i n g s i n t h e t h e o r y o f u s i n g l u n g s t r e t c h as t h e s e n s i t i v e component o f a b r e a t h - b y - b r e a t h c o n t r o l system. I n f l a t i o n o f t h e l u n g - a i r s a c s y s t e m i s , however, accompanied by a m a s s i v e v a g a l a f f e r e n t d i s c h a r g e and d e b a t e has c e n t e r e d around t h e m o d a l i t y f o r t h i s r e s p o n s e : mechano-r e c e p t o r s t i m u l a t i o n ; CC^ r e c e p t o r s t i m u l a t i o n o r s t i m u l a t i o n o f CC^ s e n s i t i v e m e c h a n o r e c e p t o r s . H i s t o l o g i c a l s t u d i e s ( K i n g et^ a l . , '74) i n d i c a t e t h e most l i k e l y c a n d i d a t e s t o a c t as pulmonary r e c e p t o r s a r e g r a n u l a r c e l l s . These c e l l s l o o k l i k e b o t h n e u r i t e r e c e p t o r c e l l complexes such as c a r o t i d body c h i e f c e l l s w h i c h a r e presumed t o be c h e m o s e n s i t i v e , and l i k e M e r k e l c e l l s w h i c h a r e known t o be m e c h a n o s e n s i t i v e . C o n s e q u e n t l y t h e h i s t o l o g i c a l e v i d e n c e c o n t r i b u t e s n o t h i n g c o n c l u s i v e t o t h i s c o n t r o v e r s y . The m a j o r i t y of i n -v e s t i g a t o r s , however, a g r e e w i t h t h e c l a s s i f i c a t i o n s y s t e m o f Molony ('74) w h i c h d i s t i n g u i s h e s between: t y p e I c e l l s , CC^ s e n s i t i v e w i t h some m e c h a n o s e n s i t i v i t y ; and t y p e I I c e l l s , s e n s i t i v e t o m e c h a n i c a l s t i m u l a t i o n o n l y (Molony, '74; Fedde eit a l . , '74a; S c h e i d jst a l . , '74; Osborne and B u r g e r , '74; B u r g e r e t a l . , '74). By f a r t h e m a j o r i t y o f r e c e p t o r s r e c o r d e d from a r e o f t y p e I (60-80%) (Molony, '74; Fedde e t a l . , '74a) and a r e l o c a t e d i n t h e m a j o r a i r w a y s ( S c h e i d e t a l . , ' 7 4 ) . Type I I r e c e p t o r s a r e t h o u g h t t o c o m p r i s e o n l y a s m a l l f r a c t i o n o f t h e t o t a l r e c e p t o r p o p u l a t i o n (18-20%) and have been t e n t a t i v e l y i s o l a t e d t o t h e o b l i q u e septum (not i n 7 t h e l u n g ) ( S c h e i d e_t aJL., ' 74). T h e i r l o c a t i o n t e n d s t o i n d i c a t e t h a t t h e y a r e u n l i k e l y t o f u n c t i o n l i k e s t r e t c h r e c e p t o r s i n a H e r i n g - B r e u e r t y p e r e f l e x . The o t h e r m a j o r c l a s s i f i c a t i o n scheme f o r a v i a n pulmonary r e c e p t o r s ( L e i t n e r and Roumy, '74) i s base d on d i s c h a r g e p a t t e r n s o f s l o w l y and r a p i d l y a d a p t i n g r e c e p t o r s r e s p o n d i n g t o i n f l a t i o n , d e f l a t i o n , o r b o t h i n f l a t i o n and d e f l a t i o n . I t has.been p o i n t e d out (Molony, '74; Fedde e t a l . , '74b; B u r g e r e t a l . , '74) t h a t t h e p a t t e r n and magnitude o f CC^ r e c e p t o r d i s c h a r g e would depend on t h e CC^ c o n c e n t r a t i o n a t t h e r e c e p t o r s i t e , a f u n c t i o n o f t h e amount o f gas exchange around t h e s i t e and t h e magnitude o f t h e gas f l o w a t t h e s i t e t h r o u g h o u t t h e v e n t i l a t o r y c y c l e . I t i s p o s s i b l e t h a t a r e a s o f t h e d o r s o -b r o n c h i o f t h e a v i a n l u n g a r e e x p o s e d t o h i g h CC^ l e v e l s d u r i n g i n f l a t i o n o r d e f l a t i o n a l o n e o r b o t h i n f l a t i o n . a n d d e f l a t i o n . A l t h o u g h t h e m a j o r i t y o f e v i d e n c e s u p p o r t s t h e i n v o l v e m e n t o f pulmonary CC>2 r e c e p t o r s i n t h e r e f l e x c o n t r o l o f v e n t i l a t i o n i n b i r d s , t h e s e s t u d i e s p o s e a f u r t h e r q u e s t i o n . A r e t h e s e r e c e p t o r s w h i c h m o n i t o r pulmonary CC^ l e v e l s a c t i n g as p a r t o f an i n s p i r a t o r y - i n h i b i t o r y r e f l e x s i m i l a r t o mammal-i a n pulmonary m e c h a n o r e c e p t o r s , o r a r e t h e y p a r t o f a r e f l e x c o n t r o l o f b l o o d o r pulmonary gas CG^? S e v e r a l i n v e s t i g a t o r s (Fedde e t a l . , '74b; S c h e i d e t a l . , '74) s u p p o r t t h e v i e w t h a t CG^ r e c e p t o r s m o n i t o r m e t a b o l i c changes a t d i f f e r e n t t i m e s i n t h e r e s p i r a t o r y c y c l e and may be i n v o l v e d i n e s t a b l i s h i n g l o c a l a i r f l o w p a t t e r n s w i t h i n t h e l u n g o r v e n t i l a t i o n - p e r f u s i o n m a t c h i n g t o r e g u l a t e b l o o d C0 2 p a r t i a l p r e s s u r e s ( P C Q ). B u r g e r e t a l . ('74) s u g g e s t t h a t t h e s e r e c e p t o r s may have o r i g i n a l l y m o n i t o r e d t i s s u e o r b l o o d P b u t b e c a u s e v e n t i l a t i o n removes p u l m o n a r y a r t e r i a l CO2, t h e y a l s o i n d i r e c t l y 8 m o n i t o r v e n t i l a t i o n . Dynamic r e s p o n s e s must have e v o l v e d t o p e r m i t i n t r a -pulmonary r e c e p t o r s t o f u r t h e r m o n i t o r b r e a t h s i z e and f l o w r a t e . Osborne and B u r g e r ('74) have p o i n t e d out t h a t by m o n i t o r i n g t h e d i s a p p e a r -ance o f dead space CO2 i n i n s p i r a t i o n , t h e y may, i n e f f e c t , m o n i t o r r a t e and d e p t h o f i n s p i r a t i o n and may p r o d u c e r e s p i r a t o r y i n h i b i t i o n s i m i l a r t o t h a t a c h i e v e d by mammalian pulmonary m e c h a n o r e c e p t o r s . F u r t h e r s u p p o r t f o r t h i s argument comes f r o m c a r o t i d body d e n e r v a t i o n s t u d i e s i n ducks (Jones & P u r v e s , '70a, b) w h i c h i n d i c a t e t h a t t h e speed o f t h e v e n t i l a t o r y r e s p o n s e t o t r a n s i e n t CO2 changes i s r e d u c e d by c a r o t i d body d e n e r v a t i o n . T h i s i n d i c a t e s t h a t i n t h e absence o f t h e c a r o t i d b o d i e s t h e r e i s no r a p i d v e n t -i l a t o r y r e s p o n s e t o CO2 and a f f e r e n t i n f o r m a t i o n from pulmonary CO2 r e c e p t o r s must t h e r e f o r e be i n v o l v e d i n a H e r i n g - B r e u e r t y p e r e f l e x o n l y . P a r t i a l l y as a r e s u l t o f t h e s e a v i a n s t u d i e s , i n v e s t i g a t o r s have begun t o re-examine t h e e f f e c t s o f c h a n g i n g gas c o n c e n t r a t i o n s ( p a r t i c u l a r l y C ^ ) on t h e r e s p o n s e o f mammalian pulmonary s t r e t c h r e c e p t o r s . There have been many s c a t t e r e d r e p o r t s i n t h e l i t e r a t u r e o f l u n g s e n s i t i v i t y t o CO2 b u t n o t h i n g c o n c l u s i v e ( A d r i a n , '33; P i - S u n e r , '47; H o r t o l o m e i et^ a l . , '56; P e t i t , '60; B i s h o p & B a c h o f e n , '73). The o l d e r l i t e r a t u r e s u g g e s t s t h a t t h e r e i s no e f f e c t o f i n h a l e d CO2 on t h e s t r e t c h r e c e p t o r d i s c h a r g e o f t h e c a t and r a b b i t . M u s t a f a and P u r v e s ('72), however, have r e c e n t l y shown t h a t CO2 does r e d u c e s t r e t c h r e c e p t o r d i s c h a r g e i n r a b b i t s . I n c r e a s i n g the.P i n t h e l u n g from 27 t o 67 mm He r e s u l t e d i n a d e c r e a s e d a v e r a g e LU2 r a t e o f d i s c h a r g e a t peak i n f l a t i o n (-25% r e d u c e d ) , an a l t e r e d i n t e r v a l h i s t o g r a m , a d e c r e a s e d d i s c h a r g e r a t e a t any g i v e n l u n g volume, a r i s e i n t h e m e c h a n i c a l t h r e s h o l d and no e f f e c t on t o t a l l u n g r e s i s t a n c e (TLR). These r e s u l t s have been c o n f i r m e d by o t h e r s f o r t h e dog (Sant'Ambrogio £t: a l . , '74) and t h e y a r e c o m p a t i b l e w i t h more r e c e n t s t u d i e s o f B a r t o l i e t a l . ('73, ' 7 4 ) . B a r t o l i and co-workers have f u r t h e r shown t h a t t h e changes i n f r e s u l t i n g f r o m t h e e f f e c t s o f C 0 o on pulmonary s t r e t c h r e c e p t o r s r e s p . 2 a r e due t o changes i n i m p l y i n g t h e e f f e c t i s m e d i a t e d p r i m a r i l y by t h e t o n i c component o f t h e i n f l a t i o n r e s p o n s e . T h i s has been c o n f i r m e d by P h i l l i p s o n ('74). R e c e n t l y , i t has been shown t h a t l i z a r d s appear t o p o s s e s s b o t h CC^ r e c e p t o r s w h i c h have f u n c t i o n a l c h a r a c t e r i s t i c s s i m i l a r t o t h o s e i n b i r d s and m e c h a n o r e c e p t o r s w i t h f u n c t i o n a l c h a r a c t e r i s t i c s s i m i l a r t o pulmonary s t r e t c h r e c e p t o r s i n mammals (Fedde e t a l . , '77; S c h e i d e t a l . , '77). Thus i t appears t h a t a l l l u n g e d v e r t e b r a t e s show a v e n t i l a t o r y r e s p o n s e t o imposed changes i n l u n g volume. I t a l s o appears t h a t l i z a r d s , b i r d s and mammals p r o v i d e s i m i l a r v a g a l i n p u t s t o t h e r e s p i r a t o r y c e n t r e s o f t h e b r a i n t h r o u g h o u t t h e b r e a t h i n g c y c l e , r e g a r d l e s s o f t h e n a t u r e o f t h e s t i m u l u s s p e c i f i c i t y o f t h e r e c e p t o r s i n v o l v e d . T h i s w o u l d seem t o i m p l y t h a t t h e a r r h y t h m i c b r e a t h i n g p a t t e r n o f t u r t l e s i s p r i m a r i l y t h e p r o d u c t o f t h e c e n t r a l r e s p i r a t o r y g e n e r a t o r . I t i s known t h a t t u r t l e s l a c k a pneumotaxic c e n t r e w i t h i n t h e pons o f t h e b r a i n s t e m (Lumsden, '23a). I t i s s u g g e s t e d t h a t t h e n o r m a l f u n c t i o n o f t h e pneumotaxic c e n t r e i n mammals i s t o r e d u c e t h e t h r e s h o l d o f t h e r e s p i r a t o r y c e n t r e s w h i c h has t o be exceeded by an i n h i b i t o r y s i g n a l b e f o r e i n s p i r a t i o n i s t e r m i n a t e d ( B r a d l e y , '77). There i s a l s o some e v i d e n c e t h a t t h i s c e n t r e may be i m p o r t a n t i n t e r m i n a t i n g e x p i r a t i o n (Cohen, '71). As a consequence, s e c t i o n o f t h e b r a i n stem i n t h e m i d - p o n t i h e r e g i o n i n mammals pro d u c e s s l o w , deep b r e a t h i n g (Tang, '67) a l t h o u g h t h i s t e n d s t o r e v e r t t o n o r m a l w i t h t i m e ( S t . John e t a l . , '72). I t i s a l s o w e l l e s t a b l i s h e d t h a t 10 a f f e r e n t I n f o r m a t i o n from pulmonary r e c e p t o r s i n mammals i s i m p o r t a n t f o r t e r m i n a t i o n o f i n s p i r a t i o n and e x p i r a t i o n ( B r e u e r , 1868; Knox, '73). The i n f l u e n c e s o f b o t h t h e pneumotaxic c e n t r e and t h e l u n g r e c e p t o r s must i n t e r -a c t and t h e consequences w i l l be r e f l e c t e d i n t h e r a t e and de p t h o f b r e a t h -i n g . When d e s t r u c t i o n o f t h e pneumotaxic c e n t r e i s combined w i t h vagotomy, a p n e u s i s ensues ( S t e l l a , ' 38). As m e n t i o n e d e a r l i e r , many a u t h o r s have n o t e d a s u p e r f i c i a l s i m i l a r i t y between t h e b r e a t h i n g p a t t e r n i n t u r t l e s and a p n e u s i s i n mammals (Lumsden, '23a; R a n d a l l e t a l , , '44; Lucey & House, '77). C l o s e r a n a l y s i s r e v e a l s , however, t h a t t h i s i s n o t a t r u e a p n e u s i s i n t u r t l e s as t h e n o n v e n t i l a t o r y p e r i o d i s n o t a s s o c i a t e d w i t h t o n i c o r t e t a n i c c o n t r a c t i o n o f i n s p i r a t o r y m u s c l e s as o c c u r s i n mammals ( S e a r s , '77; S t . J o h n , '77). F u r t h e r , t h i s a r r h y t h m i c b r e a t h i n g p a t t e r n i n t u r t l e s o c c u r s w i t h i n t a c t v a g a l n e r v e s y e t b e a r s no s i m i l a r i t y t o t h e p a t t e r n o f b r e a t h i n g i n mammals w i t h o n l y t h e p neumotaxic c e n t r e a b l a t e d (Tang, '67). O b v i o u s l y t h e n , t h e v a g a l n e r v e s o f t u r t l e s have a d i f f e r e n t i n f l u e n c e on t h e b r e a t h i n g p a t t e r n t h a n i n mammals and t h e r e i s no e v i d e n c e t o s u g g e s t whether t h i s i s a t t r i b u t a b l e t o t h e n a t u r e o f t h e v a g a l i n p u t and/or t o t h e c e n t r a l i n t e g r a t i o n o f t h i s i n f o r m a t i o n . V e r y l i t t l e i n f o r m a t i o n e x i s t s c o n c e r n i n g v a g a l i n f l u e n c e s on r e s p i r a -t i o n i n t u r t l e s . I t has been shown t h a t an i n v e r s e l i n e a r r e l a t i o n e x i s t s between t o t a l l u n g volume and b r e a t h i n g f r e q u e n c y ( M i l s o m & Jo h a n s e n , '75) and t h a t vagotomy p r o l o n g s t h e b r e a t h l e n g t h and d e c r e a s e s b r e a t h i n g f r e q u e n c y ( F r a n k e l e t a l . , '69). There i s no doubt t h a t l u n g r e c e p t o r s p l a y an e s s e n t i a l r o l e i n r e g u l a t i n g v e n t i l a t i o n and t h i s e v i d e n c e s u g g e s t s -t h e i r r e s p o n s e s a r e q u a l i t a t i v e l y s i m i l a r t o t h o s e o f mammals. As no 11 r e c o r d i n g s have been made fr o m pulmonary r e c e p t o r s i n t u r t l e s , however, t h e i r f u n c t i o n a l c h a r a c t e r i s t i c s and s t i m u l u s s p e c i f i c i t y i s unknown. W i t h o u t t h i s i n f o r m a t i o n , t h e n a t u r e o f t h e v a g a l i n p u t cannot be a s s e s s e d . I n t h e p r e s e n t s t u d y , t h e a r r h y t h m i c b r e a t h i n g p a t t e r n o f t u r t l e s has been s t u d i e d w i t h p a r t i c u l a r r e g a r d t o t h e changes w h i c h o c c u r d u r i n g r e s p i r a t o r y s t i m u l a t i o n . The n a t u r e o f t h e v a g a l a f f e r e n t i n p u t f r o m t h e l u n g s has a l s o been s t u d i e d w i t h s p e c i a l emphasis on i t s r o l e i n t h e c o n t r o l o f t h e b r e a t h i n g p a t t e r n . I n t h e f i r s t s e c t i o n , t h e f u n c t i o n a l c h a r a c t e r i s t i c s o f pulmonary r e c e p t o r s i n t h e t u r t l e a r e a n a l y z e d . The r e c e p t o r s p e c i f i c i t y f o r v o l u m e , t r a n s p u l m o n a r y p r e s s u r e and CC^ i s d e t e r -mined and from t h i s a n a l y s i s , t h e n a t u r e o f t h e v a g a l a f f e r e n t i n p u t t o t h e r e s p i r a t o r y c e n t r e s t h r o u g h o u t t h e b r e a t h i n g c y c l e i s a s s e s s e d . I n t h e second s e c t i o n , changes i n t h e b r e a t h i n g p a t t e r n d u r i n g h y p e r c a p n i c s t i m u l a t i o n a r e s t u d i e d ; The r o l e o f i n f o r m a t i o n r e g a r d i n g r a t e and d e p t h o f l u n g i n f l a t i o n on t h e changes w h i c h o c c u r i n t h e b r e a t h i n g - p a t t e r n i s d e t e r m i n e d and an a t t e m p t i s made t o d e s c r i b e t h e c o n t r o l o f t h e a r r h y t h m i c b r e a t h i n g p a t t e r n i n terms o f p e r i p h e r a l i n p u t t o t h e r e s p i r a t o r y c e n t r e s and c e n t r a l r e s p i r a t o r y p a t t e r n g e n e r a t i o n . I n t h e t h i r d s e c t i o n , changes i n t h e r e s p i r a t o r y p a t t e r n a r e measured when l e v e l s o f a l v e o l a r P a r e changed a t c o n s t a n t a r t e r i a l P . T h i s i n f o r m a t i o n i s used t o a p p r a i s e t h e s u g g e s t i o n t h a t t h e CC^ s e n s i t i v i t y o f pulmonary r e c e p t o r s i s i m p o r t a n t i n t h e v e n t i l a t o r y r e s p o n s e t o i n h a l e d CC^. The f i n a l s e c t i o n e x t e n d s t h e a n a l y s i s o f pulmonary r e c e p t o r c h a r a c t e r i s t i c s t o t h e a m p h i b i a t o a l l o w c o m p a r i s o n o f t h e d i v e r s i t y o f r e s p o n s e s t o c h e m i c a l (CC^) and m e c h a n i c a l s t i m u l i w h i c h have a r i s e n t h r o u g h o u t t h e v e r t e b r a t e g roups. 12 SECTION I An A n a l y s i s o f S l o w l y A d a p t i n g Pulmonary S t r e t c h R e c e p t o r s i n t h e T u r t l e INTRODUCTION Pulmonary r e c e p t o r s w i t h v a r y i n g degrees o f C 0 2 _ s e n s i t i v i t y have been de m o n s t r a t e d i n a v a r i e t y o f a n i m a l s . B i r d s p o s s e s s i n t r a p u l m o n a r y chemo-r e c e p t o r s h a v i n g no a p p a r e n t m e c h a n o s e n s i t i v i t y b u t r e s p o n d i n g s o l e l y t o changes i n a i r w a y CO2 c o n c e n t r a t i o n s t h r o u g h o u t t h e b r e a t h i n g c y c l e (Fedde & P e t e r s e n , ' 70; Osborne & B u r g e r , ' 7 4 ; Fedde e_t a l _ . , ' 74a,b) . S i m i l a r r e c e p t o r s havet, been d e s c r i b e d i n t h e l i z a r d (Fedde e t a l . , ' 7 7 ) . The d i s c h a r g e o f s l o w l y a d a p t i n g pulmonary s t r e t c h r e c e p t o r s i n c a t s , dogs and r a b b i t s i s p a r t i a l l y m o d i f i e d by t h e l e v e l o f a l v e o l a r CO2 ( A d r i a n , ' 3 3 ; M u s t a f a & P u r v e s , ' 7 2 ; Schoener & F r a n k e l , ' 7 2 ; B a r t o l i e t a l . , ' 7 4 ; Sant'Ambrogio e t a l . , ' 7 4 ) as i s t h e d i s c h a r g e of b r o n c h i a l s t r e t c h r e c e p t o r s i n t h e e x t r a p u l m o n a r y a i r w a y s o f t h e dog ( B a r t l e t t & S a n t ' A m b r o g i o , ' 7 6 ) . The e f f e c t o f CO2 on pulmonary r e c e p -t o r d i s c h a r g e i n t h e b i r d i s a m a j o r , i f n o t dominant f a c t o r i n t h e r e g u l a t i o n o f a v i a n r e s p i r a t i o n (Kunz & M i l l e r , ' 7 4 a , b ) . A l t h o u g h n o t a major f a c t o r i n c o n t r o l o f mammalian r e s p i r a t i o n , t h e e f f e c t o f CO2 on t h e d i s c h a r g e o f mammalian pulmonary r e c e p t o r s does a c c o u n t f o r a v a g a l l y m e d i a t e d t a c h y p n o e i c r e s p o n s e t o i n h a l e d C02(Mustafa & P u r v e s , ' 7 2 ; B a r t o l i e t a l . , ' 7 4 ; B r a d l e y e t a l . , ' 7 6 ) . R e c e n t l y t h e r e have been p r e l i m i n a r y o b s e r v a t i o n s o f s l o w l y a d a p t -i n g pulmonary s t r e t c h r e c e p t o r s i n t u r t l e s ( M i l s o m & Jones,'76) and l i z a r d s 13 (Fedde et^ a l . , '77) w h i c h a r e t y p i c a l l y m e c h a n o s e n s i t i v e but w h i c h e x h i b i t a range o f v a r i a t i o n i n t h e i r s e n s i t i v i t y t o CC^ w h i c h encompasses th e d i f f e r e n t s e n s i t i v i t i e s t o CC^ f o u n d i n t h e a v i a n and mammalian r e c e p t o r t y p e s . A t t h e moment t h e f u n c t i o n a l s i g n i f i c a n c e o f s u c h d i v e r s i t y i s u n c l e a r . B e f o r e t h e r o l e o f t h i s d i v e r s i t y i n t h e c o n t r o l o f b r e a t h i n g c a n b e s t u d i e d , h o w e v e r , i t i s n e c e s s a r y t o c a r e f u l l y c h a r a c t e r i z e t h e s e r e c e p t o r s . As a f i r s t s t e p , t h e f o l l o w i n g s t u d y was d e s i g n e d t o p r o v i d e a d e t a i l e d a n a l y s i s o f t h e s t a t i c and dynamic c h a r a c t e r i s t i c s o f pulmonary r e c e p t o r s i n t h e t u r t l e and t h e e f f e c t s o f changes i n a i r w a y CC^ c o n c e n t r a t i o n s on t h e r e c e p t o r r e s p o n s e . / 14 METHODS Ex p e r i m e n t s were p e r f o r m e d on t h i r t y t u r t l e s (Chrysemys p i c t a , 500-1500 g) s i n g l e - p i t h e d and r e s t r a i n e d i n a v e n t r a l p o s i t i o n a t room t e m p e r a t u r e ( 2 2-23°C). A p n e u m o t a c h o g r a p h . w i t h a s i d e arm f o r t r a c h e a l p r e s s u r e measure-ment and gas s a m p l i n g was a t t a c h e d t o a t r a c h e a l c a n n u l a i n s e r t e d as low i n t h e neck as p o s s i b l e . The d i s t a l end o f t h e t r a c h e a l c a n n u l a was a t t a c h e d t o a c o n s t a n t volume p o s i t i v e p r e s s u r e r e s p i r a t i o n pump f o r t i d a l v e n t i l a t i o n ( F i g . 1 ) . A c a t h e t e r i n s e r t e d i n t o t h e a b d o m i n a l c a v i t y t h r o u g h a h o l e d r i l l e d i n t h e c a r a p a c e was s e a l e d i n p l a c e w i t h d e n t a l a c r y l i c cement. I n t r a t r a c h e a l p r e s s u r e ( P ^ ) > t a k e n as an i n d e x o f i n t r a p u l m o n a r y p r e s s u r e and i n t r a - a b d o m i n a l p r e s s u r e ( P ^ a ) were measured w i t h Statham P23V p r e s s u r e t r a n s d u c e r s . S i n c e t u r t l e s p o s s e s s a" ( ; p l e u r o - p e r i t o n e a l c a v i t y , t h e abd o m i n a l c a v i t y p r e s s u r e w i t h r e s p e c t t o t r a c h e a l p r e s s u r e measured by a H e w l e t t - P a c k a r d 267 BC d i f f e r e n t i a l p r e s s u r e t r a n s d u c e r was t a k e n as t h e t r a n s p u l m o n a r y p r e s s u r e (^ tp) (Ei§» 1 ) • T h e p r e s s u r e a c r o s s t h e pneumotacho-gra p h s c r e e n d u r i n g t r a c h e a l a i r f l o w was measured w i t h a H e w l e t t - P a c k a r d 268 BC d i f f e r e n t i a l p r e s s u r e t r a n s d u c e r and t h i s a i r f l o w s i g n a l was f e d i n t o a H e w l e t t - P a c k a r d 350-3700 A i n t e g r a t i n g p r e a m p l i f i e r t o g i v e t i d a l volume. A l l measurements; p r e s s u r e s , f l o w and volume were c o n t i n u o u s l y r e c o r d e d on m a g n e t i c t a p e and m o n i t o r e d on a Sanborn 4 c h a n n e l c h a r t r e c o r d e r w r i t i n g on r e c t i l i n e a r c o - o r d i n a t e s . The O2 and CO2 c o m p o s i t i o n o f i n s p i r e d and e x p i r e d gases was e i t h e r d e t e r m i n e d on samples t a k e n t h r o u g h t h e s i d e arm of t h e pneumotachograph and measured on a F i s h e r - H a m i l t o n gas p a r t i t i o n e r o r by c o n t i n u o u s s a m p l i n g w i t h a C e n t r o n i c 200 MGA c l i n i c a l mass s p e c t r o m e t e r (sample r a t e <10 m l / m i n ) . 15 F i g u r e 1. S c h e m a t i c d i a g r a m i l l u s t r a t i n g e x p e r i m e n t a l a r r a n g e -ment d u r i n g a c u t e e x p e r i m e n t s . F o r d e s c r i p t i o n , see t e x t . e l e c t r o d e s 17 E i t h e r t h e r i g h t o r l e f t v a g o s y m p a t h e t i c n e r v e was c u t h i g h i n t h e n e c k , d i s s e c t e d f r e e o f s u r r o u n d i n g t i s s u e and p l a c e d on a d i s s e c t i o n p l a t f o r m . S m a l l f i l a m e n t s were d i s s e c t e d from t h e p r o x i m a l c u t end o f t h e n e r v e and s i n g l e u n i t . a c t i o n p o t e n t i a l s from s l o w l y a d a p t i n g s t r e t c h r e c e p t o r s were r e c o r d e d by c o n v e n t i o n a l means u s i n g b i p o l a r s i l v e r e l e c t r o d e s . T h i s a c t i v i t y was a m p l i f i e d , m o n i t o r e d w i t h an o s c i l l o s c o p e , a u d i o - a m p l i f i e r and i n s t a n t a n e o u s r a t e m e t e r , and r e c o r d e d on m a g n e t i c t a p e . L o c a t i o n o f Pulmonary R e c e p t o r s S l o w l y - a d a p t i n g s t r e t c h r e c e p t o r d i s c h a r g e was a t t r i b u t e d t o pulmonary r e c e p t o r s i f t h e d i s c h a r g e was m o d u l a t e d by a r t i f i c i a l v e n t i l a t i o n and u n a f f e c t e d by pulmonary a r t e r y o c c l u s i o n . F o r c o n f i r m a t i o n , t h e p r e c i s e l o c a t i o n s o f s e v e n t e e n s l o w l y a d a p t i n g s t r e t c h r e c e p t o r s were f u r t h e r d e t e r m i n e d i n t e n t u r t l e s . The c h e s t o f t h e s e a n i m a l s was opened by r e m o v a l o f t h e c a r a p a c e u s i n g a n e c r o p s y saw and t h e l u n g s were exposed by s u r g i c a l r e m o v a l o f a l l o b s t r u c t i n g v i s c e r a . W h i l e t h e a c t i v i t y o f each r e c e p t o r was b e i n g m o n i t o r e d , i t s l o n g i t u d i n a l and c i r c u m f e r e n t i a l p o s i t i o n w i t h i n t h e l u n g was d e t e r m i n e d by g e n t l e p r o b i n g w i t h a f i n e b r i s t l e . T h i s method always s u f f i c e d t o l o c a t e t h e r e c e p t o r s t o t h e m a j o r i n t e r n a l s e p t a w h i c h s u b d i v i d e t h e l u n g ( F i g . 2 ) . F o r n i n e o f t h e s e u n i t s , c o n d u c t i o n v e l o c i t i e s were a l s o measured from p h o t o g r a p h i c r e c o r d s o f evoked p o t e n t i a l s d u r i n g s i m u l t a n e o u s s t i m u l a t i o n w i t h two p a i r s o f s t i m u l a t i n g e l e c t r o d e s p l a c e d 0.5 cm a p a r t on t h e pulmonary vagus where i t emerged from t h e l u n g ( F i g . 1 ) . E x p e r i m e n t a l P r o t o c o l W h i l e m o n i t o r i n g t h e d i s c h a r g e o f each r e c e p t o r , t h e t u r t l e s were v e n t i l -a t e d w i t h m i x t u r e s o f h u m i d i f i e d a i r c o n t a i n i n g 0,5 and 10% C0„ a t pump 18 F i g u r e 2. (a) Dry mount o f l e f t l u n g showing i n t e r n a l s e p t a t i o n . (b) Schematic d i a g r a m o f l e f t l u n g showing a p p r o x i m a t e l o c a t i o n o f pulmonary r e c e p t o r s (•) l o c a t e d by p u n c t a t e s t i m u l a t i o n . (c) R e c o r d i n g o f d i s c h a r g e from a pulmonary r e c e p t o r d u r i n g p u n c t a t e s t i m u l a t i o n (arrow) w i t h a f i n e b r i s t l e . 20 f r e q u e n c i e s from 3 t o 20/min and t i d a l volumes o f 10 t o 50 m l . The pump was b r i e f l y s t o p p e d t o o b t a i n m a i n t a i n e d d e f l a t i o n o f t h e l u n g s e q u i l i b r a t e d t o a t m o s p h e r i c p r e s s u r e and i n f l a t i o n o f t h e l u n g s a t v a r i o u s volumes. The r a t e o f i n f l a t i o n t o , o r , d e f l a t i o n from t h e s e volumes was a l t e r e d by c h a n g i n g t h e pump r a t e a l o n g w i t h t h e pump i n f l a t i o n - d e f l a t i o n phase r a t i o . I n a f u r t h e r f i v e a n i m a l s , t h e i n g u i n a l and c e r v i c o - a x i l l a r y p o c k e t s were t i g h t l y packed w i t h p l a s t i c e n e and bound w i t h r e i n f o r c e d t a p e w h i c h , i n c o n j u n c t i o n w i t h t h e s h e l l , r e n d e r e d t h e body o f t h e a n i m a l t o t a l l y i n f l e x -i b l e . An a d j u s t a b l e p r e s s u r e r e s e r v o i r was c o n n e c t e d t o t h e body c a v i t y t h r o u g h t h e h o l e d r i l l e d " i n t h e c a r a p a c e so t h a t i n t r a - a b d o m i n a l p r e s s u r e c o u l d be v a r i e d a t w i l l . The v e n t i l a t i o n pump was a r r a n g e d f o r s i n u s o i d a l pumping o f a f i x e d volume ( F i g . 1) and c a r e was t a k e n t o always b e g i n t h e f i r s t i n f l a t i o n from a c o n s t a n t f u n c t i o n a l r e s i d u a l c a p a c i t y . By a d j u s t i n g t h e s t a r t i n g pump volume and/or t h e i n t r a - a b d o m i n a l c a v i t y p r e s s u r e , r e c e p t o r d i s c h a r g e c o u l d be m o n i t o r e d w h i l e t h e peak t r a n s p u l m o n a r y p r e s s u r e was v a r i e d a t any g i v e n i n f l a t i o n volume o r w h i l e t h e same peak t r a n s p u l m o n -a r y p r e s s u r e was d e v e l o p e d f o r a v a r i e t y o f i n f l a t i o n volumes. Measurements and A n a l y s i s A l l d a t a s t o r e d on m a g n e t i c t a p e was a n a l y z e d on a D i g i t a l PDP Lab 8e mi n i - c o m p u t e r u s i n g c o n v e n t i o n a l s o f t w a r e . The p r i m a r y p a r a m e t e r s measured were peak i n s p i r a t o r y d i s c h a r g e , e n d - e x p i r a t o r y d i s c h a r g e and i n s t a n t a n e o u s d i s c h a r g e r a t e t h r o u g h o u t each b r e a t h i n g c y c l e . I n s e v e r a l i n s t a n c e s b o t h t i m e i n t e r v a l h i s t o g r a m s (T.I.H.) and p o s t - s t i m u l u s t i m e h i s t o g r a m s (P.S.T.H.) were c o n s t r u c t e d f r o m 5 t o 10 s u c c e s s i v e pump c y c l e s t o a n a l y z e t h e e f f e c t s o f C0„ on t h e r e l a t i o n s between i n s t a n t a n e o u s d i s c h a r g e and i n f l a t i o n volume 21 and p r e s s u r e t h r o u g h o u t t h e phases o f t h e pump c y c l e . The a p p r o p r i a t e s i g n a l s were a l s o f e d i n t o an XY p l o t on a H e w l e t t - P a c k a r d 1201A s t o r a g e o s c i l l o s c o p e t o p r o v i d e p r e s s u r e - v o l u m e c u r v e s f o r d e t e r m i n a t i o n o f dynamic l u n g c o m p l i a n c e and t o o b s e r v e t h e degree o f h y s t e r e s i s i n t h e pulmonary r e c e p t o r d i s c h a r g e v e r s u s p r e s s u r e and v e r s u s volume p l o t s f o r any v e n t i l a t i o n c y c l e . ) 22 RESULTS Response o f Pulmonary S t r e t c h R e c e p t o r s t o Changes i n Lung Volume The d i s c h a r g e p a t t e r n i n 62 f i b r e s s t u d i e d v a r i e d f r o m a low t h r e s h o l d p a t t e r n (n = 54) l a s t i n g t h r o u g h o u t t h e r e s p i r a t o r y c y c l e t o a h i g h t h r e s h o l d p a t t e r n (n = 8) i n w h i c h d i s c h a r g e o c c u r r e d o n l y d u r i n g i n s p i r a -t i o n and t h e e a r l y p a r t o f e x p i r a t i o n . A l l f i b r e s e x h i b i t e d some a d a p t a t i o n t o a s t e p change i n p r e s s u r e o v e r a 2 kPa range b u t t h e r a t e o f a d a p t a t i o n e s t i m a t e d f r o m I n d e x 1 o f D a v i s e t a l . ('56) was a l w a y s l e s s t h a n 30%. The c o n d u c t i o n v e l o c i t i e s o f t h e s e f i b r e s , a t 23°C, ranged f r o m 3 t o 16 m/sec (mean = 7.4) s u g g e s t i n g f i b r e s 3-11 y i n d i a m e t e r ( E r l a n g e r & G a s s e r , '37). The s t e a d y d i s c h a r g e o f t h e s e f i b r e s was measured a f t e r a d a p t a t i o n i n r e s p o n s e t o m a i n t a i n e d l u n g i n f l a t i o n t o v a r i o u s volumes above e n d - e x p i r a t o r y volume. The r e l a t i o n between d i s c h a r g e r a t e and l u n g volume was l i n e a r o v e r t h e range s t u d i e d (0 t o 1.5 kPa t r a c h e a l p r e s s u r e ) ( F i g . 3 ) . A range o f i n f l a t i o n and d e f l a t i o n r a t e s c o r r e s p o n d i n g r o u g h l y w i t h t h e range found i n s p o n t a n e o u s l y b r e a t h i n g t u r t l e s was used i n t h e s e e x p e r i m e n t s (0 and 10% CO2 i n a i r r e s u l t e d i n a v e r a g e f l o w r a t e s o f 5 and 10 ml/min/kg r e s p e c t i v e l y ) . A t low i n f l a t i o n volumes t h e r e was no d i f f e r e n c e i n t h e maximal d i s c h a r g e f r e q u e n c y a t t a i n e d whether t h e i n f l a t i o n was s l o w o r r a p i d . F o r l a r g e r i n f l a t i o n v o lumes, however, maximal d i s c h a r g e r a t e was a f u n c t i o n o f b o t h t h e r a t e and volume o f i n f l a t i o n ( F i g . 4 ) . D e s p i t e t h i s , d i s c h a r g e i n any g i v e n f i b r e f e l l t o t h e same l e v e l w i t h i n 5 s e c o f m a i n t a i n e d i n f l a t i o n r e g a r d l e s s o f i n f l a t i o n r a t e ( F i g . 4, b - d ) . D i s c h a r g e p e r u n i t volume o r p e r u n i t p r e s s u r e a t m i d - i n f l a t i o n showed v e r y l i t t l e r a t e dependency i m p l y -i n g t h a t t h e graded o v e r s h o o t i n maximal d i s c h a r g e r a t e seen a t d i f f e r e n t 23 F i g u r e 3. The e f f e c t o f c h a n g i n g i n s p i r e d C0„ (F_. . ) on t h e r e l a t i o n -C 0 2 s h i p between d i s c h a r g e f r e q u e n c y of pulmonary s t r e t c h r e c e p -t o r s Cn =-62) and i n t r a t r a c h e a l p r e s s u r e a t l e a s t 30 sec a f t e r c o m p l e t i o n o f i n f l a t i o n t o v a r i o u s volumes. Curves and e q u a t i o n s a r e from l i n e a r r e g r e s s i o n a n a l y s i s o f v a l u e s r e c o r d e d a t t h r e e l e v e l s of i n s p i r e d C 0 9 (0, 5 and 1 0 % ) . i 25 F i g u r e 4. The e f f e c t o f c h a n g i n g t h e r a t e o f l u n g i n f l a t i o n on i n f l a t i o n volume, t r a n s p u l m o n a r y p r e s s u r e , i n t r a t r a c h e a l p r e s s u r e and t h e d i s c h a r g e r a t e and e l e c t r o n e u r o g r a m o f a s i n g l e pulmonary s t r e t c h r e c e p t o r , a. and b. r e p r e s e n t i n f l a t i o n s o f t h e same a p p r o x i m a t e r a t e b u t t o d i f f e r e n t m aximal l u n g volumes, b., c. and d. a r e t h e r e s p o n s e s t o l u n g i n f l a t i o n s o f e q u a l volume a t v a r y i n g r a t e s . The r a t e s o f change i n volume, p r e s s u r e and d i s c h a r g e a r e t a b u l a t e d b e n e a t h w i t h t h e r e l a t i v e s e n s i t i v i t y ( d i s c h a r g e p e r u n i t volume o r p e r u n i t p r e s s u r e a t m i d - i n f l a t i o n ) o f t h e f i b r e . a . b. V o l u m e (ml) 2 0 r T r a n s - p u l m o n a r y p r e s s u r e ( kPa ) O I n t r a - t r a c h e a l p r e s s u r e ( kPa ) O L -D i s c h a r g e r a t e (Hz) 20 eng. a b c d r a te of vol ume i n c r e a s e (ml / sec) 11 - 3 12-5 6-3 3 3 r a t e of p r e s s u r e i nc r e a s e l-t.p. | Tp.p. (kPa/sec) 1-2 1-5 0 9 0-4 1-2 1-5 0-9 0-4 10 sec r a t e of d i s c h a r g e i n c r e a s e —I discharge / unit vo l ume m i d - i nf la t ion d i s cha rge / unit p res su re mid - i n f l a t i on ( /sec) ( Hz / ml ) (Hz / kPa) 10-3 14 1 8 0 4-2 0-76 0 81 0-96 0-83 9- 2 10- 5 11- 0 10- 3 27 i n f l a t i o n r a t e s was a f u n c t i o n o f r a t e dependent changes i n l u n g m e c h a n i c s . D e f l a t i o n r a t e had no e f f e c t on e n d - e x p i r a t o r y d i s c h a r g e r a t e . E f f e c t o f Changing I n s p i r e d CO^ C o n c e n t r a t i o n on Pulmonary  S t r e t c h R e c e p t o r D i s c h a r g e The d i s c h a r g e r a t e o f a l l f i b r e s d u r i n g m a i n t a i n e d l u n g i n f l a t i o n t o v a r i o u s volumes d e c r e a s e d as t h e l e v e l o f i n s p i r e d CC^ i n c r e a s e d ( F i g . 3 ) . The r e l a t i o n between r a t e o f d i s c h a r g e and t r a c h e a l p r e s s u r e remained l i n e a r , t h e g r e a t e s t changes i n r e c e p t o r d i s c h a r g e o c c u r r i n g o v e r t h e l o w e r range o f a i r w a y CC>2 c o n c e n t r a t i o n s . When a n i m a l s were g i v e n CC^ d u r i n g pump v e n t i l a t i o n , a r e d u c t i o n i n t h e r a t e s o f p e a k - i n s p i r a t o r y and e n d - e x p i r a t o r y d i s c h a r g e were n o t i c e a b l e a f t e r 2 t o 4 v e n t i l a t i o n c y c l e s ( F i g . 5 ) . As a l v e o l a r CC^ l e v e l s r o s e t o a new s t e a d y l e v e l , t h e r e was a p r o g r e s s i v e r e d u c t i o n i n t h e l e v e l s o f peak i n s p i r a t o r y and e n d - e x p i r a t o r y d i s c h a r g e ( r e d u c e d s e n s i t i v i t y ) and i n t h e on s e t o f d i s c h a r g e d u r i n g i n s p i r a t i o n ( i n c r e a s e d t h r e s h o l d ) . The r a t e s o f d e c r e a s e i n p e a k - i n s p i r a t o r y and e n d - e x p i r a t o r y d i s c h a r g e were s i m i l a r and dependent on t h e degree o f change i n i n s p i r e d CC^ l e v e l s ( F i g . 5 ) . There was a marked d e p r e s s i o n o f a c t i v i t y t h r o u g h o u t t h e e n t i r e r e s p i r a t o r y c y c l e i n 59 o f 63 s t r e t c h r e c e p t o r s s t u d i e d . F i g u r e 6 i l l u s -t r a t e s t he t y p i c a l e f f e c t s o f CC^ on one f i b r e . The t i m e i n t e r v a l h i s t o -grams (T.I.H.) r e p r e s e n t t h e d i s c h a r g e p r o f i l e s o f each o f e i g h t s u c c e s s i v e b r e a t h s d u r i n g pump v e n t i l a t i o n and t h e p o s t s t i m u l u s t i m e h i s t o g r a m s (P.S.T.H.) r e p r e s e n t t h e c u m u l a t i v e a v e r a g e o f t h e d i s c h a r g e d u r i n g t h e s e same e i g h t v e n t i l a t i o n c y c l e s p o r t r a y e d o v e r t h e t i m e c o u r s e o f a s i n g l e c y c l e . I t can be seen t h a t C 0 ? r e d u c e s d i s c h a r g e a l m o s t e q u a l l y t h r o u g h o u t 2 8 F i g u r e 5 . The t i m e r e l a t i o n s h i p between changes i n t h e l e v e l o f i n s p i r e d CC^ and pulmonary r e c e p t o r d i s c h a r g e . The upper p a i r o f t r a c e s i l l u s t r a t e s the e f f e c t s o f i n t r o d u c i n g 1 0 % C O 2 t o t h e i n s p i r e d gas and t h e second p a i r o f t r a c e s i l l u s t r a t e s t h e r e t u r n t o v e n t i l a t i o n w i t h a i r . I n b o t h p a i r s o f r e c o r d i n g s , t h e t o p t r a c e i s t h e i n t r a t r a c h e a l p r e s s u r e and t h e l o w e r t r a c e t h e d i s c h a r g e o f a s i n g l e pulmonary r e c e p t o r d u r i n g pump v e n t i l a t i o n . The t i m e r e l a t i o n s h i p s p o r t r a y e d below t h e s e t r a c e s r e p r e s e n t t h e b r e a t h by b r e a t h e n d - i n s p i r a t o r y ( s o l i d symbols) and end-e x p i r a t o r y d i s c h a r g e (open symbols) o f t h e same r e c e p t o r f o l l o w i n g t h e i n t r o d u c t i o n ( a r r o w , l e f t ) and r e m o v a l ( a r r o w , r i g h t ) o f 5 % (dashed l i n e s ) and 1 0 % C O 2 ( s o l i d l i n e s ) t o t h e v e n t i l a t o r y gas. T r a c h e a l ^® p r e s s u r e ( k P a ) 0 V a g a l d i s c h a r g e 5 s e c 10 0 Hi lUllll ill I air-20+ V a g a l d i s c h a r g e 1 Q . r a t e ( H z ) 0-L ai r + C O . • A a i r 5 % C O . 10% c o 2 e n d e n d i n s p i r a t i o n e x p i r a t i o n IV) CD To" _o 11 Si k _rff_ _ i 1 0 10 20 S e q u e n c e of b r e a t h 30 F i g u r e 6. E f f e c t o f c h a n g i n g i n s p i r e d CO2 on pulmonary s t r e t c h r e c e p t o r d i s c h a r g e t h r o u g h o u t t h e v e n t i l a t i o n c y c l e . The t i m e i n t e r v a l h i s t o g r a m s (T.I.H.) and p o s t s t i m u l u s t i m e h i s t o g r a m s (P.S.T.H.) o f e i g h t s u c c e s s i v e b r e a t h s a r e shown a t two l e v e l s o f i n s p i r e d CO2 and t h r e e l e v e l s o f i n f l a t i o n volume ( V ^ ) . The b i n l e n g t h s f o r t h e T.I.H. were 500 msec and 150 msec f o r t h e P.S.T.H. 32 a l l phases o f t h e v e n t i l a t i o n c y c l e r e g a r d l e s s o f v e n t i l a t i o n volume. F i g u r e 7 shows t h e r e l a t i v e e f f e c t o f i n c r e a s i n g t h e i n s p i r e d C 0 2 c o n c e n t r a -t i o n on t h e p e a k - i n s p i r a t o r y and e n d - e x p i r a t o r y d i s c h a r g e o f a l l f i b r e s s t u d i e d . There i s a l a r g e range o f v a r i a t i o n i n r e s p o n s e between f i b r e s . As under s t e a d y s t a t e c o n d i t i o n s , t h e r e s p o n s e was most s e n s i t i v e o v e r t h e lo w e r range o f a i r w a y c o n c e n t r a t i o n s . R e c o v e r y t o p r e v i o u s d i s c h a r g e l e v e l s on r e t u r n t o b r e a t h i n g room a i r was much s l o w e r t h a n t h e i n h i b i t i o n o f d i s c h a r g e d u r i n g CC^ a d m i n i s t r a t i o n ( F i g . 5 ) . R e c o v e r y r a t e s were i n v e r s e l y r e l a t e d t o t h e C 0 2 l e v e l s employed and p e a k - i n s p i r a t o r y d i s c h a r g e r a t e i n v a r i a b l y r e c o v e r e d q u i c k e r t h a n end-e x p i r a t o r y d i s c h a r g e r a t e ( F i g . 5 ) . E f f e c t o f Changing t h e I n s p i r e d CO 2 C o n c e n t r a t i o n on Lung M e c h a n i c s T h e r e was l i t t l e o r no e f f e c t o f low l e v e l s o f C 0 2 on l u n g c o m p l i a n c e i n t h e s e s t u d i e s . A t h i g h e r l e v e l s o f C 0 2 , i n e i g h t a n i m a l s examined, t h e r e was u s u a l l y a s l i g h t d e c r e a s e i n dynamic c o m p l i a n c e ; a v e r a g e v a l u e s f a l l i n g f r om 0.96 ± 0.09 (S.E. o f mean) t o 0.82 ± 0.10 ml/kPa/kg i n a n i m a l s b r e a t h i n g a i r and 10% C 0 2 i n a i r r e s p e c t i v e l y ( T a b l e 1 & F i g . 8 ) . E f f e c t o f Changes i n Transpulmonary P r e s s u r e and Lung Volume  on Pulmonary. S t r e t c h R e c e p t o r D i s c h a r g e The i n d e p e n d e n t e f f e c t s o f t r a n s p u l m o n a r y p r e s s u r e and l u n g volume on pulmonary s t r e t c h r e c e p t o r d i s c h a r g e were a n a l y z e d on s i x f i b r e s i n f i v e a n i m a l s . L a r g e s c a l e changes i n t r a n s p u l m o n a r y p r e s s u r e a c h i e v e d by a l t e r -i n g i n t r a - a b d o m i n a l p r e s s u r e d u r i n g c o n s t a n t volume v e n t i l a t i o n , had v e r y l i t t l e e f f e c t on pulmonary r e c e p t o r d i s c h a r g e ( F i g . 9 ) . O n l y when t r a n s -pulmonary p r e s s u r e was r e d u c e d t o v e r y low v a l u e s was t h e r e any r e s p o n s e i n 33 F i g u r e 7. The e f f e c t o f c h a n g i n g a i r w a y CC^ c o n c e n t r a t i o n s on pulmon-a r y r e c e p t o r d i s c h a r g e d u r i n g pump v e n t i l a t i o n . Peak i n s p i r a t o r y and e n d - e x p i r a t o r y d i s c h a r g e f r e q u e n c y i n 62 pulmonary s t r e t c h r e c e p t o r s a t i n s p i r e d CC^ c o n c e n t r a t i o n s o f 0, 5 and 10% a r e shown. The mean v a l u e s f o r a l l f i b r e s a t each gas c o n c e n t r a t i o n a r e shown by t h e s o l i d symbols (@). P u m o n a r y s t r e t c h r e c e p t o r d i s c h a r g e (% peak i n s p i r a t o r y d i s c h a r g e at F = 0 ) ICQ2 35 T a b l e 1. The e f f e c t o f c h a n g i n g a i r w a y CO^ c o n c e n t r a t i o n s on dynamic l u n g c o m p l i a n c e . T u r t l e L u n g . C o m p l i a n c e ( m l / k P a / k g ) ;. A i r A i r + 10% CO 1 1.12 0.78 2 0.97 0.90 3 1.12 0.99 4 . 1.18 1.14 5 0.96 0.96 6 0.36 0.22 7 1.00 0.91 8 0.98 0.64 X 0.96 ± 0.09* 0.82 *±S.E. 36 F i g u r e 8. The e f f e c t of c h a n g i n g a i r w a y CC^ c o n c e n t r a t i o n s on p u l -monary s t r e t c h r e c e p t o r d i s c h a r g e and dynamic c o m p l i a n c e . The top t r a c e shows t h e d i s c h a r g e r a t e o f a s i n g l e pulmon-a r y s t r e t c h r e c e p t o r ; t h e m i d d l e t r a c e , i n f l a t i o n volume; and t h e bottom t r a c e , t r a n s p u l m o n a r y p r e s s u r e . 1 0 % CO2 was removed from t h e v e n t i l a t i n g gas a t t h e arrow ( i ) . The t r a c e s f o r two c y c l e s ( i n d i c a t e d ) , one b e f o r e ( l e f t ) and one a f t e r ( r i g h t ) r e m o v a l o f t h e C O 2 a r e shown on an e x t e n d e d t i m e s c a l e t o t h e s i d e s o f t h e c o n t i n u o u s r e c o r d -\ i n g s . The p r e s s u r e - v o l u m e p l o t s f o r t h e s e two b r e a t h s a r e shown below t h e e x t e n d e d t r a c e o f each r e s p e c t i v e b r e a t h a l o n g w i t h t h e i r computed dynamic c o m p l i a n c e s . B o t h p l o t s c y c l e i n a c l o c k w i s e d i r e c t i o n . 5 s e c V a g a l 1 0 d i s c h a r g e r a t e (Hz) 0 I n f l a t i o n v o l u m e ( m l ) 2 5 i — 0 0-4 T r a n s p u l m o n a r y p r e s s u r e ( k P a ) O 0 - 4 H T r a n s p u l m o n a r y p r e s s u r e ( k P a ) O—I i r O 2 5 I n f l a t i o n v o l u m e ( m l ) r 3 0 s e c 5 s e c 38 Figure 9. The e f f e c t of changes i n transpulmonary pressure at cons-tant i n f l a t i o n volume on pulmonary receptor discharge. The upper t r a c e shows the instantaneous discharge frequency of a s i n g l e pulmonary r e c e p t o r , the second t r a c e shows transpulmonary pressure, the t h i r d t r a c e shows i n f l a t i o n volume and the f o u r t h t r a c e shows the i n t r a t r a c h e a l pressure during two v e n t i l a t i o n c y c l e s to each of four peak transpulmonary pressures. s e c 2 0 - r d i s c h a r g e r a t e ( H z ) T r a n s -p u l m o n a r y Pressure ( k P a ) V o l u m e (ml) O-4-2 + O-30-0-L I n t r a -t r a c h e a l P r e s s u r e 2 ( k P a ) 4 -r 0-L CD 40 r e c e p t o r d i s c h a r g e r a t e . Changes i n v e n t i l a t i o n volume d u r i n g i n f l a t i o n t o c o n s t a n t peak t r a n s p u l m o n a r y p r e s s u r e , on t h e o t h e r hand, were always s u c c e s s f u l i n a l t e r i n g r e c e p t o r d i s c h a r g e r a t e ( F i g . 1 0 ) . F u r t h e r , d u r i n g n o r m a l b r e a t h i n g , f o r any g i v e n l u n g volume, t r a n s p u l m o n a r y p r e s s u r e i s always g r e a t e r d u r i n g i n f l a t i o n . T h i s g i v e s r i s e t o a c l o c k w i s e r o t a t i n g h y s t e r e s i s l o o p when t r a n s p u l m o n a r y p r e s s u r e i s p l o t t e d a g a i n s t volume t h r o u g h a c o m p l e t e pump c y c l e ( F i g . 1 1 ) . When t h e d i s c h a r g e r a t e o f a pulmonary r e c e p t o r i s p l o t t e d a g a i n s t t h e a s s o c i a t e d t r a n s p u l m o n a r y p r e s s u r e d u r i n g a s i n g l e v e n t i l a t i o n c y c l e , a c o u n t e r c l o c k w i s e h y s t e r e s i s l o o p r e s u l t s . A t any g i v e n l e v e l o f t r a n s p u l m o n a r y p r e s s u r e , d i s c h a r g e i s always l e s s d u r i n g i n f l a t i o n . When t h e same r e c e p t o r d i s c h a r g e i s p l o t t e d a g a i n s t t h e a s s o c i a t e d l u n g volume f o r a g i v e n pump c y c l e , t h e r e i s no h y s t e r e s i s ; d i s c h a r g e i s always t h e same a t any g i v e n l u n g volume r e g a r d l e s s o f whether t h e l u n g s a r e i n f l a t i n g o r d e f l a t i n g . 41 F i g u r e 10. The e f f e c t o f changes i n i n f l a t i o n volume w i t h i n f l a t i o n t o c o n s t a n t peak t r a n s p u l m o n a r y p r e s s u r e on pulmonary r e c e p t o r d i s c h a r g e . The upper t r a c e i s t h e i n s t a n t a n e o u s d i s -c h a r g e f r e q u e n c y o f a s i n g l e pulmonary s t r e t c h r e c e p t o r , t h e m i d d l e t r a c e i s t h e t r a n s p u l m o n a r y p r e s s u r e and t h e l o w e r t r a c e i s t h e i n f l a t i o n volume o f two v e n t i l a t i o n c y c l e s each w i t h two i n f l a t i o n volumes. 40 discharge rate 20-(Hz) 0-Transpulmonary _ Pressure ^ (kPa) 0-Volume 20 43 F i g u r e 11. The r e l a t i o n s h i p s between changes i n t r a n s p u l m o n a r y p r e s s u r e , i n f l a t i o n volume and pulmonary s t r e t c h r e c e p t o r d i s c h a r g e d u r i n g a s i n g l e b r e a t h . (a) P l o t o f t r a n s p u l m o n a r y p r e s s u r e v e r s u s i n f l a t i o n volume d u r i n g a s i n g l e b r e a t h . (b) P l o t o f d i s c h a r g e f r e -quency o f a s i n g l e pulmonary s t r e t c h r e c e p t o r v e r s u s t r a n s p u l m o n a r y p r e s s u r e d u r i n g t h e same b r e a t h , (c) P l o t o f t h e same changes i n d i s c h a r g e f r e q u e n c y v e r s u s i n f l a t i o n volume d u r i n g t h e same b r e a t h . The tim e r e l a t i o n o f each p l o t i s i n d i c a t e d by t h e arrows 45 DISCUSSION The f u n c t i o n a l c h a r a c t e r i s t i c s o f s l o w l y a d a p t i n g pulmonary s t r e t c h r e c e p t o r s i n t u r t l e s a r e v e r y s i m i l a r t o t h o s e o f mammals. Most r e c e p t o r s e x h i b i t e d t o n i c a c t i v i t y a t r e s t i n g l u n g volume ( z e r o t r a n s m u r a l p r e s s u r e ) as do most i n t r a - and e x t r a p u l m o n a r y s t r e t c h r e c e p t o r s i n mammals,(Mustafa & P u r v e s , '72; P a i n t a l , '73; M i s e r o c c h i and Sant'Ambrogio, '74; B a r t l e t t e t a l . , ' 76). The r e c e p t o r s e x h i b i t e d a l i n e a r r e s p o n s e t o i n c r e a s i n g l u n g volume o v e r t h e range s t u d i e d . A t low i n f l a t i o n volumes peak d i s c h a r g e was u n a f f e c t e d by t h e i n f l a t i o n r a t e , however, as t h e i n f l a t i o n volume i n c r e a s e d , peak d i s c h a r g e i n c r e a s e d w i t h b o t h t h e r a t e and degree o f i n f l a t i o n . R egard-l e s s o f t h e i n f l a t i o n r a t e , d i s c h a r g e a lways f e l l " t o a t o n i c l e v e l dependent o n l y on l u n g volume. S i m i l a r r e s u l t s a r e w e l l documented i n t h e c a t and dog ( A d r i a n , '33; D a v i s e t a l . , '56). The axons a r i s i n g from pulmonary r e c e p t o r s a r e o f s i m i l a r d i a m e t e r i n t u r t l e s and mammals ( P a i n t a l , '73) y e t t h e c o n d u c t i o n v e l o c i t i e s i n t u r t l e s (x = 7.4 m/sec) a r e much s l o w e r t h a n t h o s e o f mammals (36-39 m/sec) ( P a i n t a l , '73). A l s o , t h e d i s c h a r g e f r e q u e n -c i e s o f t h e r e c e p t o r s i n t u r t l e s were much l o w e r and t h e s e n s i t i v i t y o f t h e r e c e p t o r s ( d e f i n e d as t h e change i n d i s c h a r g e r a t e f o r a g i v e n change i n pulmonary p r e s s u r e d u r i n g i n f l a t i o n ) was a l s o l e s s ; 15.1 Hz/kPa i n t h e t u r t l e compared t o 30 Hz/kPa i n t h e dog ( M i s e r o c c h i & Sant'Ambrogio, '74; B r a d l e y et^ a l . , ' 76). These d i f f e r e n c e s , however, a r e p r o b a b l y a t t r i b u t a b l e t o t h e much l o w e r body t e m p e r a t u r e o f t h e t u r t l e s (22-23°C). I n mammals, t h e d i s c h a r g e o f pulmonary s t r e t c h r e c e p t o r s a s s o c i a t e d w i t h any g i v e n l u n g volume d u r i n g l u n g i n f l a t i o n a ppears t o c o n s i s t o f a component a s s o c i a t e d w i t h t h e a c t u a l volume and a component a s s o c i a t e d w i t h a i r 46 f l o w ( D a v i s e t a l . , '56). S i n c e t h i s same r e l a t i o n a l s o e x i s t s between t r a n s m u r a l p r e s s u r e and volume and f l o w , many i n v e s t i g a t o r s have s u c c e s s -f u l l y c o r r e l a t e d r e c e p t o r d i s c h a r g e w i t h t h e l e v e l and r a t e o f change ( d P / d t ) o f t r a n s p u l m o n a r y p r e s s u r e , b o t h i n v i v o ( D a v i s e t a l . , '56; Sant'Ambrogio e t a l . , '74; B a r t l e t t e t a l . , '76) and i n v i t r o ( B r a d l e y & S c h e u r m i e r , '77). Two s e p a r a t e l i n e s o f e v i d e n c e p r e s e n t e d i n t h i s s t u d y , however, show t h a t i n t h e t u r t l e , pulmonary s t r e t c h r e c e p t o r d i s c h a r g e i s a s s o c i a t e d o n l y w i t h l u n g volume. Wide s c a l e a l t e r a t i o n s i n t h e t r a n s p u l -monary p r e s s u r e d e v e l o p e d d u r i n g l u n g v e n t i l a t i o n f a i l e d t o have much n o t i c e a b l e e f f e c t on peak r e c e p t o r d i s c h a r g e f r e q u e n c y as l o n g as t h e i n f l a t i o n volume was h e l d c o n s t a n t ( F i g . 9 ) . When, on t h e o t h e r hand, a c o n s t a n t l e v e l o f t r a n s p u l m o n a r y p r e s s u r e was d e v e l o p e d w i t h v a r y i n g i n f l a t i o n volumes, peak r e c e p t o r d i s c h a r g e r a t e always r e f l e c t e d t h e changes i n i n f l a t i o n volume ( F i g . 1 0 ) . F u r t h e r , i n t h e absence o f such m a n i p u l a t i o n s , b u t d u r i n g n o r m a l pump v e n t i l a t i o n , pulmonary s t r e t c h r e c e p t o r d i s c h a r g e f r e q u e n c y always f o l l o w e d t h e changes i n l u n g volume e x a c t l y w h i l e b o t h d i s c h a r g e and volume l a g g e d b e h i n d t h e changes i n t r a n s m u r a l p r e s s u r e ( F i g . 1 1 ) . There i s no q u e s t i o n t h a t t h e r e s p o n s e o f a r e c e p t o r t o t r a n s p u l m o n -a r y p r e s s u r e changes r e q u i r e s t h a t a n e t f o r c e be d e v e l o p e d a c r o s s t h e s e p t a o r w a l l c o n t a i n i n g t h e r e c e p t o r . The e x t e n t t o w h i c h t h i s o c c u r s d e c r e a s e s w i t h each g e n e r a t i o n o f a i r w a y d i v i s i o n (Fung, '75). I n t u r t l e s , t h e p r i m a r y b r o n c h i ( f i r s t g e n e r a t i o n ) do^not d i v i d e but p a s s t h e l e n g t h o f t h e l u n g and end. Each bronchus r e m a i n s p a t e n t but i s p e r f o r a t e d a l o n g i t s l e n g t h p r o v i d i n g a c c e s s t o t h e 8 t o 10 m a j o r chambers o f t h e l u n g ( F i g . 2a, b) (Gans & Hughes, '67). Each chamber i s f u r t h e r s u b d i v i d e d by 47 numerous s e p t a and i t i s on t h e s e s e p t a t h a t t h e r e c e p t o r s a r e l o c a t e d ( F i g . 2 b ) . I n v i e w o f t h e f a c t t h e s e s e p t a must be exposed t o t h e same p r e s s u r e and f l o w changes on each s i d e d u r i n g i n f l a t i o n (Fung, ' 7 5 ) , i t i s not s u r p r i s i n g t h a t t h e r e c e p t o r s were i n s e n s i t i v e t o changes i n t r a n s p u l m o n -a r y p r e s s u r e . I n l i g h t o f t h i s , however, t h e mechanism b e h i n d t h e r e s p o n s e t o r a t e o f i n f l a t i o n r e c o r d e d a t h i g h i n f l a t i o n volumes remains u n c l e a r . I t i s p o s s i b l e t h a t t h e r e s p o n s e a r i s e s f r o m t r a n s i e n t changes i n t h e l o n g i -t u d i n a l t e n s i o n o f t h e s e p t a w h i c h o n l y appear d u r i n g a i r f l o w once t h e l u n g s a r e d i s t e n d e d beyond a c e r t a i n p o i n t . F o r t h e moment, t h i s must r e m a i n s p e c u l a t i v e . The a v e r a g e response o f pulmonary r e c e p t o r s i n t h e t u r t l e t o CC^ was a l s o s i m i l a r t o t h a t r e p o r t e d f o r mammals. The s e n s i t i v i t y o f r e c e p t o r s t o s t a t i c volume i n f l a t i o n s was r e d u c e d t o 9.2 Hz/kPa and 6.8 Hz/kPa when a n i m a l s were v e n t i l a t e d w i t h 5 and 10% CO2 r e s p e c t i v e l y . These v a l u e s r e p r e s e n t 39 and 55% r e d u c t i o n s , s l i g h t l y g r e a t e r t h a n t h e 20-40% r e d u c t i o n s r e p o r t e d i n r a b b i t s , c a t s and dogs v e n t i l a t e d w i t h 7-9% CO2 i n O2 ( M u s t a f a & P u r v e s , '72; Kunz e t a l . , '76; B r a d l e y e_t a l . , '76). As r e p o r t e d f o r t h e dog ( B r a d l e y e t a l . , '76) t h e e f f e c t s were n o t l i n e a r b u t were g r e a t e r o v e r t h e l o w e r r a n g e o f CO2 c o n c e n t r a t i o n s . The t i m e c o u r s e o f t h e i n c r e a s e i n m e c h a n i c a l t h r e s h o l d and d e c r e a s e i n s e n s i t i v i t y o f r e c e p t o r s when CO2 was a d m i n i s t e r e d d u r i n g pump v e n t i l a t i o n were s i m i l a r i n b o t h t u r t l e s and mammals ( M u s t a f a & P u r v e s , ' 7 2 ; Schoener & F r a n k e l , ' 7 2 ; B r a d l e y e t a l . , ' 7 6 ) as was t h e t i m e c o u r s e o f t h e i r r e c o v e r y when CO2 was removed ( A d r i a n , ' 3 3 ; M u s t a f a & P u r v e s , ' 7 2 ; B r a d l e y e^ t a l . , ' 7 6 ; B a r t l e t t & S a n t ' A m b r o g i o , ' 7 6 ) . I n t e r e s t i n g l y , however, a l t h o u g h t h e r a t e o f r e s p o n s e o f peak i n s p i r a t o r y and e n d - e x p i r a t o r y d i s c h a r g e were s i m i l a r when CO2 was a d m i n i s t e r e d , peak i n s p i r a t o r y r a t e o f t e n r e c o v e r e d more q u i c k l y t h a n t h e 48 e n d - e x p i r a t o r y d i s c h a r g e r a t e when C 0 2 was removed. H y p e r c a p n i a has been f o u n d t o i n h i b i t s t r e t c h r e c e p t o r d i s c h a r g e l e s s d u r i n g t h e i n s p i r a t o r y phase o f a r t i f i c i a l v e n t i l a t i o n t h a n d u r i n g t h e e x p i r a t o r y phase i n mammals ( M u s t a f a & P u r v e s , '72; Sant'Ambrogio e t a l . , '74; B r a d l e y e t a l . , '76; B a r t l e t t & S a n t ' A m b r o g i o , '76) b u t t h i s o n l y o c c u r s d u r i n g t h i s r e c o v e r y phase i n t h e t u r t l e . D u r i n g pump v e n t i l a t i o n , as w i t h s t a t i c l u n g i n f l a t i o n s , t h e s e n s i t i v i t y o f r e c e p t o r d i s c h a r g e t o C 0 2 was g r e a t e r o v e r t h e l o w e r range o f CC^ concen-t r a t i o n s . T h i s t o o has been o b s e r v e d i n mammals ( B r a d l e y e t a l . , ' 7 6 ; B a r t -l e t t & S a n t ' A m b r o g i o , 1 7 6 ) . Over a range o f C 0 2 c o n c e n t r a t i o n s from 0 t o 5%, t h e r e was an a v e r a g e r e d u c t i o n o f 10% o f c o n t r o l i n peak d i s c h a r g e r a t e p e r 10 mm Hg i n c r e a s e i n P and 7% o f c o n t r o l i n e n d - e x p i r a t o r y d i s c h a r g e p e r 2 10 mm Hg i n c r e a s e i n " P c o . These v a l u e s t e n d t o be s l i g h t l y g r e a t e r t h a n t h o s e r e p o r t e d f o r mammals ( M u s t a f a & P u r v e s , '72; B r a d l e y e t a l . , '76). I t seems e v i d e n t t h a t C 0 2 a c t s d i r e c t l y on t h e pulmonary r e c e p t o r s i n mammals r a t h e r t h a n on l u n g volume o r t r a n s p u l m o n a r y p r e s s u r e as C 0 2 a p p e a r s t o have l i t t l e d i r e c t o r r e f l e x e f f e c t on e i t h e r t h e f u n c t i o n a l r e s i d u a l c a p a c i t y (FRC) ( M u s t a f a & P u r v e s , '72) o r t h e t o t a l l u n g r e s i s t a n c e (TLR) ( M u s t a f a _ P u r v e s , '72; Sant'Ambrogio et_ a l . , '74; B r a d l e y e t a l . , '76). T h i s would c e r t a i n l y a ppear t o be t h e c a s e i n t u r t l e s s i n c e , any C 0 2 i n d u c e d d e c r e a s e i n l u n g r e s i s t a n c e must appear as a d e c r e a s e i n t r a n s p u l m o n a r y p r e s s u r e d u r i n g c o n s t a n t volume v e n t i l a t i o n and i t has been shown t h a t changes i n t r a n s p u l m o n a r y p r e s s u r e have v e r y l i t t l e e f f e c t on r e c e p t o r d i s c h a r g e . F u r t h e r , h i g h l e v e l s o f C 0 2 d e c r e a s e dynamic l u n g c o m p l i a n c e i n t h e t u r t l e ( F i g . 8) ( T a b l e 1 ) , t h u s i f t h e r e c e p t o r s were s e n s i t i v e t o t r a n s p u l m o n a r y p r e s s u r e , t h i s e f f e c t w o u l d t e n d t o mask, not enhance, t h e 4 9 e f f e c t s o f t h e C 0 2 -I t i s i n t e r e s t i n g t o n o t e t h a t t h e q u a n t i f i c a t i o n o f t h e dP/dt s e n s i -t i v i t y o f s t r e t c h r e c e p t o r s i n mammals has been f o c u s e d p r i m a r i l y upon t r a c h e a l ( B a r t l e t t ' e t ' _ a l . , '76; B r a d l e y & S c h e u r m i e r , '77) o r t y p e I ( M i s e r o c c h i et_ al., '73) r e c e p t o r s i n t h e e x t r a p u l m o n a r y a i r w a y s and t h a t t h e s e r e c e p t o r s a r e i n s e n s i t i v e t o C C ^ The pulmonary r e c e p t o r s w h i c h a r e s e n s i t i v e t o CG^ appear t o be t h e b r o n c h i a l ( B a r t l e t t & Sant'Ambrogio, '76) o r t y p e I I r e c e p t o r s ( M i s e r o c c h i e t a l . , '73) w h i c h . a r e t h e e x c l u s i v e r e c e p t o r t y p e i n t h e i r p r i m a r y l o c a t i o n , t h e s m a l l b r o n c h i ( > 1 mm d i a m e t e r ) . T h i s i s a p o s i t i o n where l u n g s t r e t c h ( c i r c u m f e r e n t i a l and l o n g i t u d i n a l ) and n o t t r a n s p u l m o n a r y p r e s s u r e w i l l be t h e p r i m a r y s t i m u l u s . On t h e b a s i s o f f u n c t i o n a l c h a r a c t e r i s t i c s , l o c a t i o n and s e n s i t i v i t y t o i t w o u l d appear t h a t t u r t l e pulmonary s t r e t c h r e c e p t o r s d i f f e r o n l y q u a n t i -t a t i v e l y f rom t h e s e mammalian i n t r a p u l m o n a r y ( b r o n c h i a l ) s t r e t c h r e c e p t o r s . The range o f s e n s i t i v i t y t o C O 2 e x h i b i t e d by t u r t l e pulmonary r e c e p t o r s was much g r e a t e r t h a n t h a t found i n mammals and a l t h o u g h some (n = 3) were i n s e n s i t i v e t o C O 2 s e v e r a l were t o t a l l y i n h i b i t e d t h r o u g h o u t a l l phases o f t h e v e n t i l a t i o n c y c l e a t C O 2 l e v e l s o f 5 t o 1 0 % ( F i g . 5, 7 ) . The degree o f s e n s i t i v i t y t o C O 2 e x h i b i t e d by t h e s e few r e c e p t o r s i s e q u a l t o t h a t o f t h e i n t r a p u l m o n a r y C O 2 r e c e p t o r s w h i c h have been d e s c r i b e d i n b i r d s and l i z a r d s (Fedde et^ a l . , '74a; Fedde e_t. _ a l . ,' 77) . These pulmonary C O 2 r e c e p t o r s a r e e x t r e m e l y s e n s i t i v e t o C O 2 and a r e a p p a r e n t l y i n s e n s i t i v e t o changes i n l u n g volume and t r a n s m u r a l p r e s s u r e (Osborne & B u r g e r , '74; Fedde e t a l . , '74a). I t i s b e l i e v e d t h a t t h e s e r e c e p t o r s m o n i t o r t h e r a t e and degree o f l u n g v e n t i l a t i o n by t h e f l u c t u a t i o n s i n i n t r a p u l m o n a r y C O 2 c o n c e n t r a t i o n t h r o u g h -o u t t h e r e s p i r a t o r y c y c l e (Molony, '74; Fedde et_ al., '74b) and a r e a ma j o r 50 f a c t o r i n t h e r e g u l a t i o n o f a v i a n r e s p i r a t i o n (Osborne & B u r g e r , '74; Kunz & M i l l e r , 74a,b). The f i n d i n g t h a t s u c h l e v e l s o f CO2 s e n s i t i v i t y a r e n o t w i t h o u t e q u a l i n t h e pulmonary s t r e t c h r e c e p t o r s o f o t h e r v e r t e -b r a t e s p l a c e s new emphasis on a t t e m p t s t o d e s c r i b e t h e p h y s i o l o g i c a l r o l e s and f u n c t i o n a l c h a r a c t e r i s t i c s o f t h e s e r e c e p t o r g roups. 51 SECTION I I The R o l e o f Pulmonary A f f e r e n t I n f o r m a t i o n and H y p e r c a p n i a i n C o n t r o l o f t h e B r e a t h i n g P a t t e r n i n t h e T u r t l e \ INTRODUCTION A p a t t e r n o f a r r h y t h m i c b r e a t h i n g c o n s i s t i n g o f a s e r i e s o f one t o s e v e r a l b r e a t h s s e p a r a t e d by a h i g h l y v a r i a b l e , r e s p i r a t o r y pause ( f r o m a few seconds t o s e v e r a l h o u r s ) commencing a t e n d - i n s p i r a t i o n has been d e s c r i b e d f o r most s p e c i e s o f r e p t i l e s ( t u r t l e s , McCutcheon,'43; Gans & Hughes, '67; c r o c o d i l e s , N a i f e h e t a l . , ' 7 0 ^ G a n s & C l a r k , '76; s n a k e s , ' G l a s s & Jo h a n s e n , ' 7 6 ) . D e s p i t e t h e s i m i l a r i t i e s between t h i s b r e a t h i n g p a t t e r n and v a r i o u s p a t t e r n s o f abnormal b r e a t h i n g i n mammals w i t h d e f e c t i v e c e n t r a l r e s p i r a t o r y c o n t r o l , e v i d e n c e i n d i c a t e s t h a t r e p t i l e s a c c u r a t e l y a d j u s t v e n t i l a t i o n t o m a i n t a i n b l o o d pH a t a s p e c i f i c , t e mperature-dependent v a l u e ( c f . H o w e l l & Rahn, '76, f o r r e v i e w ) . A l t h o u g h many s t u d i e s have a n a l y z e d t h e e f f e c t s o f r e s p i r a t o r y s t i m u l i s u c h as h y p o x i a , h y p e r c a p n i a and t e m p e r a t u r e i n r e p t i l e s , t h e s e s t u d i e s p r i m a r i l y a s s e s s t h e e f f e c t s upon m e t a b o l i s m , t o t a l pulmonary v e n t i l a t i o n o r d i v e l e n g t h i n a q u a t i c s p e c i e s ( c f . Wood & L e n f a n t , '76, f o r r e v i e w ) . V e r y l i t t l e i s known about t h e f a c t o r s and mechanisms c o n t r o l l i n g t h e a r r h y t h m i c b r e a t h i n g p a t t e r n . H y p o x i a a p p ears t o be the major f a c t o r c o n t r o l l i n g r e s p i r a t i o n i n r e p -t i l e s ( N i e l s e n , '61a,b; Templeton & Dawson, '63; G l a s s & Johansen, '76; Wood & L e n f a n t , '76) i n c l u d i n g t u r t l e s ( L e n f a n t e_t a l . , ' 7 0 ) , y e t many o f t h e s e a n i m a l s e x h i b i t an i n c r e d i b l e t o l e r a n c e t o a n o x i a ( B e l k i n , '63a,b, '68). 52 R e p o r t s on t h e e f f e c t s o f h y p e r c a p n i a on v e n t i l a t i o n i n t u r t l e s range from a s l i g h t o r moderate i n c r e a s e ( M i l l e n e t a l . , ' 6 3 ; Wood & L e n f a n t , '76) t o a p o w e r f u l s t i m u l a t i o n ( J a c k s o n e t a l . , ' 7 4 ) . Many o f t h e s e d i s c r e p a n c i e s a r i s e from s p e c i e s d i f f e r e n c e s , l e v e l s o f a n a e s t h e s i a and t h e p r e s e n c e o r absence o f avenues o f cutaneous gas exchange. F u r t h e r m o r e , i n many s t u d i e s o n l y v e n t i l a t i o n r a t e s and n o t volumes were measured w h i c h makes a c c u r a t e c o m p a r i s o n a l m o s t i m p o s s i b l e . The p r e s e n t s t u d y was u n d e r t a k e n t o d e s c r i b e and a n a l y z e t h e b r e a t h i n g p a t t e r n i n t h e t u r t l e Chrysemys p i c t a . The r o l e o f pulmonary a f f e r e n t i n f o r -m a t i o n c a r r i e d i n t h e vagus n e r v e , w i t h and w i t h o u t c o n c o m i t a n t h y p e r c a p n i a ( a t n o r m o x i a and c o n s t a n t t e m p e r a t u r e ) , have been s t u d i e d as a f i r s t s t e p i n a s s e s s i n g t h e mechanisms i n v o l v e d i n t h e c o n t r o l o f t h i s b r e a t h i n g p a t t e r n . 53 METHODS S u r g i c a l P r o c e d u r e s E x p e r i m e n t s were p e r f o r m e d on u n a n a e s t h e t i z e d , l i g h t l y r e s t r a i n e d specimens o f t h e f r e s h w a t e r t u r t l e , Chrysemys p i c t a (600-1200 g) a t room t e m p e r a t u r e (22-23°C). The optimum t e m p e r a t u r e range f o r t h i s t e m p e r a t e s p e c i e s i s 20-25°C ( C a g l e , ' 5 4 ) . U s i n g a c o m b i n a t i o n o f c o l d (1-4 h r a t -20 C) and l o c a l a n a e s t h e s i a ( 2 % L i d o c a i n e h y d r o c h l o r i d e ) , a t r a c h e a l c a n n u l a was i n s e r t e d and t h e v a g i b i l a t e r a l l y exposed. A pneumotachograph w i t h a s i d e arm f o r t r a c h e a l p r e s s u r e measurement and gas s a m p l i n g was a t t a c h e d t o t h e t r a c h e a l c a n n u l a and, i n t h e m a j o r i t y o f c a s e s , t h e open end o f t h e pneumotachograph was a t t a c h e d t o a p l a s t i c T - p i e c e . One arm o f t h e T was open t o atmosphere and t h e o t h e r was a t t a c h e d t o a gas s u p p l y . U s i n g a system o f a i r f l o w m e t e r s , the c o m p o s i t i o n o f t h e gas f l o w i n g p a s t t h e end o f t h e t r a c h e a l c a n n u l a c o u l d be a l t e r e d thus c o n t r o l l i n g t h e c o m p o s i t i o n o f t h e i n s p i r e d a i r when t h e t u r t l e b r e a t h e d . Due t o t h i s p r o c e d u r e , t h e l u n g r e m a i n e d open t o atmosphere t h r o u g h o u t t h e r e s p i r a t o r y pause and th u s l u n g volume r e t u r n e d t o a c o n s t a n t f u n c t i o n a l r e s i d u a l volume d u r i n g t h i s p e r i o d . I n t h e i n t a c t a n i m a l , t h e r e s p i r a t o r y pause o c c u r s a t e n d - i n s p i r a t i o n and t h e e s t a b l i s h e d l u n g volume i s not o n l y v a r i a b l e b u t i n f l u e n c e s t h e r e s p i r a -t o r y f r e q u e n c y ( M i l s o m & Jo h a n s e n , ' 7 5 ) . To e s t a b l i s h w hether t h e m a i n t e n -ance o f a c o n s t a n t f u n c t i o n a l r e s i d u a l c a p a c i t y by our p r o c e d u r e i n f l u e n c e d t h e b r e a t h i n g p a t t e r n o r t i m e c o u r s e o f t h e v a r i o u s r e s p i r a t o r y p h a s e s , t h e d i s t a l end o f t h e t r a c h e a l c a n n u l a was r e a t t a c h e d t o t h e c u t p e r i p h e r a l end o f t h e t r a c h e a on s e v e r a l o c c a s i o n s . These a n i m a l s b r e a t h e d t h r o u g h an i n t a c t g l o t t i s and showed no d i f f e r e n c e s i n t h e measured v a r i a b l e s from t h e 54 e x p e r i m e n t a l a n i m a l s . However, as a consequence o f h o l d i n g t h e e n d - i n s p i r a -t o r y ( b r e a t h h o l d ) volume c o n s t a n t , any e f f e c t s o f C 0 2 , vagotomy, t i d a l volume o r b r e a t h h o l d l e n g t h on t h i s volume w i l l n o t have been o b s e r v e d . R e c o r d i n g T e c h n i q u e s T r a c h e a l p r e s s u r e was measured w i t h a Statham P23V p r e s s u r e t r a n s d u c e r and the p r e s s u r e drop a c r o s s t h e pneumotachograph s c r e e n d u r i n g t r a c h e a l a i r f l o w w i t h a H e w l e t t - P a c k a r d 268 BC d i f f e r e n t i a l p r e s s u r e t r a n s d u c e r . The a i r f l o w s i g n a l was f e d t h r o u g h a H e w l e t t - P a c k a r d 350-3700 A i n t e g r a t i n g p r e a m p l i f i e r t o g i v e t i d a l volume and a l l measurements; p r e s s u r e , f l o w and volume, were c o n t i n u s o u l y r e c o r d e d on a Sanborn 4 c h a n n e l c h a r t r e c o r d e r w r i t i n g on r e c t i l i n e a r c o - o r d i n a t e s . The 0 2 and C 0 2 c o m p o s i t i o n o f i n s p i r e d and e x p i r e d gases was e i t h e r d e t e r m i n e d on samples t a k e n t h r o u g h t h e s i d e arm o f t h e t r a c h e a l c a n n u l a and measured on a F i s h e r - H a m i l t o n gas p a r t i t i o n -e r o r by c o n t i n u o u s s a m p l i n g w i t h a C e n t r o n i c 200 MGA c l i n i c a l mass s p e c t r o -meter (sample r a t e <10 m l / m i n ) . P r o t o c o l A n i m a l s were a l l o w e d t o r e c o v e r from a n a e s t h e s i a f o r 4 t o 6 h o u r s b e f o r e e x p e r i m e n t a t i o n began. The a n i m a l s were s u r r o u n d e d by opaque s c r e e n s t o s h i e l d them from a l l a c t i v i t i e s o f t h e e x p e r i m e n t e r s and when r e s t i n g q u i e t l y were p r e s e n t e d w i t h m i x t u r e s o f 0, 5 o r 10% C 0 2 ' i n a i r t o b r e a t h e f o r o n e - h a l f t o one hour p e r i o d s i n random o r d e r . A l l measurements were r e c o r d e d c o n t i n u o u s l y b u t d a t a were s e l e c t e d f o r a n a l y s i s o n l y a f t e r t h e r e s p o n s e s t o each gas m i x t u r e had s t a b i l i z e d . I t must be s t r e s s e d t h a t due t o t h e i n c o m p l e t e l y d i v i d e d v e n t r i c l e o f t h e t u r t l e h e a r t t h e r e can be r i g h t t o l e f t s h u n t i n g o f b l o o d caused by pulmonary a r t e r i a l v a s o c o n s t r i c -t i o n ( W h i t e , '76; M i l s o m e t a l . , '77) and t h u s t u r t l e s may m a i n t a i n a 55 c o n s i d e r a b l e p a r t i a l p r e s s u r e d i f f e r e n c e between a l v e o l a r and a r t e r i a l CC^ (AaACC^) f o r l o n g p e r i o d s o f ti m e f o l l o w i n g changes i n t h e c o m p o s i t i o n o f i n s p i r e d gases ( G l a s s e t a l . , ' 78). C o n s e q u e n t l y c a r e was t a k e n t o a s s u r e t h a t ample t i m e was a l l o w e d f o r s t a b l e r e s p o n s e s t o d e v e l o p b e f o r e any measurements were made. On t h e second day o f e x p e r i m e n t a t i o n , t h e v a g i were s e c t i o n e d under l o c a l a n a e s t h e t i c and the above p r o t o c o l r e p e a t e d . Measurements and A n a l y s i s The i n t e r v a l s o f t h e v a r i o u s r e s p i r a t o r y phases were a l l measured fr o m t h e a i r f l o w r e c o r d i n g s . The s l o p e s o f a l l graphs were computed by s i m p l e l i n e a r r e g r e s s i o n a n a l y s i s o f t h e d a t a o r l o g a r i t h m i c t r a n s f o r m a -t i o n s o f t h e d a t a on a PDP 12 l a b m i n i - c o m p u t e r . U n l e s s o t h e r w i s e s t a t e d a l l v a l u e s a r e means ± S.E. 56 RESULTS The P a t t e r n o f B r e a t h i n g (a) I n t a c t A n i m a l s A f t e r a v a r i a b l e p e r i o d o f b r e a t h h o l d i n g i n t h e i n s p i r a t o r y p o s i t i o n ( n o n - v e n t i l a t o r y p e r i o d , NVP) a s e r i e s o f one t o s e v e r a l b r e a t h s w o u l d com-mence w i t h an a c t i v e e x p i r a t i o n and t e r m i n a t e a g a i n i n t h e e n d - i n s p i r a t o r y phase ( F i g . 12a and 1 3 ) . T h i s c o n s t i t u t e s t h e v e n t i l a t o r y p e r i o d (VP) ( F i g . 1 3 ) . Each v e n t i l a t o r y p e r i o d commenced w i t h a c t i v e e x p i r a t i o n r e g a r d l e s s o f whether t h e a n i m a l had an i n t a c t o r b y p a s s e d g l o t t i s ( i . e . r e g a r d l e s s o f l u n g v o l u m e ) . The t i m e i n t e r v a l o f each b r e a t h ( T t Q t ) c o n s i s t e d o f t h e e x p i r a t o r y i n t e r v a l (T ) and t h e subsequent i n s p i r a t o r y i n t e r v a l (T ) w h i c h u s u a l l y c o n s i s t e d o f an a c t i v e i n s p i r a t o r y phase (T-.') ( t h i s phase wo u l d be s i m i l a r t o t h e T j i n mammals), and a s h o r t i n t e r v a l d u r i n g w h i c h t h i s b r e a t h was h e l d a t e n d - i n s p i r a t i o n (T -T_.') ( F i g . 1 3 ) . T a b l e 2 l i s t s t h e mean v a l u e s (±S.E.) o f s e v e r a l o f t h e v a r i a b l e s meas-u r e d f o r each t r i a l (n) i n s i x t e e n a n i m a l s . I n c r e a s i n g t h e p e r c e n t a g e o f CC>2 i n t h e i n s p i r e d gas (ET^Q ) r e s u l t e d i n an i n c r e a s e d f r e q u e n c y o f b r e a t h -i n g ( f , c a l c u l a t e d on a l l b r e a t h s t a k e n p e r u n i t t i m e ) , t i d a l volume (V_) and t o t a l pulmonary v e n t i l a t i o n ( V _ ) . The i n c i d e n c e o f v e n t i l a t o r y p e r i o d s Ji (VP-min "'") i n c r e a s e d as a r e s u l t o f t h e l a r g e r e d u c t i o n i n t h e d u r a t i o n o f t h e n o n - v e n t i l a t o r y p e r i o d (T_j__,) w h i l e t h e number o f b r e a t h s w i t h i n each v e n t i l a t o r y p e r i o d ( B r e a t h s ' V P ~ 1 ) a l s o i n c r e a s e d ( F i g . 1 2 a ) . S i n c e t h e mean d u r a t i o n o f each i n d i v i d u a l b r e a t h (T ) d i d n o t change n o t i c e a b l y , t h e f r e q u e n c y o f b r e a t h s w i t h i n each v e n t i l a t o r y p e r i o d ( f y p ) remained unchanged 57 F i g u r e 1 2 . R e p r e s e n t a t i v e r e c o r d s o f t h e b r e a t h i n g p a t t e r n o f a t u r t l e and t h e e f f e c t s o f vagotomy and h y p e r c a p n i a on t h i s p a t t e r n . V a l u e s o f F and F_ a p p l y t o b o t h o2 co2 t h e prevagotomy and postvagotomy r e c o r d s . 5 9 F i g u r e 13. S c h e m a t i c d i a g r a m o f t h e b r e a t h i n g p a t t e r n i n a t u r t l e i l l u s t r a t i n g t h e v a r i o u s r e s p i r a t o r y i n t e r v a l s . See t e x t f o r t h e e x p l a n a t i o n o f a l l a b b r e v i a t i o n s . out i n 5 s e c V e n t i l a t o r y P e r i o d Breathing cycle M T 1 tot 'VP N o n - V e n t i l a t o r y P e r i o d •NVP T a b l e 2. V e n t i l a t o r y v a r i a b l e s o f t u r t l e s b r e a t h i n g a i r and CO^ gases. a i r 5% CC »2 + a i r 10% CO 2 + a i r n p r e vagotomy n p o s t vagotomy n p r e vagotomy p o s t n vagotomy p r e n vagotomy p o s t n vagotomy f (min 31 1.8 10 0.9 16 3.9 4 1.6 19 5.4 5 1.8 ±0.2 ±0.1 ±0.6 ±0.3 ±0.8 ±0.6 f v p (min 1 ) 13.5 9.0 12.2 9.0 13.5 8.4 ±2.0 ±1.6 ±2.7 ±1.4 ±2.0 ±2.0 V T (ml BTPS-Kg" 1) 13.5 50.6 17.5 82.4 32.6 112.7 ±0.9 ±8.4 ±2.0 ±24.2 ±4.8 ±20.8 V (ml BTPS-min" 1 •Kg-1) 24.8 ±3.4 44.9 ±6.1 65.5 ±11.4 125.1 ±46.8 167.6 ±35.8 184.9 ±47.4 VP*min 1 0.9 0.4 1.4 0.5 1.8 0.5 ±0.1 ±0.1 ±0.3 ±0.1 ±0.3 ±0.1 B r e a t h s - V P - 1 2.4 2.6 3.1 3.2 3.2 3.4 ±0.2 ±0.4 ±0.4 ±0.5 ±0.3 ±0.5 T t o t ( S 6 C ) 5.6 6.5 6.0 6.5 5.6 7.1 ±0.5 ±0.5 ±0.7 ±0.5 ±0.5 ±0.9 T E ( s e c ) 2.6 2.1 2.5 2.3 2.3 2.1 ±0.1 ±0.1 ±0.1 ±0.2 ±0.1 ±0.1 T j ' ( s e c ) 2.6 2.5 2.8 2.6 2.4 2.8 ±0.1 ±0.1 ±0.1 ±0.2 ±0.1 ±0.1 T j ( s e c ) 3.2 4.3 3.7 3.6 3.0 5.1 ±0.1 ±0.3 ±0.2 ±0.4 ±0.1 ±0.6 T v p ( s e c ) 14.0 16.6 18.9 21.1 18.1 24.3 ±2.0 ±2.1 ±3.2 ±4.3 ±2.2 ±4.2 TNVP < s e c> 93.9 169.6 41.2 124.3 24.5 127.8 ±18.0 ±39.6 ±10.8 ±36.3 ±7.8 ±45.1 TVP+NVP ( s e c ) 110.1 185.6 63.4 145.5 48.2 152.8 ±18.0 ±40.5 ±12.5 ±33.2 ±9.0 ±43.1 T TP VP+NVP 16.2 ±2.4 9.5 ±1.4 36.1 ±4.1 17.4 ±5.0 49.6 ±5.9 25.5 ±10.6 6 2 w h i l e the d u r a t i o n o f each v e n t i l a t o r y p e r i o d (T^p) i n c r e a s e d as a r e s u l t o f t h e l a r g e r number o f b r e a t h s i n each. As a consequence o f t h i s i n c r e a s e i n and t h e d e c r e a s e i n t h e subsequent T N V p » t n e p e r c e n t a g e o f t i m e s p e n t T a c t i v e l y b r e a t h i n g ( V P . - i ™ ) i n c r e a s e d . VP + NVP I t i s i n t e r e s t i n g t o n o t e t h a t t h e o b s e r v e d i n c r e a s e s i n t i d a l volume when a n i m a l s were b r e a t h i n g t h e CC^ gas m i x t u r e s were due t o an i n c r e a s e i n b o t h f o r c e ( F i g . 12a) and r a t e ( c o n s t a n t T t > T a b l e 2) o f e x p i r a t i o n and i n s p i r a t i o n . D e s p i t e t h e i n c r e a s e d r e s p i r a t o r y d r i v e the b r e a t h i n g p a t t e r n r emained a r r h y t h m i c . A t t h e maximum v e n t i l a t o r y f r e q u e n c i e s measured under s e v e r e h y p e r c a p n i c s t r e s s ( F T = 15%) b r e a t h i n g became v i r t u a l l y iC02 e u p n e i c as T ^ ^ was e l i m i n a t e d and f approached fyp> t h e d u r a t i o n o f each b r e a t h i n g c y c l e s t i l l r e m a i n i n g unchanged, (b) A f t e r B i l a t e r a l Vagotomy The e f f e c t o f vagotomy on t h e b r e a t h i n g p a t t e r n , s t u d i e d i n f i v e a n i m a l s ( F i g . 12b, T a b l e 2), was t o d e c r e a s e t h e r e s p i r a t o r y f r e q u e n c y and i n c r e a s e t h e t i d a l volume and t o t a l pulmonary v e n t i l a t i o n . The number o f b r e a t h s p e r v e n t i l a t o r y p e r i o d i n c r e a s e d s l i g h t l y b u t t h e f r e q u e n c y o f v e n t i l a t o r y p e r i o d s d e c r e a s e d . The a v e r a g e b r e a t h l e n g t h was p r o l o n g e d and thus t h e b r e a t h i n g f r e q u e n c y w i t h i n each v e n t i l a t o r y p e r i o d d e c r e a s e d . The n e t e f f e c t o f t h e s e changes was a s l i g h t l e n g t h e n i n g o f T^ p b u t s i n c e T ^ p was g r e a t l y l e n g t h e n e d t h e a c t u a l t i m e s p e n t a c t i v e l y b r e a t h i n g d e c r e a s e d . I n c r e a s i n g t h e CC^ con-c e n t r a t i o n i n t h e i n s p i r e d a i r o f t h e s e v a g o t o m i z e d a n i m a l s produced s i m i l a r t r e n d s t o t h o s e o b s e r v e d i n : i n t a c t a n i m a l s . A l t h o u g h t h e amount o f t i m e s p e n t a c t i v e l y b r e a t h i n g was l e s s a f t e r vagotomy, t h e p r e s e n c e o f 5 and 10% CO2 i n t h e i n s p i r e d a i r p r o d u c e d p r o p o r t i o n a t e l y t h e same e f f e c t i n n o r m a l 63 and v a g o t o m i z e d a n i m a l s , a d o u b l i n g and t r i p l i n g r e s p e c t i v e l y o f t h e amount of t i m e s p e n t a c t i v e l y b r e a t h i n g compared t o v e n t i l a t i o n w i t h room a i r . The t i d a l volume i n i n t a c t a n i m a l s b r e a t h i n g room a i r was a p p r o x i m a t e l y 10% o f t h e l u n g volume , ( M i l s o m , ' 7 5 ) . F o l l o w i n g vagotomy, t i d a l volume i n c r e a s e d t o 39% o f t h e l u n g volume. A d d i t i o n o f 10% C 0 2 t o t h e i n s p i r e d gas m i x t u r e r a i s e d t h e s e v a l u e s t o 25% and 87% f o r i n t a c t and v a g o t o m i z e d a n i m a l s r e s p e c t i v e l y . The R e l a t i o n s h i p s Between V_, V_ and f (a) I n t a c t A n i m a l s There i s a l l i n e a r ' r e l a t i o n s h i p between V_ p l o t t e d as a f u n c t i o n o f V_, ("Hey P l o t " ) i n agreement w i t h r e s u l t s o b t a i n e d a t low l e v e l s o f v e n t i l a t i o n i n humans ( M i l i c - E m i l i & C a j a n i , '57; Hey e t a l . , '66) and a n a e s t h e t i z e d mammals ( E u l e r e t a l . , '70) ( F i g . 14a, open c i r c l e s ) . H i g h e r l e v e l s o f v e n t i l a t i o n were o b t a i n e d by h y p e r c a p n i c s t i m u l a t i o n w h i c h , c o n s i s t e n t w i t h mammalian s t u d i e s (Hey e t a l . , '66; E u l e r e_t a l . , '70) d i d n o t a l t e r t h e V_, V\_ r e l a t i o n . T h i s r e l a t i o n has a v e r y s t e e p s l o p e s i n c e i n t h e unanaes-t h e t i z e d t u r t l e , V_, v a r i e s l i t t l e and i n c r e a s e s i n V_, a r e p r i m a r i l y t h e i Ji r e s u l t o f i n c r e a s e s i n f ( F i g . 1 4 b ) . The i n f l e x i o n o r c u r v i l i n e a r bend found i n t h e V_, V\_ r e l a t i o n o f man and o t h e r mammals a t r o u g h l y h a l f v i t a l _j T c a p a c i t y (Hey e t a l . , '66; E u l e r e_t a l . , '70) was n o t fo u n d i n t u r t l e s . A l t h o u g h i t i s p o s s i b l e t h a t C 0 2 s t i m u l a t i o n a l o n e i s i n a d e q u a t e t o r r a i s e v e n t i l a t i o n above t h e c r i t i c a l l e v e l a t w h i c h suc h i n f l e x i o n s o c c u r ( E u l e r e t a l . , ' 7 0 ) , t h e p r i m a r y dependence o f i n c r e a s e s i n V on i n c r e a s e s i n f above t h i s c r i t i c a l l e v e l i n mammals p a r a l l e l s t h e s i t u a t i o n w h i c h n o r m a l l y e x i s t s a t a l l l e v e l s o f V i n t u r t l e s s u g g e s t i n g t h a t such an i n f l e x i o n 64 F i g u r e 14. The r e l a t i o n s h i p s between (a) Vg and V\_, (b) V-, and f , and ( c ) V _ , a n d l / f f o r t u r t l e s b r e a t h i n g a i r and CO- gas m i x t u r e s . V a l u e s r e c o r d e d i n i n t a c t t u r t l e s a r e r e p r e -s e n t e d by open symbols, t h o s e r e c o r d e d postvagotomy by c l o s e d symbols. I n c, v a l u e s r e c o r d e d w h i l e t u r t l e s b r e a t h e d a i r ( 0 ) , 5% C 0 2 , ( A ) , and 10% C 0 2 (?) a r e shown s e p a r a t e l y . 65 66 s i m p l y does n o t e x i s t . The p l o t o f V a g a i n s t 1/f ( F i g . 14c) c l e a r l y i l l u s t r a t e s t h e marked dependence o f i n c r e a s e s i n V„ on i n c r e a s e s i n f . The n e g a t i v e c o r r e l a t i o n Hi i n t h i s r e l a t i o n i s c o n s i s t e n t w i t h s t u d i e s on C C ^ - v e n t i l a t i o n r e s p o n s e s i n a n a e s t h e t i z e d mammals ( E u l e r e t a l . , '70). The use o f h y p e r c a p n i a as a r e s p i r a t o r y s t i m u l a n t does n o t a l t e r e i t h e r t h e V^, f o r t h e V T , 1/f r e l a t i o n , (b) A f t e r B i l a t e r a l Vagotomy A l l t h r e e r e l a t i o n s r e m a i n l i n e a r f o l l o w i n g vagotomy ( F i g . 14, c l o s e d c i r c l e s ) . The s l o p e o f t h e V„, V_ r e l a t i o n d e c r e a s e s ; t h a t o f t h e V„, f r e l a t i o n i n c r e a s e s , and t h i s i n c r e a s e d dependency o f l e v e l s o f V_ on changes i n V^ i s c l e a r l y e v i d e n t f r o m t h e i n c r e a s e d n e g a t i v e s l o p e o f t h e V^, 1/f r e l a t i o n . These r e s u l t s i n d i c a t e t h a t some a c c e l e r a t i o n o f r e s p i r a t o r y f r e q u e n c y i n r e s p o n s e t o i n c r e a s e d c h e m i c a l d r i v e remains a f t e r t h e v a g i a r e c u t . T h i s i s n o t t h e case i n a n a e s t h e t i z e d c a t s where r e s p i r a t o r y f r e q u e n c y r e m a i n s c o n s t a n t f o l l o w i n g vagotomy and i n c r e a s e s i n V^ , r e s u l t s o l e l y from i n c r e a s e s i n V^ ( E u l e r e t a l . , '70). The s l o p e c o n s t a n t o f t h e V„, V r e l a t i o n has t h e d i m e n s i o n o f f r e q u e n c y (Hey e_t a l . , '66) and c o n s e q u e n t l y t h a t o f t h e V , f r e l a t i o n has t h e dimen-Jt-J s i o n o f t i d a l volume. I t can be seen from t h e c a l c u l a t i o n s o f E u l e r e t a l . ('70) t h a t e x t r a p o l a t i o n o f a l l t h e s e r e l a t i o n s , p r e and p o s t vagotomy, t o z e r o v a l u e w i l l n o t pass t h r o u g h t h e o r i g i n u n l e s s t h e f r e q u e n c y i n t h e case o f t h e V-, V^ r e l a t i o n o r V^ i n t h e c a s e o f t h e V„, f r e l a t i o n r emains Hi 1 1 b c o n s t a n t as V„ i n c r e a s e s . T h i s i s t h e b a s i s f o r t h e x i n t e r c e p t o f each o f E. t h e s e c u r v e s . I t s h o u l d be n o t e d t h a t t h e r e s p i r a t o r y f r e q u e n c y computed d u r i n g t h e v e n t i l a t o r y p e r i o d ( f y p ) r e m a i n s c o n s t a n t d e s p i t e ( X ^ - i n d u c e d v e n t i l a t i o n r e s p o n s e s ( T a b l e 2) and t h a t a l t h o u g h vagotomy r e s u l t s i n a 67 d e c r e a s e d f t h i s f r e q u e n c y a l s o remains c o n s t a n t as v e n t i l a t i o n i s i n c r e a s -ed by h y p e r c a p n i a . The D u r a t i o n o f t h e B r e a t h (a) I n t a c t A n i m a l s D u r i n g r e s t i n g , spontaneous b r e a t h i n g , t h e l e n g t h o f each b r e a t h was v a r i a b l e and t h e l e n g t h s o f each i n t e r v a l c o m p r i s i n g t h e b r e a t h , w e r e p o s i -t i v e l y c o r r e l a t e d t o t h e t o t a l l e n g t h o f t h e b r e a t h ( F i g . 15, open c i r c l e s ) . The s t r o n g e s t c o r r e l a t i o n was between T _ and T_ ( r = 0.975, F i g . 15b) b t o t I d e s p i t e weaker c o r r e l a t i o n s between T and t h e component i n t e r v a l s o f T_ ( r f o r T T ' = 0.839, F i g . 15a, f o r T <_, T T—T T' = 0.655, F i g . 1 5 d ) . I t o t ' I ' b ' t o t I I » & / The c o r r e l a t i o n between T and T_ was always weaker ( r = 0.844, F i g . 15c) t o t E J t h a n between T ^ and T_. S t i m u l a t i o n o f b r e a t h i n g by C 0 o had no e f f e c t on t o t I 2 t h e s e r e l a t i o n s ( T a b l e 2, F i g . 1 5 ) . (b) A f t e r B i l a t e r a l Vagotomy F o l l o w i n g vagotomy T t was p r o l o n g e d s l i g h t l y ( T a b l e 2 ) . I n mammals t h e r e i s a l e n g t h e n i n g o f b o t h T T and T_ ( C l a r k and E u l e r , ' 7 2 ) , b u t i n t h e 1 E t u r t l e s t h e ran g e s o f T ' and T„ were r e d u c e d and t h e i r mean v a l u e s were r e l a t i v e l y unchanged. As a consequence t h e c o r r e l a t i o n between T t and t h e i n t e r v a l s o f a c t i v e i n s p i r a t i o n and e x p i r a t i o n were r e d u c e d ( r = 0.32 and 0.30 r e s p e c t i v e l y ) ( F i g . 15, c l o s e d c i r c l e s ) . The b r e a t h l e n g t h was p r o l o n g e d p r i m a r i l y t h r o u g h t h e i n c r e a s e i n T_-T-.' w h i c h became s t r o n g l y c o r r e l a t e d t o T ( r = 0.898) th u s t h e s t r o n g l i n e a r c o r r e l a t i o n remained between T ^ and t o t 6 t o t Tj. ( r = 0.971). S t i m u l a t i o n o f b r e a t h i n g by CO2 s t i l l had no e f f e c t on t h e s e r e l a t i o n s ( T a b l e 2 ) . 68 F i g u r e 15. The r e l a t i o n s h i p s between T t and i t s component i n t e r -v a l s f o r i n t a c t t u r t l e s (open c i r c l e s and s o l i d r e g r e s s i o n l i n e s ) and v a g o t o m i z e d t u r t l e s ( c l o s e d c i r c l e s and b r o k e n r e g r e s s i o n l i n e s ) b r e a t h i n g a i r and C0„ gas m i x t u r e s . 70 t T_,, T_ and T„, T T' R e l a t i o n s h i p s —SL — i —_, — 1 (a) I n t a c t A n i m a l s A l t h o u g h T_ showed t h e s t r o n g e s t c o r r e l a t i o n t o T , t h e r e was a b e t -& I ° t o t t e r c o r r e l a t i o n between T T' and T„ ( r = 0.80) t h a n between T T and T„ ( r = 0.68) ( F i g . 1 6 ) . F u r t h e r , a t h i g h e r v a l u e s o f T-., t h e r e was an i n f l e x i o n i n t h i s r e l a t i o n so t h a t e x p i r a t o r y d u r a t i o n changed r e l a t i v e l y l i t t l e w i t h f u r t h e r i n c r e a s e s i n i n s p i r a t o r y d u r a t i o n . These r e s u l t s a r e . s i m i l a r t o r e s u l t s o b t a i n e d i n p a r a l y z e d a r t i f i c i a l l y v e n t i l a t e d c a t s where was r e g u -l a t e d by e l e c t r i c a l s t i m u l a t i o n o f t h e s e v e r e d v a g a l n e r v e s ( G l a r k and E u l e r , '72) and p r o v i d e d l a r g e r T.- v a l u e s t h a n c o u l d be o b t a i n e d i n r e b r e a t h i n g e x p e r i m e n t s and r e v e a l e d a s i m i l a r i n f l e x i o n . had no e f f e c t on t h e s e r e l a t i o n s ( T a b l e 2 ) . (b) A f t e r B i l a t e r a l Vagotomy F o l l o w i n g vagotomy, however, a good c o r r e l a t i o n r e m a i n s between T^ and T-.' ( F i g . 16) as i n c a t s (Widdicombe and W i n n i n g , ' 7 4 ) . T h i s c o r r e l a t i o n i s t h u s e s t a b l i s h e d by some c e n t r a l mechanism and i f t h e e x p i r a t o r y i n t e r v a l i s d e t e r m i n e d by t h e e v e n t s t e r m i n a t i n g t h e a c t i v e i n s p i r a t o r y i n t e r v a l i n t u r t l e s as s u g g e s t e d by some r e s e a r c h e r s f o r mammals ( C l a r k and E u l e r , ' 7 2 ), t h e s e e v e n t s must a r i s e c e n t r a l l y u n i n f l u e n c e d by volume f e e d b a c k i n f o r m a -t i o n f r o m t h e l u n g s . Lung volume f e e d b a c k i n f o r m a t i o n i s n e c e s s a r y , however, f o r t h e p o s i -t i v e c o r r e l a t i o n between T_. and T_, ( F i g . 1 6 ) . I t s i n f l u e n c e , t h e r e f o r e , must be on t h e T J _ - J - ' i n t e r v a l and must be due t o t o n i c r a t h e r t h a n p h a s i c i n p u t as changes i n volume f e e d b a c k i n f o r m a t i o n r e s u l t i n g f r o m f l u c t u a t i o n s i n V_, i n d u c e d by h y p e r c a p n i a i n i n t a c t a n i m a l s a r e w i t h o u t e f f e c t on t h e r e l a t i o n . 71 F i g u r e 16. The r e l a t i o n s h i p between t h e e x p i r a t o r y (T ) and i n s p i r a -t o r y i n t e r v a l s (T^' and T^) f o r i n t a c t t u r t l e s (open c i r c l e s and s o l i d r e g r e s s i o n l i n e s ) and v a g o t o m i z e d t u r t l e s ( c l o s e d c i r c l e s and b r o k e n r e g r e s s i o n l i n e s ) b r e a t h i n g a i r and C 0 9 gas m i x t u r e s . 7 2 73 < \ N e i t h e r t h e T ' , T„ n o r T T, T„ r e l a t i o n s w h i c h do e x i s t a f t e r vagotomy a r e a f f e c t e d by l e v e l s o f CC^ ( F i g . 1 6 ) . The E f f e c t s o f T i d a l Volume (a) I n t a c t A n i m a l s V^ was m a i n t a i n e d w i t h i n n a r r o w l i m i t s d e s p i t e a w i d e range o f s p o n t a n -e o u s l y o c c u r r i n g changes i n Tg, T^, T^ .1 and T t ( F i g . 1 7 ) . I n c r e a s e s i n V g and V^ r e s u l t i n g from CO^ b r e a t h i n g d i d n o t a l t e r t h e s e r e l a t i o n s h i p s ( F i g . 17, T a b l e 2 ) . F i g u r e - 1 8 shows a s t r o n g l i n e a r c o r r e l a t i o n between t h e i n s p i r -a t o r y i n t e r v a l and t h e l o g o f t h e r e c i p r o c a l o f t h e i n s p i r a t o r y f l o w r a t e ( r = 0.845) as w e l l as between t h e e x p i r a t o r y i n t e r v a l and t h e l o g o f t h e r e c i p r o c a l o f t h e e x p i r a t o r y f l o w r a t e ( r = 0.810). The l i n e a r s l o p e s o f t h e s e r e l a t i o n s h i p s r e p r e s e n t c o n s t a n t volume i s o p l e t h s i n d i c a t i n g t h a t T^ and T„ a r e a d j u s t e d t o m a i n t a i n a c o n s t a n t V o v e r w i d e l y r a n g i n g i n s p i r a t o r y h i i and e x p i r a t o r y a i r f l o w r a t e s . T h i s d i f f e r s m a r k e d l y from t h e s t r o n g p o s i t i v e c o r r e l a t i o n s shown between V„, and T T and V„, and T„ i n s p o n t a n e o u s l y b r e a t h i n g s t e a d y s t a t e c o n -1 l 1 hi d i t i o n s i n man (Newsom D a v i s and S t a g g , ' 7 5 ) , and i n t a c t a n i m a l s (Widdicombe and W i n n i n g , '74). CO2 has no e f f e c t on t h e s e r e l a t i o n s i n man (Newsom D a v i s and S t a g g , '75) b u t d e c r e a s e s T i n i n t a c t c a t s (Widdicombe and h i W i n n i n g , '74). (b) A f t e r B i l a t e r a l Vagotomy When a l l v o l u me-feedback i n f o r m a t i o n was removed, t h e i n t e r v a l s o f a c t i v e i n s p i r a t i o n and e x p i r a t i o n were r e s t r i c t e d i n range and i n d e p e n d e n t of t i d a l volume ( F i g . 1 7 ) . They no l o n g e r s t r o n g l y c o r r e l a t e d w i t h t h e i n s p i r a t o r y and e x p i r a t o r y f l o w r a t e s ( F i g . 18).and t h u s t i d a l volumes i n c r e a s e d , p r e sumably i n d i r e c t p r o p o r t i o n t o t h e c e n t r a l r e s p i r a t o r y d r i v e . 74 F i g u r e 17. The r e l a t i o n s h i p s between t i d a l volume and t h e i n s p i r a t o r y i n t e r v a l s , e x p i r a t o r y i n t e r v a l and t o t a l b r e a t h l e n g t h f o r i n t a c t a n i m a l s (open c i r c l e s and s o l i d r e g r e s s i o n l i n e ) and v a g o t o m i z e d t u r t l e s ( c l o s e d c i r c l e s ) b r e a t h i n g a i r and C 0 9 gas m i x t u r e s . 75 4 ° o _ o •- J3 Jo > E 76 F i g u r e 18. The r e l a t i o n s h i p between t h e i n s p i r a t o r y i n t e r v a l and i n s p i r a t o r y f l o w r a t e (open c i r c l e s and s o l i d r e g r e s s i o n l i n e s ) and between t h e e x p i r a t o r y i n t e r v a l and e x p i r a -t o r y f l o w r a t e ( c l o s e d c i r c l e s and b r o k e n r e g r e s s i o n l i n e ) i n i n t a c t (a) and v a g o t o m i z e d (b) t u r t l e s b r e a t h -i n g a i r and C 0 9 gas m i x t u r e s . a b Interval (sec) 78 U n l i k e mammals, T ' and T v a l u e s were n o t e q u a l t o maximum prevagotomy v a l u e s ( B r a d l e y e t a l . , '74a, b ) . There appears t o be some s l i g h t l e n g t h e n -i n g o f T T and hence T ^ a t h i g h e r v a l u e s o f V_ i n d u c e d by C 0 o s t i m u l a t i o n I t o t T J 2 w h i c h must have been due t o c e n t r a l e f f e c t s o f t h e CC^. S i m i l a r r e s u l t s have been o b t a i n e d d u r i n g a p n e u s t i c b r e a t h i n g i n v a g o t o m i z e d a n i m a l s d e c e r e b r a t e a t t h e l e v e l o f t h e pons (Tang, '67; S t e l l a , '38a, b; Lumsden, '22, 23a, b ) . The D u r a t i o n o f t h e N o n v e n t i l a t b r y P e r i o d (a) I n t a c t A n i m a l s The T ^ p was p o o r l y c o r r e l a t e d t o V T ( r = 0.495), however a g e n e r a l t r e n d f o r T ^ p t o d e c r e a s e and t o i n c r e a s e as v e n t i l a t i o n was s t i m u l a t e d by CO2 d i d e x i s t ( F i g . 1 9 ) . T ^ p was n o t c o r r e l a t e d t o t h e f o l l o w i n g o r p r e c e d i n g V,j,. Such a c o r r e l a t i o n can a l s o n o t be fo u n d i n t h e d a t a o f G l a s s and Johansen ('76) f o r t h e snake, a l t h o u g h i t i s argued t o be p r e s e n t i n t h e l i z a r d by Jammes and Grimaud ('76). The T N V p was, however, s t r o n g l y c o r r e l a t e d t o f ( l o g f = 1.533 - 0.688 l o g T N V p ; r = 0.899) and t o V E ( l o g V E = 2.995 - 0.857 l o g T ^ ; r = 0.875) ( F i g . 1 9 ) . The s i m i l a r i t y between t h e s e two r e l a t i o n s d e p i c t s t h e major r o l e o f changes i n f r e q u e n c y i n d e t e r m i n i n g changes i n V„ i n t h e n o r m a l i n t a c t a n i m a l . S i n c e T . and hence f T T_, remained r e l a t i v e l y c o n s t a n t as V„ t o t VP E i n c r e a s e d d u r i n g CO2 b r e a t h i n g , T ^ p was t h e major d e t e r m i n a n t o f f . CO2 d i d n o t a l t e r t h e shape o f t h e s e r e l a t i o n s . (b) A f t e r B i l a t e r a l Vagotomy v F o l l o w i n g vagotomy t h e same weak c o r r e l a t i o n e x i s t e d between V^ and T ^ p ( r = 0.490), t h u s t h e g e n e r a l t r e n d o f d e c r e a s i n g T ^ p w i t h i n c r e a s i n g V^ , 79 F i g u r e 19. R e l a t i o n s h i p between t h e l e n g t h of t h e n o n v e n t i l a t o r y p e r i o d and (a) t i d a l volume, (b) r e s p i r a t o r y f r e q u e n c y and (c) m i n u t e v e n t i l a t i o n i n i n t a c t (open c i r c l e s ) and v a g o t o m i z e d ( c l o s e d c i r c l e s ) t u r t l e s b r e a t h i n g a i r and C 0 9 gas m i x t u r e s . 80 o •o m o o o o o CO rvj CO ^ o 81 d u r i n g CO^ b r e a t h i n g was i n d e p e n d e n t of volume f e e d b a c k i n f o r m a t i o n and must have r e s u l t e d from t h e e f f e c t s o f t h e CO- on p e r i p h e r a l o r c e n t r a l chemorecept-o r s . Vagotomy d i d n o t a f f e c t t h e T,-^, f r e l a t i o n s h i p b u t s h i f t e d t h e T ^ p , V-, c u r v e up and t o t h e r i g h t ( l o g V_, = 4.549 - 1.194 l o g T ; r = 0.765). I n t h e absence o f l u n g volume f e e d b a c k , t i d a l volume i n c r e a s e d d r a m a t i c a l l y and V_, was met a t l o w e r r e s p i r a t o r y f r e q u e n c i e s . C o n s e q u e n t l y , mean T__^ _, r a r e l y f e l l below a v a l u e o f 75 s e c o n d s . 82 DISCUSSION I n r e s t i n g , s p o n t a n e o u s l y b r e a t h i n g a n i m a l s , t h e r e s p i r a t o r y p a t t e r n was c o n s i s t e n t w i t h t h a t r e c o r d e d b y - o t h e r r e s e a r c h e r s ( s e e Gans, ' 7 8 , f o r r e v i e w ) ( F i g . 1 2 ) . The mean v a l u e s r e c o r d e d f o r f , V_, and V_, ( T a b l e 2) f a l l w i t h i n t h e l o w e r r a n g e o f v a l u e s r e p o r t e d i n t h e l i t e r a t u r e f o r t h e s e v a r i a b l e s i n t u r t l e s (McCutcheon, '43; M i l l e n et a l . , '63; F r a n k e l e t a l . , '69; J a c k s o n , '71, '73; J a c k s o n e t a l . , '74). I t i s more d i f f i c u l t t o f i n d r e p o r t e d v a l u e s f o r c o m p a r i s o n o f t h e o t h e r r e s p i r a t o r y v a r i a b l e s b u t p u b l i s h e d v a l u e s o f T , B r e a t h s - V P - 1 , and T V P (McCutcheon,' 43; B e l k i n , * 6 8 ; F r a n k e l t o t 1UU VP + NVP e t a l . , . '69; B u r g g r e n , ' 7 2 ; L u c e y and House, '77) encompass t h o s e v a l u e s r e p o r t e d h e r e ( T a b l e 2 ) . C 0 2 has c o n s i s t e n t l y been r e p o r t e d as a r e s p i r a -t o r y s t i m u l a n t i n t u r t l e s l e a d i n g t o i n c r e a s e s i n b o t h f and V_, ( R a n d a l l e t a l . , '44; M i l l e n e t a l . , '63; F r a n k e l e t a l . , '69; J a c k s o n e t a l . , '74) a l t h o u g h t h e r e i s some d i s c r e p a n c y - c o n c e r n i n g t h e s e n s i t i v i t y o f t u r t l e s t o C 0 o as a s t i m u l u s and t h e m a g n i t u d e o f t h e v e n t i l a t o r y - r e s p o n s e i t c a u s e s . M i l l e n e t a l . ('63) r e p o r t t h a t 6% C 0 2 i n t r o d u c e d i n t o t h e i n s p i r a t o r y gas m i x t u r e o f Pseudemys s c r i p t a ( t e m p e r a t u r e n o t r e p o r t e d ) i n c r e a s e d v e n t i l a t i o n -1 o n l y s l i g h t l y (V_, i n c r e a s e d f r o m 31 t o 41 ml-min , t h e body w e i g h t o f t h e a n i m a l s i s n o t r e p o r t e d ) , whereas J a c k s o n e_t a l . ('74), u s i n g t h e same gas m i x t u r e and t h e same a n i m a l , r e p o r t a 10 X i n c r e a s e i n v e n t i l a t i o n (V E i n c r e a s e d from 23.8 t o 215 m l ' m i n - _ 1 - k g - 1 ) . The 3 X and 7 X i n c r e a s e s i n pulmonary m i n u t e v e n t i l a t i o n w h i c h we o b s e r v e d when Chrysemys p i c t a were exposed t o 5 and 10% CO- r e s p e c t i v e l y i n t h e i n s p i r e d a i r ( T a b l e 2) s u p p o r t 83 t h e c o n t e n t i o n o f J a c k s o n e t a l . ('74) t h a t CC^ i s a p o w e r f u l r e s p i r a t o r y s t i m u l a n t . F u r t h e r , t h e s e l e v e l s o f CG^ a r e w e l l w i t h i n p h y s i o l o g i c a l l e v e l s c o n s i d e r i n g r e p o r t e d v a l u e s f o r Pa,-,,-, o f 100 t o 130 mm Hg f o l l o w i n g 2 h r of 2 d i v i n g i n Pseudemys s c r i p t a ( R o b i n e t a l . , '64; J a c k s o n & S i l v e r b l a t , ' 74). I t s h o u l d be n o t e d t h a t Pseudemys s c r i p t a has now been r e c l a s s i f i e d as Chrysemys s c r i p t a and t h e r e appear t o be few e c o l o g i c a l o r p h y s i o l o g i c a l d i f f e r e n c e s between t h i s s p e c i e s and t h e Chrysemys p i c t a used i n t h e p r e s e n t s t u d y . S e v e r a l s t u d i e s i n d i c a t e t h a t 0 2 d e p l e t i o n and C 0 2 a c c u m u l a t i o n p l a y a major r o l e i n d e t e r m i n i n g t h e l e n g t h o f the b r e a t h h o l d between v e n t i l a t o r y p e r i o d s i n t u r t l e s (Lumsden, '23b, c; L e n f a n t e t a l . , ' 7 0 ) . A l t h o u g h each v e n t i l a t o r y p e r i o d must t h e n s u f f i c e t o r a i s e 0 2 l e v e l s , d e c r e a s e C 0 2 l e v e l s and e n a b l e b r e a t h h o l d i n g t o be resumed, i t i s d i f f i c u l t t o a s s e s s how i n d i v i d u a l b r e a t h s a r e r e g u l a t e d p a r t i c u l a r l y s i n c e changes i n appear i n d e p e n d e n t o f changes i n T , T , T ', o r T_ ( F i g . 1 7 ) . I n s p o n t a n e o u s l y t o t l l i i b r e a t h i n g , e u c a p n i c a n i m a l s , t i d a l volume i s h e l d w i t h i n v e r y n a rrow l i m i t s by a d j u s t i n g T^, T^' and T^ . (hence T t o t ) t o t n e h i g h l y v a r i a b l e r a t e s o f e x p i r a t i o n and i n s p i r a t i o n ( F i g . 1 8 ) . Lung volume f e e d b a c k i s n e c e s s a r y t o e s t a b l i s h t h e c o r r e l a t i o n s o f T ' and T„ w i t h T T and T ^ and t o m a i n t a i n t h e I E I t o t r e l a t i v e c o n s t a n c y o f t i d a l volume ( F i g s . 15, 16, and 17) as a l l a r e e l i m i n -a t e d by vagotomy. A l t h o u g h t h e r e i s much v a r i a b i l i t y i n e x p i r a t o r y and i n s p i r a t o r y f l o w r a t e s , t h e r e i s a l s o m o d u l a t i o n o f t h e gas f l o w r a t e s by v a g a l v o l u m e - r e l a t e d i n f o r m a t i o n shown by t h e i n c r e a s e i n gas f l o w r a t e s f o l l o w i n g vagotomy ( F i g . 1 8 ) . I n mammals, a f t e r vagotomy t h e T T v a l u e s a r e p r o l o n g e d t o t h e maximum Table 3: Comparison of ventilatory control in mammals and turtles. Intact Mammals Vagotomized NPBM ablated Vagotomized + N^ BM ablated Turtles Intact Vagotomized tTI tTE tTI tTI V T , T I r e l & , t e d VT, TE r e l a - t e d Tg^Tj r e l a t e d i n s p i r a t o r y a c t i v i t y unchanged tVT ( f unrelated unrelated re l a t e d i n s p i r a t o r y a c t i v i t y unchanged tVT related related related i n s p i r a t o r y a c t i v i t y unchanged r r unrelated unrelated T-, independent * of Tj r e l a t i o n constant r e l a t i o n constant r e l a t e d A I E unchanged i n s p i r a t o r y a c t i v i t y increased K if unrelated unrelated T p independent * of T j no e f f e c t T, f a i r flow 1 rates tVT some |Tj some |Tg | a i r flow rates C0 2 Effects reduced depthf TV_ | f exaggerated A r a t e j no ef f e c t Tj no effe c t Tj Tg constant Tg constant | a i r flow | a i r flow rates rates ff tVT K tf no e f f e c t Tj no e f f e c t T^  I a i r flow rates some T E constant T a i r flow rates 85 r e c o r d e d p r i o r t o vagotomy and s u g g e s t a maximum i n t e r v a l s e t by c e n t r a l mechanisms and n o r m a l l y o v e r r i d d e n by p e r i p h e r a l i n p u t s . The i n c r e a s e i n V_, f o l l o w i n g vagotomy i s due s o l e l y t o t h i s p r o l o n g a t i o n o f T._, t h e l e v e l s o f i n s p i r a t o r y a c t i v i t y r e m a i n c o n s t a n t ( E u l e r and T r i p p e r i b a c h , '76a, b) ( T a b l e 3 ) . I n t u r t l e s , however, t h e v a l u e s o f T T' and T„ a f t e r vagotomy a r e f r e q u e n t l y i n t h e mid-range o f t h o s e r e c o r d e d b e f o r e vagotomy ( F i g . 17, T a b l e 3 ) . The l a r g e o v e r a l l i n c r e a s e i n V_, and i t s v a r i a b i l i t y a r e n o t due t o p r o l o n g a t i o n o f T._ a t a c o n s t a n t l e v e l o f i n s p i r a t o r y a c t i v i t y as i n mammals b u t s o l e l y t o i n c r e a s e d l e v e l s o f i n s p i r a t o r y a c t i v i t y . The f a c t t h a t T T and T„"values a r e n o t e q u i v a l e n t t o t h e maximum v a l u e s measured J. hi b e f o r e vagotomy s u g g e s t s t h a t volume i n f o r m a t i o n p r o l o n g s as w e l l as s h o r t e n s t h e a c t i v e i n s p i r a t o r y and e x p i r a t o r y i n t e r v a l s . When t h e F T i s i n c r e a s e d f r o m 0 t o 10%, t i d a l volume i n c r e a s e s 1C02 r o u g h l y 2.5 t i m e s y e t T , T T, T ' and T do n o t change a p p r e c i a b l y ( T a b l e t o t l _L E 2) and t h e s l o p e s o f t h e i r r e l a t i o n s w i t h V_, r e m a i n a t z e r o ( F i g . 1 7 ) . There i s an i n c r e a s e i n , t h e e x p i r a t o r y and i n s p i r a t o r y f l o w r a t e s ( F i g . 12) and t h e i n c r e a s e i n t i d a l volume must r e s u l t f r o m c e n t r a l e x c i t a t i o n o f t h e motor o u t p u t t o i n s p i r a t o r y and e x p i r a t o r y m u s c l e s . T h i s i s l i n k e d t o b o t h a p r o p o r t i o n a t e r i s e i n t h e c e n t r a l t i d a l volume t h r e s h o l d such t h a t an i n c r e a s e d t i d a l volume i s v e n t i l a t e d w i t h i n t h e same b r e a t h l e n g t h Crtot) and t o d i r e c t i n h i b i t i o n o f pulmonary s t r e t c h r e c e p t o r d i s c h a r g e by CO2 ( S e c t i o n I ) . Such i n h i b i t i o n w i l l l e a d t o i n c r e a s e s i n t i d a l volume n e c e s s -a r y t o r e s t o r e volume f e e d b a c k i n f o r m a t i o n t o p r e v i o u s l e v e l s . I n mammals CO2 does n o t change t h e i n s p i r a t o r y t h r e s h o l d c u r v e and d e p r e s s i o n o f pulmon-a r y s t r e t c h r e c e p t o r d i s c h a r g e i s i m p l i c a t e d as t h e s o l e cause f o r h y p e r c a p -n i c i n d u c e d i n c r e a s e s i n V T ( B r a d l e y e t a l . , ' 7 5 ) . 86 The c o n t r i b u t i o n o f t h e changes i n t i d a l volume t o t h e i n c r e a s e i n m i n u t e v e n t i l a t i o n d u r i n g h y p e r c a p n i a , however, i s s m a l l ; t h e major c o n t r i b u -t i o n comes f r o m an i n c r e a s e i n r e s p i r a t o r y f r e q u e n c y . These predominant changes i n r e s p i r a t o r y f r e q u e n c y o c c u r d e s p i t e t h e c o n s t a n c y o f i n d i v i d u a l b r e a t h l e n g t h s ( T a b l e 2 ) . Thus t h e f r e q u e n c y o f b r e a t h s w i t h i n each v e n t i l -a t o r y p e r i o d ( f y p ) r e m a i n s c o n s t a n t w h i l e t h e number o f b r e a t h s p e r v e n t i l a t -o r y p e r i o d i n c r e a s e s 50% and t h e l e n g t h o f t h e n o n v e n t i l a t o r y p e r i o d d e c r e a s e s ; t h e number o f v e n t i l a t o r y p e r i o d s p e r m i n u t e d o u b l i n g when F J „ Q i s i n c r e a s e d from 0 t o 10% ( T a b l e 2 ) . Under c o n d i t i o n s o f s e v e r e h y p e r c a p n i c s t r e s s ( F >15%) f approaches f T T T ) and T approaches T T. The r e l a t i v e c o n t r i b u t i o n s o f f and V_, t o V_, i n t u r t l e s a r e s i m i l a r t o i J-t h e r e s p o n s e s o f mammals f o l l o w i n g b r a i n s t e m l e s i o n s i n t h e r e t i c u l a r f orma-t i o n o f t h e c a u d a l pons and r o s t r a l m e d u l l a ( S t . J o h n , '77) w h i c h te n d s t o c o n f i r m t h a t t u r t l e s l a c k a pneumotaxic c e n t r e i n the'pons (Lumsden, ' 2 3 c ) . The H e r i n g - B r e u e r r e f l e x i n mammals n o r m a l l y c o n t r o l s t h e r e s p i r a t o r y r a t e and, i n t h e absence o f t h e pneumotaxic c e n t r e , m a i n t a i n s a r e l a t i v e l y c o n s t a n t d e p t h o f b r e a t h i n g (Tang, '67). C o n s e q u e n t l y , i n t h e absence o f an i n t e g r a l p a r t o f t h e mammalian r e s p i r a t o r y g e n e r a t o r , c e n t r a l r e s p i r a t o r y i n t e g r a t i o n and t h e e f f e c t s o f v a g a l i n p u t f r o m pulmonary r e c e p t o r s and o f h y p e r c a p n i a on t h i s i n t e g r a t i o n a r e bound t o be d i f f e r e n t under s t e a d y s t a t e c o n d i t i o n s i n t u r t l e s and mammals. The d i f f e r e n c e s , however, a r e due t o more tha n j u s t t h e absence o f a pneumotaxic c e n t r e f o r a l t h o u g h d e s t r u c t i o n o f t h e pneumo-t a x i c c e n t r e i n mammals r e s u l t s i n l a r g e r V_, T T and T v a l u e s , t h e V /T T i 1 E i 1 and V_,/ ( 1 / f ) c u r v e s a r e o f a shape s i m i l a r t o t h o s e found i n t h e i n t a c t a n i m a l , m e r e l y b e i n g s h i f t e d t o t h e r i g h t ( B r a d l e y , '77) ( T a b l e 3 ) . The 87 absence o f such s t r o n g p o s i t i v e c o r r e l a t i o n s i n t u r t l e s i s n o t due t o a l a c k o f c e n t r a l i n t e g r a t i o n o f v a g a l volume r e l a t e d i n f o r m a t i o n as shown by t h e c a r e f u l r e g u l a t i o n o f t i d a l volume and t h e f a c t vagotomy d r a s t i c a l l y changes the c o r r e l a t i o n s w h i c h do e x i s t ( F i g s . 14 and 1 7 ) . I t i s a l s o n o t due t o any m a j o r d i f f e r e n c e s i n t h e n a t u r e o f t h e v a g a l volume i n f o r m a t i o n . The f u n c t i o n a l c h a r a c t e r i s t i c s o f pulmonary s t r e t c h r e c e p t o r s i n t h e t u r t l e d i f f e r o n l y q u a n t i t a t i v e l y f r o m t h o s e of mammalian pulmonary s t r e t c h - r e c e p t o r s ( S e c t i o n I ) . I t seems a p p a r e n t t h a t c e n t r a l i n t e g r a t i o n o f t h e v a g a l volume s i g n a l must be q u i t e d i f f e r e n t i n t h e s e two groups o f a n i m a l s under s t e a d y s t a t e c o n d i t i o n s . A f t e r vagotomy changes i n due t o h y p e r c a p n i a a r e due p r i m a r i l y t o changes i n ( F i g . 1 4 ) . As i n mammals (Widdicombe and W i n n i n g , '74; B r a d l e y et^ a l . , '74a, b, ' 7 5 ) , changes i n r e s p i r a t o r y r a t e a r e g r e a t l y r e d u c e d ( F i g . 14, T a b l e 3 ) . T i d a l volume i s now p r i m a r i l y d e t e r m i n e d by t h e i n s p i r a t o r y and e x p i r a t o r y r a t e s and t h e l e n g t h o f t h e a c t i v e i n s p i r a t o r y and e x p i r a t o r y i n t e r v a l s . The i n c r e a s e s i n r e s p i r a t o r y f r e q u e n c y w h i c h o c c u r d u r i n g h y p e r -c a p n i a a r e p r i m a r i l y due t o d e c r e a s e s i n T ^ p ( T a b l e 2, F i g . 1 9 ) ; t h e b r e a t h h o l d i n t e r v a l . On vagotomy, t h i s i n t e r v a l i n l e n g t h e n e d b u t s i n c e t h e d egree o f c o r r e l a t i o n between T ^ p and i s unchanged, i t must stem e i t h e r f rom r e m o v a l o f e x c i t a t o r y t o n i c v a g a l i n f l u e n c e from t h e mechanism i n i t i a t -i n g i n s p i r a t i o n f o l l o w i n g b r e a t h h o l d i n g o r a d e c r e a s e d s e n s i t i v i t y of t h i s mechanism t o 0^ d e p l e t i o n and a c c u m u l a t i o n . H y p e r c a p n i a s t i l l a c t s t o s h o r t e n t h i s i n t e r v a l b u t i t s e f f e c t s on r e s p i r a t o r y f r e q u e n c y a r e somewhat o f f s e t by t h e i n c r e a s e d b r e a t h l e n g t h and r e d u c e d f ^ p compounded by an i n c r e a s e d number o f b r e a t h s w i t h i n each v e n t i l a t o r y p e r i o d ( T a b l e 2 ) . 88 A p n e u s i s i n mammals i s c h a r a c t e r i z e d by a marked p r o l o n g a t i o n o f T^ ., hence T , and c o n c o m i t a n t l y , an e l e v a t i o n o f t i d a l volume and r e d u c t i o n t o t of r e s p i r a t o r y f r e q u e n c y ( S t . John and Wang, ' 7 7 ) . D u r i n g a p n e u s i s , i n s p i r a t o r y d e p t h and d u r a t i o n a r e t o t a l l y dependent on c e n t r a l e f f e c t s o f F-., (Lumsden, '23a,b,c; S t e l l a , '38a,b; Tang, ' 6 2 ) . I n : t u r t l e s , f o l l o w i n g co 2 vagotomy, a l l measured v a r i a b l e s behaved i n a manner s i m i l a r t o t h o s e o f mammals d u r i n g a p n e u s i s . I n mammals d u r i n g a p n e u s i s , T^ i s r e l a t i v e l y con-s t a n t and i n d e p e n d e n t o f T-j. b u t somehow r e l a t e d t o t h e d u r a t i o n o f t h e r i s i n g phase o f b r a i n s t e m c e n t r a l i n s p i r a t o r y a c t i v i t y r a t h e r t h a n t h e t o t a l d u r a t i o n o f t h e i n s p i r a t o r y phase ( E u l e r j j t a l . , ' 7 6 ) . The e x p i r a t o r y i n t e r -v a l i n t u r t l e s , f o l l o w i n g vagotomy, a l s o r e m a i n s r e l a t i v e l y c o n s t a n t , i s inde p e n d e n t o f T-j. b u t i s c o r r e l a t e d t o T^ .' , t h e a c t i v e i n s p i r a t o r y i n t e r v a l . Thus, i n t h e absence o f a p o n t i n e pneumotaxic c e n t r e (Lumsden, *23a,b,c) s i m p l e vagotomy i n t u r t l e s r e s u l t s i n an a p n e u s i s comparable t o t h a t f ound i n mammals ( T a b l e 3 ) . 89 SECTION I I I The R o l e o f Pulmonary R e c e p t o r C h e m o s e n s i t i v i t y i i i t h e V e n t i l a t o r y Response t o I n h a l e d C 0 9 INTRODUCTION I n v e r t e b r a t e s , pulmonary r e c e p t o r s w i t h d i s c h a r g e c o r r e l a t e d t o t h e r a t e and degree o f l u n g v e n t i l a t i o n a r e o f two major t y p e s : i n t r a p u l m o n a r y c h e m o r e c e p t o r s and i n t r a p u l m o n a r y s t r e t c h r e c e p t o r s . The i n t r a p u l m o n a r y c h e m o receptors w h i c h have been found i n t h e l u n g s o f b i r d s (Fedde & P e t e r s o n , '7.0; Osborne' & B u r g e r , .'.74) and l i z a r d s (Fedde et^ a l . , '77) a r e p r i m a r i l y s e n s i t i v e t o changes i n a i r w a y C 0 2 c o n c e n t r a t i o n . I n t r a p u l m o n a r y s t r e t c h r e c e p t o r s have been d e s c r i b e d i n t h e l u n g s o f mammals ( A d r i a n , '33), t u r t l e s ( M i l s o m & J o n e s , ' 7 6 ) , l i z a r d s (Fedde e_t a l . , '77) and f r o g s ( T a g l i e t t i & C a s s e l a , '66) and a l t h o u g h t h e adequate s t i m u l u s f o r t h e s e r e c e p t o r s i s m e c h a n i c a l d e f o r m a t i o n , t h e y have been shown t o p o s s e s s v a r y i n g degrees o f C 0 2 s e n s i t i v i t y ( M u s t a f a & P u r v e s , '72; M i l s o m & J o n e s , '76; [ S e c t i o n i ] ; Fedde e t a l , , '77; M i l s o m & J o n e s , '77; [ S e c t i o n I V ] ) . The p r i m a r y f u n c t i o n s u g g e s t e d f o r t h e s e r e c e p t o r s i n mammals (Widdicombe, '64) and b i r d s (Osborne & M i t c h e l l , '77) i s t o r e f l e x l y c o n t r o l t h e t i d a l volume t o r e s p i r a t o r y f r e q u e n c y r a t i o and m i n i m i z e t h e work o f b r e a t h i n g ( O t i s e t a l . , '50) o r f o r c e o f c o n t r a c t i o n o f t h e r e s p i r a t o r y m u s c l e s (Mead, ' 6 0 ) . R e c e n t l y , however, due t o t h e i r C 0 2 s e n s i t i v i t y , pulmonary r e c e p t o r s have been i m p l i -c a t e d i n t h e r e g u l a t i o n o f Pa d u r i n g m u s c u l a r e x e r c i s e , i n t r a v e n o u s C0„ 90 l o a d i n g e x p e r i m e n t s and CO^ i n h a l a t i o n e x p e r i m e n t s (Wasserman e t a l . , '67; Wassermari e t a l . , '75; Osborne & M i t c h e l l , '77). To d a t e , t h i s p r o p o s e d r o l e f o r i n t r a p u l m o n a r y r e c e p t o r s remains u n r e s o l v e d . The f o l l o w i n g s t u d y i s a c o m p a r i s o n o f t h e v e n t i l a t o r y r e s p o n s e s o f t u r t l e s t o changes i n t h e i n t r a -pulmonary CO- c o n t e n t o f v a s c u l a r l y i s o l a t e d l u n g s (hence c o n s t a n t Pa ) w i t h t h e r e s p o n s e s t o changes i n t h e i n t r a p u l m o n a r y CO2 c o n t e n t o f i n t a c t l u n g s and an assessment of t h e r o l e o f CO2 r e l a t e d r e c e p t o r i n f o r m a t i o n c a r r i e d f r o m pulmonary r e c e p t o r s w i t h i n t h e vagus n e r v e i n t h i s a n i m a l ' s r e s p o n s e t o h y p e r c a p n i a . 91 METHODS I n one s e r i e s o f a c u t e e x p e r i m e n t s , t u r t l e s (Chrysemys p i c t a , 600-1200 g) were s i n g l e - p i t h e d and t i d a l l y v e n t i l a t e d w i t h a c o n s t a n t volume, p o s i t i v e p r e s s u r e r e s p i r a t i o n pump. S i n g l e and m u l t i - f i b r e n e r v e a c t i v i t y i n phase w i t h a r t i f i c i a l v e n t i l a t i o n was r e c o r d e d i n v a g a l s l i p s u s i n g b i p o l a r s i l v e r e l e c t r o d e s . N e u r a l a c t i v i t y was a m p l i f i e d ( T e k t r o n i x FM 122 p r e a m p l i f i e r ) , v i s u a l l y d i s p l a y e d on an o s c i l l o s c o p e and a u d i b l y m o n i t o r e d . R e c e p t o r s were l o c a l i z e d by p u n c t a t e s t i m u l a t i o n w i t h a f i n e p r o b e t o major s e p t a w i t h i n t h e l u n g s . The p r e s s u r e drop a c r o s s a pneumotachograph d u r i n g t r a c h e a l a i r f l o w was r e c o r d e d w i t h a H e w l e t t - P a c k a r d 268 BC d i f f e r e n t i a l p r e s s u r e t r a n s d u c e r and i n t r a t r a c h e a l p r e s s u r e g e n e r a t e d d u r i n g v e n t i l a t i o n was measured w i t h a Statham P23V p r e s s u r e t r a n s d u c e r . The a i r f l o w s i g n a l was f e d t h r o u g h a H e w l e t t - P a c k a r d 350-3700A i n t e g r a t i n g p r e a m p l i f i e r t o g i v e t i d a l volume and a l l measurements, p r e s s u r e , f l o w , volume and r e c e p t o r d i s c h a r g e were s t o r e d on m a g n e t i c t a p e f o r l a t e r a n a l y s i s on a D i g i t a l PDP Lab 8e m i n i - c o m p u t e r u s i n g c o n v e n t i o n a l s o f t w a r e . The v e n t i l a t i n g gas m i x t u r e was a l t e r e d from 0 t o 10% CO2 i n compressed a i r u s i n g p r e m i x e d gases and t h e 0^ and CO2 composi-t i o n o f t h e i n s p i r e d and e x p i r e d gases was d e t e r m i n e d e i t h e r on samples t a k e n t h r o u g h t h e s i d e arm o f t h e t r a c h e a l c a n n u l a and measured on a F i s h e r - H a m i l t o n gas p a r t i t i o n e r "or by c o n t i n u o u s s a m p l i n g w i t h a C e n t r o n i c 200 MGA c l i n i c a l mass s p e c t r o m e t e r (sample r a t e < 1 0 ml-min "*") . F o r f u r t h e r d e t a i l s , see S e c t i o n I . A f u r t h e r s e r i e s o f c h r o n i c e x p e r i m e n t s was p e r f o r m e d on u n a n a e s t h e t i z e d , l i g h t l y r e s t r a i n e d specimens (500-1500 g) a t room t e m p e r a t u r e (22-23°C). 92 S u r g e r y was p e r f o r m e d under e i t h e r a c o m b i n a t i o n o f c o l d (1-4 h r a t -20 C) and l o c a l a n a e s t h e s i a ( 2 % L i d o c a i n e h y d r o c h l o r i d e ) , o r g e n e r a l a n a e s t h e s i a (1.5-2 ml-100 g - 1 10% MS222 i n j e c t e d I . P . ) . A window was removed from t h e p l a s t r o n above t h e a r e a o f t h e h e a r t u s i n g a n e c r o p s y saw a l l o w i n g b o t h p r i m a r y b r o n c h i t o be c a n n u l a t e d s e p a r a t e l y . An o c c l u s i o n c a t h e t e r was a l s o p l a c e d around t h e l e f t pulmonary a r t e r y and t h e l e f t vagus was exposed and l o o s e l y s n a r e d f o r s e c t i o n i n g a t a l a t e r t i m e . A l l c a t h e t e r s were l e d out t h r o u g h t h e s k i n a t t h e base o f t h e neck and t h e window was r e p l a c e d and s e a l e d w i t h c o t t o n w o o l and d e n t a l a c r y l i c cement. Pneumotachograph c u f f s and s i d e arms f o r t r a c h e a l p r e s s u r e measurement and gas s a m p l i n g were a t t a c h e d t o t h e b r o n c h i a l c a n n u l a e and t h e d i s t a l ends o f t h e s e c a n n u l a e were a t t a c h e d t o T c o n n e c t i o n s . One arm o f each T c o n n e c t i o n was open t o atmosphere and t h e r e m a i n i n g arm was a t t a c h e d t o a r e s p i r a t o r y gas s u p p l y . U s i n g a sy s t e m o f gas f l o w m e t e r s , t h e c o m p o s i t i o n o f t h e r e s p i r a t o r y gas f l o w i n g p a s t t h e end o f each b r o n c h i a l c a n n u l a c o u l d be i n d e p e n d e n t l y a l t e r e d t h u s s e p a r a t e l y c o n t r o l l i n g t h e c o m p o s i t i o n o f t h e i n s p i r e d gas g o i n g t o each l u n g when t h e t u r t l e b r e a t h e d . T r a c h e a l p r e s s u r e , a i r f l o w and t i d a l volume were measured as d e s c r i b e d e a r l i e r and c o n t i n u o u s l y r e c o r d e d on a Sanborn 7700 c h a r t r e c o r d e r . Gas samples were a l s o measured as has been d e s c r i b e d f o r t h e a c u t e e x p e r i m e n t s . A n i m a l s were a l l o w e d t o r e c o v e r f u l l y f r o m a n a e s t h e s i a ( u s u a l l y 24 h r s ) b e f o r e e x p e r i m e n t a t i o n began. The t u r t l e s were s h i e l d e d from a l l a c t i v i t i e s o f t h e e x p e r i m e n t e r s and w h i l e r e s t i n g q u i e t l y , t h e o c c l u s i o n c u f f was i n f l a t e d i s o l a t i n g t h e b l o o d s u p p l y o f t h e l e f t l u n g . The b r e a t h i n g p a t t e r n was t h e n a l l o w e d t o s t a b i l i z e b e f o r e p r e s e n t a t i o n o f t h e t e s t gas samples. 93 L e f t pulmonary a r t e r y o c c l u s i o n , i n i t s e l f , d i d n o t u s u a l l y a l t e r t h e n o r m a l b r e a t h i n g p a t t e r n . The a n i m a l s were p r e s e n t e d w i t h a i r t o b o t h l u n g s and 10% 602 i n a i r t o each l u n g w h i l e t h e o t h e r r e c e i v e d room a i r . Each combin-a t i o n was p r e s e n t e d f o r a one-hour p e r i o d and t h e p r e s e n t a t i o n sequence was v a r i e d . A l l v a r i a b l e s were r e c o r d e d c o n t i n u o u s l y d u r i n g t h i s t i m e b u t d a t a were s e l e c t e d f o r a n a l y s i s o n l y a f t e r t h e r e s p o n s e s t o each gas m i x t u r e had s t a b i l i z e d . F o l l o w i n g t h i s s e r i e s o f e x p e r i m e n t s t h e l e f t vagus was s e c t i o n -ed under l o c a l a n a e s t h e s i a and on t h e f o l l o w i n g day t h e above p r o t o c o l was r e p e a t e d . 94 RESULTS F i g u r e 20 shows t h e d i s c h a r g e p r o f i l e o f a t y p i c a l r e c e p t o r i n r e s p o n s e t o l u n g i n f l a t i o n s o f v a r y i n g volumes w i t h 0% and w i t h 10% C O 2 i n t h e v e n t i l -a t i o n gas m i x t u r e . From r e c o r d i n g s o f 62 f i b r e s i n 30 t u r t l e s v e n t i l a t e d w i t h room a i r ( S e c t i o n I ) pulmonary s t r e t c h r e c e p t o r d i s c h a r g e r a t e was b e s t d e f i n e d by t h e e q u a t i o n : R a t e = 3.46 + 1 1 . 6 ITP where ITP i s t h e i n t r a t r a c h e a l p r e s s u r e , i n k P a , g e n e r a t e d d u r i n g l u n g i n f l a -t i o n . A t any l e v e l o f l u n g i n f l a t i o n r e c e p t o r d i s c h a r g e was r e d u c e d by a p p r o x i m a t e l y 55% when 10% C O 2 was p r e s e n t i n t h e v e n t i l a t i o n gas ( F i g . 2 0 ) . Under t h e s e c i r c u m s t a n c e s ( S e c t i o n I ) pulmonary s t r e t c h r e c e p t o r d i s c h a r g e r a t e was b e s t d e f i n e d a s : R a t e = 1 . 5 6 + 5 . 2 ITP. The r e l a t i v e r o l e o f t h e s e r e c e p t o r s i n t h e o v e r a l l r e s p o n s e o f t u r t l e s (n = 12) t o C O 2 can be s e e n - f r o m F i g u r e 21. When 10% C O 2 was p r e s e n t e d t o t h e v a s c u l a r l y i s o l a t e d l e f t l u n g , t h e r e was a 47% i n c r e a s e i n m i n u t e v e n t i l -a t i o n ( V _ ) , a 20% i n c r e a s e i n t i d a l volume (V„,) and a 23% i n c r e a s e i n r e s p i r -i i 1 a t o r y f r e q u e n c y ( f ) . F o l l o w i n g u n i l a t e r a l ( l e f t ) vagotomy t h e r e was an i n c r e a s e i n r e s t i n g m i n u t e v e n t i l a t i o n , however, t h e r e was no l o n g e r any r e s p i r a t o r y r e s p o n s e f o l l o w i n g i n t r o d u c t i o n o f 10% C O 2 t o t h e v a s c u l a r l y , n e u r a l l y i s o l a t e d l e f t l u n g . When th e same l e v e l s o f C O 2 were i n t r o d u c e d t o t h e v a s c u l a r l y i n t a c t r i g h t l u n g t h e r e was a 272% i n c r e a s e i n V , a 56% i n c r e a s e i n V^, and a 108% i n c r e a s e i n f . A l t h o u g h u n i l a t e r a l l e f t vagotomy a l t e r e d t h e r e l a t i v e r o l e s o f V and f , t h e r e was no r e d u c t i o n i n t h e r e s p o n s e i n m i n u t e v e n t i l a t i o n t o i n c r e a s i n g l e v e l s o f C O r , i n t h e i n t a c t r i g h t l u n g . 95 F i g u r e 20. Response o f a s i n g l e pulmonary s t r e t c h r e c e p t o r t o l u n g i n f l a t i o n t o d i f f e r e n t volumes w i t h d i f f e r e n t gas m i x -t u r e s . The upper t r a c e i n each r e c o r d shows t h e i n t r a -1 t r a c h e a l p r e s s u r e i n kPa and t h e l o w e r t r a c e t h e r e c e p t o r d i s c h a r g e f o r each i n f l a t i o n . The r e s p o n s e s a t t h r e e d i f f e r e n t i n f l a t i o n volumes (30, 40 and 50 c c ) w i t h e i t h e r 0% C 0 2 ( l e f t hand t r a c e s ) o r 10% C 0 2 ( r i g h t hand t r a c e s ) , a r e shown. 9 7 F i g u r e 2 1 . L e v e l s of m i n u t e v e n t i l a t i o n , t i d a l volume and r e s p i r a -t o r y f r e q u e n c y o f t u r t l e s w i t h b l o o d f l o w t o one l u n g o c c l u d e d and b o t h p r i m a r y b r o n c h i c a n n u l a t e d . Responses a r e shown o f t u r t l e s b r e a t h i n g a i r i n b o t h l u n g s o r 1 0 % C O 2 i n a i r i n one l u n g and a i r i n t h e o t h e r , b e f o r e and a f t e r vagotomy. V a l u e s a r e t h e means ± S.E. 8 0 - r ( m , V E m i n - f 0 + l - m i n 0-L 3 2 + ( m l ) 1 6 -0_. 4 T f ( m i n - 1 ) 2 + 0-L a i r b o t h l u n g s 1 0 % co2 l e f t l u n g a i r b o t h l u n g s 1 0 % co2 r i g h t l u n g a i r b o t h l u n g s u n i l a t e r a l ( l e f t ) v a g o t o m y CD 00 ) r e p o s t 99 DISCUSSION Our r e s u l t s i n d i c a t e t h a t t u r t l e s i n c r e a s e t h e i r m i n u t e v e n t i l a t i o n s l i g h t l y i n r e s p o n s e t o i n c r e a s e s i n t h e i n t r a p u l m o n a r y C 0 2 c o n c e n t r a t i o n o f a v a s c u l a r l y i s o l a t e d l u n g . The l a c k o f any r e s p o n s e t o i n c r e a s e d l e v e l s o f C 0 2 i n t h i s l u n g f o l l o w i n g vagotomy i n d i c a t e s t h a t pulmonary r e c e p t o r s a r e t h e a f f e r e n t l i m b o f t h i s r e s p o n s e . I n t r a p u l m o n a r y c h e m o r e c e p t o r s have n o t been d e m o n s t r a t e d i n t u r t l e s b u t t h e d i s c h a r g e r a t e s o f pulmonary s t r e t c h r e c e p t o r s a r e i n h i b i t e d by i n c r e a s -i n g l e v e l s o f i n t r a p u l m o n a r y C 0 2 ( F i g . 20; S e c t i o n I ) . I n s p o n t a n e o u s l y b r e a t h i n g , i n t a c t a n i m a l s t h e s e r e c e p t o r s s e r v e t o r e g u l a t e t i d a l volume w i t h i n n a r row l i m i t s and p l a c e t h e emphasis o f r e s p i r a t o r y r e s p o n s e s t o h y p e r c a p n i a on changes i n r e s p i r a t o r y f r e q u e n c y . T h i s i s c l e a r l y i l l u s t r a t e d by t h e - m a j o r ^ r o l e p l a y e d by changes i n t i d a l volume i n t h e r e s p i r a t o r y r e s p o n s e t o v e n t i l a t i o n o f t h e v a s c u l a r l y i n t a c t l u n g w i t h 10% C 0 2 f o l l o w i n g vagotomy. C o n s e q u e n t l y , t h e i n h i b i t i o n o f t h e d i s c h a r g e o f pulmonary s t r e t c h r e c e p t o r s d u r i n g h y p e r c a p n i a w i l l n e c e s s i t a t e g r e a t e r t i d a l volumes t o p r o -v i d e t h e same volume i n f o r m a t i o n t o t h e b r a i n and p r o b a b l y a c c o u n t s f o r t h e i n c r e a s e o b s e r v e d i n t i d a l volume when C 0 2 was i n s p i r e d by t h e v a s c u l a r l y i s o l a t e d l u n g . The e f f e c t o f C 0 2 i n i n c r e a s i n g t i d a l volume i n t h i s s i t u a -t i o n w i l l be p a r t i a l l y o f f s e t by t h e i n c r e a s e d d i s c h a r g e coming f r o m r e c e p t o r s i n t h e i n t a c t l u n g w h i c h i s b e i n g v e n t i l a t e d w i t h a i r a t t h e s e i n c r e a s e d t i d a l volumes. However, when t h e same l e v e l s o f C 0 2 were i n s p i r e d by t h e i n t a c t l u n g w h i l e t h e v a s c u l a r l y i s o l a t e d l u n g i n s p i r e d a i r , p r e sumably p r o v i d i n g t h e same t o t a l pulmonary r e c e p t o r i n p u t as i n t h e r e v e r s e s i t u a t i o n , t h e t i d a l volume r e s p o n s e was more t h a n t w i c e as g r e a t . Whether t h i s 100 i n c r e a s e d r e s p o n s e i s due t o c e n t r a l o r p e r i p h e r a l e f f e c t s o f t h e i n c r e a s e i n b l o o d P , i t i n d i c a t e s t h a t C0~ must a l s o a c t t o f u n c t i o n a l l y r a i s e t h e c e n t r a l i n s p i r a t o r y volume t h r e s h o l d i n t u r t l e s , i n d i r e c t c o n t r a s t t o t h e s i t u a t i o n f o u n d i n a n a e s t h e t i z e d c a t s ( C l a r k & E u l e r , '72; B r a d l e y e t a l . , ' 7 4 ) . I t has been r e p o r t e d i n mammals t h a t pulmonary s t r e t c h r e c e p t o r d i s c h a r g e d u r i n g e x p i r a t i o n p r o l o n g s t h e ^ d u r a t i o n o f e x p i r a t i o n ( H e r i n g & B r e u e r , 1868; Knox, '73; M i s e r o c c h i & M i l i c - E m i l i , '75; D'Angelo & A g o s t o n i , '75). I t has t h e r e f o r e been s u g g e s t e d t h a t t h e i n h i b i t i o n o f e n d - e x p i r a t o r y d i s c h a r g e o f pulmonary s t r e t c h r e c e p t o r s by CO2 i s r e s p o n s i b l e f o r t h e i n c r e a s e d f r e q u e n c y o f b r e a t h i n g o b t a i n e d when CO2 i s i n h a l e d by dogs on c a r d i o p u l m o n a r y bypass ( B a r t o l i et^ a l . , '74; B r a d l e y e t a l . , ' 7 6 ) . T h i s i s a c h i e v e d p r i m a r i l y by s h o r t e n i n g t h e e x p i r a t o r y i n t e r v a l ( B a r t o l i e t a l . , '74). I t i s p o s s i b l e t h a t t h e s m a l l i n c r e a s e i n b r e a t h i n g f r e q u e n c y o b s e r v e d i n t h e t u r t l e when CO2 was i n h a l e d i n t o a v a s c u l a r l y i s o l a t e d l u n g a l s o r e s u l t e d from d e p r e s s i o n o f e n d - e x p i r a t o r y d i s c h a r g e o f pulmonary s t r e t c h r e c e p t o r s . I n t h e t u r t l e , however, t h e b r e a t h i n g f r e q u e n c y i s i n c r e a s e d t h r o u g h a s h o r t e n i n g o f t h e p e r i o d s o f i n t e r m i t t e n t b r e a t h h o l d i n g r a t h e r t h a n changes i n t h e r a t e o f a c t i v e v e n t i l a t i o n ( S e c t i o n I I ) , t h u s i f t h e changes i n b r e a t h i n g f r e q u e n c y a r e t h e r e s u l t o f d e c r e a s e d pulmonary r e c e p t o r d i s c h a r g e , t h e c e n t r a l i n t e g r a -t i o n o f t h i s i n f o r m a t i o n must be s l i g h t l y d i f f e r e n t i n t h e t u r t l e and mammal: The p r i m a r y v e n t i l a t o r y r e s p o n s e o f i n t a c t t u r t l e s t o i n c r e a s i n g l e v e l s o f F J Q Q i s an i n c r e a s e i n r e s p i r a t o r y f r e q u e n c y . S i n c e t h i s component o f t h e v e n t i l a t o r y r e s p o n s e t o CO2 i s g r e a t l y r e d u c e d when CO2 i s i n s p i r e d o n l y by t h e v a s c u l a r l y i s o l a t e d l u n g , t h e t o t a l v e n t i l a t o r y r e s p o n s e t o C0 9 under 101 t h e s e c o n d i t i o n s i s r e l a t i v e l y s m a l l . Thus i t would appear t h a t t h e CO2 s e n s i t i v i t y o f pulmonary r e c e p t o r s does c o n t r i b u t e t o t h e i n c r e a s e seen i n t i d a l volume d u r i n g h y p e r c a p n i a b u t i s o f l i t t l e s i g n i f i c a n c e t o t h e o v e r a l l r e s p o n s e o f t h e t u r t l e t o i n c r e a s e d ^J_QQ • There i s no e v i d e n c e t h a t t u r t l e s p o s s e s s i n t r a p u l m o n a r y c h e m o r e c e p t o r s b u t i f t h e y a r e p r e s e n t i n t u r t l e s as i n l i z a r d s (Fedde \et a l . , '77) t h e y w o u l d appear t o have o n l y t h e same r e -f l e x f u n c t i o n s as i n t r a p u l m o n a r y m e c h a n o r e c e p t o r s . 102 SECTION IV Carbon D i o x i d e S e n s i t i v i t y o f Pulmonary R e c e p t o r s i n t h e F r o g INTRODUCTION The pulmonary r e c e p t o r s o f mammals r e s p o n d p r i m a r i l y t o t h e t r a n s p u l m o n -a r y p r e s s u r e d e v e l o p e d d u r i n g each b r e a t h i n g c y c l e b u t t h e i r d i s c h a r g e i s p a r t i a l l y m o d i f i e d by h i g h l e v e l s o f a l v e o l a r CO2 ( D a v i s e t a l . , '56; M u s t a f a & P u r v e s , '72; B r a d l e y e_tj a l . , '76). A v i a n pulmonary r e c e p t o r s have l i t t l e o r no m e c h a n o s e n s i t i v i t y r e s p o n d i n g p r i m a r i l y t o changes i n a i r w a y CO2 c o n c e n t r a t i o n t h r o u g h o u t t h e b r e a t h i n g c y c l e (Fedde e t a l . , '74a,b). T u r t l e pulmonary r e c e p t o r s a r e t y p i c a l l y m e c h a n o s e n s i t i v e b u t e x h i b i t a range o f v a r i a t i o n i n t h e i r s e n s i t i v i t y t o CO2 w h i c h encompasses t h e d i f f e r e n t s e n s i -t i v i t i e s t o CO2 found i n t h e a v i a n and mammalian r e c e p t o r t y p e s ( S e c t i o n I ) . I t b e g i n s t o appear t h a t t h e d i v e r g e n t r e c e p t o r t y p e s found i n b i r d s and mammals may have a r i s e n , p h y l o g e n e t i c a l l y , f r o m a common, l e s s s p e c i a l i z e d r e c e p t o r t y p e . A m p h i b i a e v o l v e d from t h e e v o l u t i o n a r y stem l i n e a t an e a r l y d a t e , p o s s e s s s t r u c t u r a l l y s i m p l e l u n g s and r e p r e s e n t some o f t h e e a r l i e s t forms o f s e m i - t e r r e s t r i a l l u n g e d v e r t e b r a t e s . T h i s s t u d y was u n d e r t a k e n t o d e t e r m i n e whether t h e r e a r e r e c e p t o r s p r e s e n t i n t h e l u n g s o f t h e s e e a r l y forms w h i c h a r e s e n s i t i v e t o C0 o. 103 METHODS Fro g s (Rana p i p i e n s ) w e i g h i n g between 120 and 160 g were used i n t h e s e e x p e r i m e n t s . The f r o g s were d o u b l e - p i t h e d and u n i d i r e c t i o n a l l y v e n t i l a t e d w i t h a c o n t i n u o u s gas f l o w under s l i g h t p o s i t i v e p r e s s u r e , a i r e n t e r i n g t h e l u n g t h r o u g h a t r a c h e a l c a n n u l a and l e a v i n g t h e l u n g by a c a n n u l a sewn i n t o t h e c a u d a l t i p o f t h e l u n g . The l u n g c o u l d be i n f l a t e d d u r i n g v e n t i l a t i o n t o any d e s i r e d volume by a l t e r i n g t h e r e s i s t a n c e o f t h e o u t f l o w c a n n u l a from t h e l u n g . S i n g l e and m u l t i - f i b r e n e r v e a c t i v i t y were r e c o r d e d from pulmonary a f f e r e n t f i b r e s i n v a g a l s l i p s u s i n g s t a n d a r d t e c h n i q u e s ( S e c t i o n I ) . The i n t r a t r a c h e a l p r e s s u r e was r e c o r d e d w i t h a Statham P23V p r e s s u r e t r a n d u c e r and w i t h n e u r a l a c t i v i t y were a m p l i f i e d , v i s u a l l y d i s p l a y e d on an o s c i l l o -scope and s t o r e d on m a g n e t i c t a p e f o r l a t e r a n a l y s i s on a PDP Lab 8e m i n i -computer u s i n g c o n v e n t i o n a l s o f t w a r e . 104 • RESULTS AND DISCUSSION On t h e b a s i s o f changes i n d i s c h a r g e f r e q u e n c y f o l l o w i n g l u n g i n f l a t i o n , f r o g pulmonary r e c e p t o r s have been c l a s s i f i e d i n t o t h r e e g r o u p s : r a t e r e c e p -t o r s , p r o p o r t i o n a l r e c e p t o r s and r a t e p l u s p r o p o r t i o n a l r e c e p t o r s (McKean, '6 9 ) . The d i s c h a r g e f r e q u e n c y o f r a t e r e c e p t o r s i s modulated s o l e l y by t h e r a t e o f i n c r e a s e i n l u n g volume. S i x o f 25 f i b r e s . r e c o r d e d f r o m were o f t h i s t y p e . A l t h o u g h t h e s e f i b r e s were c o n t i n u o u s l y a c t i v e , t h e i r s t a t i c r a t e o f d i s c h a r g e was u n a f f e c t e d by t h e volume o f t h e l u n g ; d i s c h a r g e i n c r e a s -ed o n l y d u r i n g t h e p e r i o d o f l u n g i n f l a t i o n and t h e n r e t u r n e d i m m e d i a t e l y t o t h e p r e v i o u s l e v e l o f d i s c h a r g e ( F i g . 22b) . A d d i t i o n o f 10% CO2 t o t h e v e n t i l a t i n g gas caused a d e c r e a s e i n t h e s t a t i c d i s c h a r g e r a t e o f t h e s e r e c e p t o r s o f 56% (10.6 ± 6.0 Hz f a l l i n g t o 4.7 ± 1.0 Hz) (each v a l u e . - i s t h e mean ± S.E.M.) and t h e m o d u l a t i o n i n d i s c h a r g e o c c u r r i n g d u r i n g l u n g i n f l a -t i o n was r e d u c e d by 50% (41.3 ± 7.8 Hz f a l l i n g t o 20.6 + 2.7 Hz) a t an i n f l a t i o n r a t e o f 1.0 ml"s 1 ( T a b l e 4 ) . One u n i t was p a r t i c u l a r l y s e n s i t i v e t o CO2 showing a 75% r e d u c t i o n i n s t a t i c d i s c h a r g e r a t e and a 74% r e d u c t i o n i n t h e d i s c h a r g e a s s o c i a t e d w i t h l u n g i n f l a t i o n w i t h 3% CO2 p r e s e n t i n t h e v e n t i l a t i n g gas ( F i g . 22b) and 67% and 64% r e d u c t i o n s i n d i s c h a r g e r a t e d u r i n g v e n t i l a t i o n w i t h 1% CO2 i n a i r . The d i s c h a r g e r a t e o f p r o p o r t i o n a l r e c e p t o r s i n c r e a s e s w i t h i n c r e a s i n g l u n g volume and f o r any g i v e n l u n g volume d i s c h a r g e i s m a i n t a i n e d w i t h o n l y s l i g h t d i m i n u t i o n as l o n g as t h e l u n g volume i s c o n s t a n t ( F i g . 22a). Three o f t h e r e c e p t o r s r e c o r d e d f r o m f i t t h i s c a t e g o r y . A l l u n i t s were a c t i v e d u r i n g l u n g d e f l a t i o n when i n t r a t r a c h e a l p r e s s u r e approached a t m o s p h e r i c . The a d d i t i o n o f 10% C0 9 t o 105 F i g u r e 22. a) P r o p o r t i o n a l r e c e p t o r r e s p o n s e t o l u n g i n f l a t i o n w i t h a i r ( l e f t ) and a i r + 3% C0 2 ( r i g h t ) . Upper t r a c e i s a t i m e m a r k e r , second t r a c e i s pulmonary r e c e p t o r d i s c h a r g e , t h i r d t r a c e i s a n a l o g d i s c h a r g e f r e q u e n c y and l o w e r t r a c e i s i n t r a t r a c h e a l p r e s s u r e , b) R a t e r e c e p t o r r e s p o n s e t o l u n g i n f l a t i o n w i t h a i r ( l e f t ) and a i r + 3% CO2 ( r i g h t ) . T r a c e s as i n d i c a t e d above. vagal discharge discharge rate (Hz) 5 r O L intratracheal 0.5 pressure CkPa) o 50 r -o Q5 O [ 107 t h e v e n t i l a t i n g gas caused a r e d u c t i o n i n t h i s d i s c h a r g e r a t e o f 56%. There was a r e d u c t i o n o f 45% i n t h e d i s c h a r g e r a t e a s s o c i a t e d w i t h l u n g i n f l a t i o n t o an i n t r a t r a c h e a l p r e s s u r e o f 0.5 kPa ( T a b l e 4 ) . One u n i t was v e r y s e n s i t i v e t o CO2 and was 75% r e d u c e d d u r i n g d e f l a t i o n and 61% r e d u c e d d u r i n g i n f l a t i o n t o 0.5 kPa i n t r a t r a c h e a l p r e s s u r e w i t h t h e a d d i t i o n o f 3% C0 2 ( F i g . 22a) ( 6 5 % and 43% r e d u c e d r e s p e c t i v e l y by 1% CO.). The r e m a i n i n g s i x t e e n f i b r e s r e c o r d e d were f r o m r a t e and p r o p o r t i o n a l s e n s i t i v e r e c e p t o r s w h i c h e x h i b i t e d a peak d i s c h a r g e f r e q u e n c y d u r i n g i n f l a t i o n as w e l l as an i n c r e a s e i n s t a t i c d i s c h a r g e f r e q u e n c y w i t h i n c r e a s i n g l u n g volume ( F i g . 2 3 ) . F i f t e e n u n i t s were a c t i v e d u r i n g l u n g d e f l a t i o n a t an a v e r a g e d i s c h a r g e f r e q u e n c y o f 6.9 ± 1.3 Hz w h i c h i n c r e a s e d t o an a v e r a g e d i s c h a r g e f r e q u e n c y o f 19.2 ± 3.0 Hz on l u n g i n f l a t i o n t o 0.5 kPa i n t r a t r a c h e a l p r e s s u r e . The peak d i s c h a r g e f r e q u e n c y a s s o c i a t e d w i t h a 1 ml-s ~^ r a t e o f i n f l a t i o n was 51.6 ± 7.0 Hz. W i t h 10% C0 2 p r e s e n t i n t h e v e n t i l a t i n g gas t h e s e d i s c h a r g e f r e q u e n c i e s f e l l t o average v a l u e s o f 5.3 ± 1.0 Hz, 15.4 ± 3.2 Hz and 35.8 ± 5.9 Hz r e s p e c t i v e l y r e p r e s e n t i n g 23, 20 and 31% r e d u c t i o n s i n t h e d i s c h a r g e f r e q u e n c i e s ( T a b l e 4 ) . One f i b r e i n c r e a s e d i t s d i s c h a r g e f r e q u e n c y i n t h e p r e s e n c e o f 10% C0 2. I n a l l c a s e s t h e changes i n a c t i v i t y began d u r i n g t h e f i r s t few seconds f o l l o w i n g a s t e p change i n t h e CO,- c o n t e n t o f t h e v e n t i l a t i n g gas. A l l u n i t s were i s o l a t e d t o l o c a t i o n s w i t h i n t h e l u n g by p u n c t a t e s t i m u l a t i o n . No f i b r e s c o u l d be found w h i c h were s e n s i t i v e t o s t e p changes i n C0 2 c o n t e n t i n t h e v e n t i l a t i n g gas but i n s e n s i t i v e t o m e c h a n i c a l s t i m u l i . T h i s s t u d y has shown t h a t t h e pulmonary m e c h a n o r e c e p t o r s i n t h e f r o g l u n g a r e s e n s i t i v e t o CO2 c o n c e n t r a t i o n s i n t h e l u n g s and a i r w a y s ; i n some 108 F i g u r e 23. R ate and p r o p o r t i o n a l r e c e p t o r ~ r e s p o n s e t o l u n g i n f l a t i o n w i t h a i r ( l e f t ) and a i r + 10% C 0 2 ( f i g h t ) . Upper t r a c e i s a t i m e m a r k e r , second t r a c e i s pulmon-a r y r e c e p t o r d i s c h a r g e , t h i r d t r a c e i s a n a l o g d i s - . \ •.• c h a r g e f r e q u e n c y and l o w e r t r a c e i s i n t r a t r a c h e a l p r e s s u r e . t i m e (s) Table A: Effect of rate and degree of lung I n f l a t i o n and of C0 2 on frog pulmonary receptor discharge. D i s c h a r g e R a t e o f R e c e p t o r s (Hz) F i b r e i 2 3 R e c e p t o r Type . P r o p o r t i o n a l s e n s i t i v e Air A i r + 10% C0 2 D e f l a t i o n 1.8 4.3 2.5 I n f l a t i o n t o 0.5 k P a a t 1 m l . s -1 6.0 5.5 3.8 D e f l a t i o n 1.0 0.3 2.3 I n f l a t i o n t o 0.5 k P a a t 1 m l . s -1 2.8 2.5 3.3 1 2 5 6 Rate s e n s i t i v e 2 . 9 5.1 35.5 0.8 7.5 2.0 6.3 8.8 76.3 27.5 20.0 30.0 35.0 47-5 1.2 0 0.8 5.0 2.5 3.8 10.0 2 . 9 0 25.0 22.5 17.5 12.5 30.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R a t e + P r o p o r t i o n a l s e n s i t i v e 10.2 39.4 3-8 9.0 13.5 7.3 11.8 7.8 14.3 11.0 2.0 0.3 0.5 3.3 14.3 13.5 2.5 I6.3 34.8 36.5 17.0 24.8 15.8 43.3 18.8 5.3 4.8 2.8 16.5 36.5 27.8 9.3 40.0 54.0 72.5 27.5 72.5 35.0 90.0 47.5 22.5 10.0 4o.o 57.5 70.0 75.0 32.5 7.8 24.0 49.8 3-7 3.8 7.5 11.3 2.5 7.5 6.3 9-5 9-0 1.8 0.3 0. 8 2.5 7-5 11.8 3.3 5.7 16.5 29.0 36.8 5.3 26.3 11.0 38.3 23.3 4.0 2.0 4.5 4.0 12.5 24.5 2.8 16.1 17.9 27.5 45.0 62.5 17.5 62.5 15.0 87.5 27.5 27.5 2.5 30.0 15.0 45.0 45.0 12.5 34.8 R a t e + P r o p o r t i o n a l s e n s i t i v e 4.5 10.0 17.5 9.5 16.0 35.0 I l l i n s t a n c e s e x t r e m e l y s e n s i t i v e t o v e r y low l e v e l s o f CC^ comparable t o t h e p h y s i o l o g i c a l l e v e l s r e c o r d e d i n f r o g s ( E m i l i o , '74). These r e s u l t s a r e s i m i l a r t o t h o s e r e c o r d e d f r o m t h e t u r t l e ( S e c t i o n I ) and s u p p o r t t h e s u g g e s t i o n t h a t a pulmonary r e c e p t o r w i t h d i s t i n c t mechano- and chemosensi-t i v e p r o p e r t i e s may r e p r e s e n t t h e f u n c t i o n a l p r e c u r s o r o f t h e v a r i e t y o f more s p e c i a l i z e d pulmonary r e c e p t o r t y p e s w h i c h appear i n modern day v e r t e b r a t e s . 112 GENERAL DISCUSSION A n a l y s i s o f t h e a r r h y t h m i c b r e a t h i n g p a t t e r n i n t h e t u r t l e has shown t h a t t h e change i n m i n u t e v e n t i l a t i o n d u r i n g h y p e r c a p n i a was p r i m a r i l y t h e r e s u l t o f a change i n r e s p i r a t o r y f r e q u e n c y . T h i s was n o t due t o a change i n t h e r a t e o f a c t i v e v e n t i l a t i o n b u t s o l e l y t o s h o r t e n i n g o f t h e n o n v e n t i l -a t o r y p e r i o d . A t any l e v e l o f r e s p i r a t o r y d r i v e , t i d a l volume was m a i n t a i n e d w i t h i n n a r r ow l i m i t s by a d j u s t i n g t h e l e n g t h o f t h e a c t i v e i n s p i r a t o r y and e x p i r a -t o r y i n t e r v a l t o compensate f o r a l a r g e v a r i a b i l i t y i n t h e i n s p i r a t o r y and e x p i r a t o r y gas f l o w r a t e . T h i s l a r g e v a r i a b i l i t y i s p r o b a b l y a consequence o f t h e changes i n g r a v i t a t i o n a l and m e c h a n i c a l f a c t o r s w h i c h a f f e c t b r e a t h i n g . T u r t l e s l a c k an i n d e p e n d e n t pulmonary chamber whose volume can be s e p a r a t e l y v a r i e d by m u s c u l a r a c t i v i t y . V e n t i l a t i o n i s t h u s e f f e c t e d by m u s c l e s w h i c h a l t e r t h e volume o f t h e g e n e r a l body c a v i t y and t h e s e a l t e r a t i o n s a r e i n t u r n t r a n s m i t t e d t o t h e l u n g s r e s u l t i n g i n b r e a t h i n g movements. There appear t o be f o u r m a j o r m u s c l e s i n v o l v e d , t h e s e r r a t u s m a j o r and p e c t o r a l i s m u s c l e s o f t h e p e c t o r a l g i r d l e and t h e t r a n s v e r s u s abdominus and o b l i q u u s abdominus m u s c l e s o f t h e p o s t e r i o r f l a n k c a v i t y . W i t h t h e p o s s i b l e e x c e p t i o n o f t h e m u s c u l a r i s t r a n s v e r s u s abdominus, none o f t h e s e m u s c l e s a r e p u r e l y r e s p i r a t o r y i n f u n c t i o n but s u b s e r v e o t h e r r o l e s , p a r t i c u l a r l y l o c o m o t i o n ( G a n s & H u g h e s , ' 6 7 ) . C o n s e q u e n t l y , w i t h e a c h b r e a t h , t h e l o c a t i o n o f t h e l i m b s and l i m b g i r d l e s (Gans & Hughes, '6 7 ) , t h e p o s s i b l e d e p t h o f t h e t u r t l e below t h e w a t e r s u r f a c e from w h i c h i t i s b r e a t h i n g ( G a u n t & G a n s , ' 6 9 ) , and t h e tonus and p o s i t i o n o f t h e v i s c e r a 1 1 3 w h i c h hang suspended by c o n n e c t i v e t i s s u e s h e e t s below t h e l u n g s (Gans & Hughes, ' 6 7 ) , w i l l a l l a f f e c t t h e r a t e s o f e x p i r a t i o n and i n s p i r a t i o n f o r any g i v e n l e v e l o f ' r e s p i r a t o r y motor o u t p u t . A t any l e v e l o f r e s p i r a t o r y d r i v e , m i n u t e v e n t i l a t i o n i s g r e a t e r i n v a g o t o m i z e d a n i m a l s a l t h o u g h t h e amount o f t i m e s p e n t a c t i v e l y b r e a t h i n g i s r e d u c e d . A l t h o u g h oxygen c o n s u m p t i o n was n o t q u a n t i t a t i v e l y measured and a n a l y z e d , i t w o u l d appear ( v a l u e s i n F i g . 1 2 ) t h a t VQ a l s o i n c r e a s e s s u g g e s t i n g t h a t c o n t r o l o f v e n t i l a t i o n p r i m a r i l y t h r o u g h changes i n r a t h e r t h a n f i n c r e a s e s . t h e work o f b r e a t h i n g . I t i s q u i t e f e a s i b l e t h a t a l t e r a t i o n s i n T^^p a r e l e a s t c o s t l y i n terms o f r e s p i r a t o r y work and u n t i l b r e a t h h o l d i n g i s e l i m i n a t e d changes i n T , T^' and T^, as w e l l as l a r g e r changes i n V^, w i l l n o t appear. T h i s argument i m p l i e s t h a t t h e change i n t h e f o r c e o f m u s c l e c o n t r a c t i o n r e q u i r e d t o i n c r e a s e r e s p i r a t o r y f l o w r a t e s such t h a t f i n c r e a s e s by s h o r t e n i n g T . and i n c r e a s e s w i t h i n t h i s s h o r t e n e d i n t e r v a l , a r e e n e r g e t i c a l l y more c o s t l y t h a n t a k i n g a n o t h e r b r e a t h a t t h e same r a t e and d e p t h . I t s h o u l d be n o t e d t h a t under s e v e r e h y p e r c a p n i c s t r e s s ( F xco ^5%)» f approaches f and b r e a t h i n g becomes v i r t u a l l y e u p n e i c . A l t h o u g h d a t a were not c o l l e c t e d and a n a l y z e d from a n i m a l s under such c o n d i t i o n s , i t i s c l e a r t h a t f u r t h e r i n c r e a s e s i n V„ must e i t h e r r e s u l t s o l e l y from changes i n V o r f p must i n c r e a s e . I t i s p o s s i b l e t h a t once t h e b r e a t h i n g rhythm becomes c o n t i n u o u s , p o s i t i v e c o r r e l a t i o n s o f V^, and f w i t h t h e r e s p i r a t o r y i n t e r v a l s may appear as i n mammals. T h i s s u g g e s t s t h a t t h e a p p a r e n t d i f f e r e n c e i n c e n t r a l i n t e g r a t i o n of v a g a l a f f e r e n t i n f o r m a t i o n between mammals and t u r t l e s may stem from t h e 114 r e d u c e d m e t a b o l i c demand o f t u r t l e s . I t s h o u l d be remembered, however, t h a t i n t u r t l e s , b r e a t h h o l d i n g i s a r e l a t i v e l y ; p a s s i v e phenomenon f o r a t t h e end of a c t i v e i n s p i r a t i o n t h e g l o t t i s c l o s e s and t h e r e s p i r a t o r y m u s c l e s r e l a x (McCutcheon, '43; Gans & Hughes, ' 6 7 ) . T h i s p e r i o d i s t e r m i n a t e d by an a c t i v e e x p i r a t i o n . Models o f c e n t r a l i n t e g r a t i o n o f r e s p i r a t i o n i n mammals i n c o r p o r a t e an i n s p i r a t o r y " o f f s w i t c h " as t h e major c o n t r o l element d e t e r m i n -i n g c e s s a t i o n o f a c t i v e i n s p i r a t i o n and t h e b e g i n n i n g o f p a s s i v e e x p i r a t i o n ( B r a d l e y e t a l . , '75; E u l e r e t a l . , '76b). F u r t h e r , i t has been ar g u e d t h a t i n mammals ( G a u t i e r e t a l . , '73) t h e e x p i r a t o r y i n t e r v a l t r i g g e r s t h e n e x t i n s p i r a t i o n when;lung volume f a l l s , t o FRC. I n t u r t l e s t h e r e ' i s no c o r r e l a -t i o n between T^ and V^, FRC can be h i g h l y v a r i a b l e ( M i l s o m & J o h a n s e n , ' 7 5 ) / and a r t i f i c i a l l y o p e n i n g t h e l u n g s t o atmosphere a t t h e end o f each i n s p i r a -t i o n r e d u c e s l u n g volume t o FRC o r below d u r i n g b r e a t h h o l d i n g b u t does n o t t r i g g e r i n s p i r a t i o n . T h i s p l u s t h e p r e s e n c e o f b o t h an a c t i v e and a p a s s i v e i n s p i r a t o r y phase f o l l o w e d by a c t i v e e x p i r a t i o n d u r i n g s t e a d y s t a t e s p o n t a n -eous b r e a t h i n g i n t u r t l e s i n d i c a t e t h a t e i t h e r t h e i n s p i r a t o r y " o f f s w i t c h " i s much more complex i n t u r t l e s t h a n mammalian models i n d i c a t e , o r t h a t an e x p i r a t o r y "on s w i t c h " may e x i s t , o r b o t h . A l t h o u g h t h e r e s p i r a t o r y d r i v e n e c e s s a r y t o pr o d u c e a c o n t i n u o u s p a t t e r n o f b r e a t h i n g i n t u r t l e s f a r exceeds p h y s i o l o g i c a l l e v e l s , answers t o t h e s e s p e c u l a t i v e q u e s t i o n s may p r o v i d e i n s i g h t s i n t o t h e changes w h i c h o c c u r i n t h e c o n t r o l o f t h e v e n t i l a t o r y p a t t e r n when m e t a b o l i c demand exceeds t h e p o i n t a t w h i c h f r e q u e n t i n t e r m i t t e n t b r e a t h h o l d i n g i s f e a s i b l e . 115 SUMMARY 1. i s shown t o be a s t r o n g r e s p i r a t o r y s t i m u l a n t i n t u r t l e s . was i n c r e a s e d 3 and 7 t i m e s above r e s t i n g v a l u e s by t h e p r e s e n c e o f 5 and 10% CO2 r e s p e c t i v e l y i n t h e i n s p i r e d gas. 2. I n c r e a s e s i n V„ were p r i m a r i l y t h e r e s u l t o f changes i n r e s p i r a t o r y hi f r e q u e n c y . T i d a l volume was m a i n t a i n e d w i t h i n n a r row l i m i t s by a d j u s t -i n g t h e l e n g t h o f t h e a c t i v e i n s p i r a t o r y and e x p i r a t o r y i n t e r v a l t o compensate f o r a l a r g e v a r i a b i l i t y i n t h e i n s p i r a t o r y and e x p i r a t o r y gas f l o w r a t e s . T h i s mechanism was dependent upon l u n g volume i n f o r m a -t i o n c a r r i e d w i t h i n t h e vagus n e r v e . 3. The f r e q u e n c y o f b r e a t h i n g w i t h i n e ach v e n t i l a t o r y p e r i o d remained c o n s t a n t d u r i n g h y p e r c a p n i a ; t h e i n c r e a s e seen i n r e s p i r a t o r y f r e q u e n c y was due s o l e l y t o s h o r t e n i n g o f t h e n o n v e n t i l a t o r y p e r i o d . 4. R e s p i r a t o r y f r e q u e n c y and t i d a l volume were c o n t r o l l e d s e p a r a t e l y and i n d e p e n d e n t o f t h e b r e a t h l e n g t h ( T t Q t ) , t h e i n s p i r a t o r y i n t e r v a l (T^.), t h e a c t i v e i n s p i r a t o r y i n t e r v a l ( T ' ) , and t h e e x p i r a t o r y i n t e r v a l (T ) . J. hi 5. A l a r g e i n c r e a s e i n accompanied vagotomy and was due t o an i n c r e a s e i n c e n t r a l r e s p i r a t o r y a c t i v i t y . The i n s p i r a t o r y and e x p i r a t o r y i n t e r -v a l s were n o t p r o l o n g e d . 6. F o l l o w i n g vagotomy, changes i n V„ due t o h y p e r c a p n i a stemmed p r i m a r i l y hi from changes i n V T w h i l e changes i n r e s p i r a t o r y f r e q u e n c y were m a r k e d l y r e d u c e d . 116 S i n g l e f i b r e d i s c h a r g e was r e c o r d e d f r o m s l o w l y a d a p t i n g pulmonary s t r e t c h r e c e p t o r s i n s i n g l e - p i t h e d f u r t l e s and t h e f u n c t i o n a l c h a r -a c t e r i s t i c s o f t h e s e r e c e p t o r s were d e r i v e d f r o m s t a t i c and dynamic l u n g i n f l a t i o n s . Change i n l u n g volume was t h e s o l e s t i m u l u s o f t h e s e r e c e p t o r s ; t h e r o l e and degree o f change i n t r a n s p u l m o n a r y p r e s s u r e were w i t h o u t d i r e c t e f f e c t on r e c e p t o r d i s c h a r g e . A l l o t h e r f u n c t i o n a l c h a r a c t e r i s t i c s , i n c l u d i n g s e n s i t i v i t y t o C 0 2 , d i f f e r e d o n l y q u a n t i t a t i v e l y from t h o s e r e c o r d e d i n pulmonary s t r e t c h r e c e p t o r s o f mammals. Most o f t h e q u a n t i t a t i v e d i f f e r e n c e s may a r i s e from t h e l o w e r body t e m p e r a t u r e o f t h e t u r t l e and t h e l o c a t i o n o f t h e r e c e p t o r s i n t h e t u r t l e l u n g . I n s e v e r a l i n s t a n c e s b o t h t o n i c and p h a s i c r e c e p t o r d i s c h a r g e were t o t a l l y i n h i b i t e d t h r o u g h o u t t h e v e n t i l a t o r y c y c l e by 5 t o 10% CO^ i n t h e i n s p i r e d gas. The h i g h d e g r e e o f s e n s i t i v i t y t o CO^ i n t h e s e few r e c e p t o r s p a r a l l e l s t h a t o f t h e i n t r a p u l m o n a r y C 0 2 r e c e p t o r s d e s c r i b e d i n b i r d s . Pulmonary m e c h a n o r e c e p t o r s i n t h e f r o g l u n g were a l s o shown t o be s e n s i t i v e t o C 0 2 , i n some i n s t a n c e s e x t r e m e l y s e n s i t i v e t o v e r y low . l e v e l s o f C 0 2 . I t i s s u g g e s t e d t h a t a pulmonary r e c e p t o r w i t h d i s t i n c t mechano- and c h e m o s e n s i t i v e p r o p e r t i e s may r e p r e s e n t t h e f u n c t i o n a l p r e c u r s o r o f t h e v a r i e t y o f more s p e c i a l i z e d pulmonary r e c e p t o r t y p e s w h i c h appear i n modern day v e r t e b r a t e s . The r o l e o f C 0 2 s e n s i t i v i t y o f pulmonary r e c e p t o r s i n t h e o v e r a l l r e s p o n s e o f t u r t l e s t o i n h a l e d C 0 2 was a l s o t e s t e d . I s o c a p n i c 117 hyperpnea associated with inhalation of by a vascularly isolated lung was small and abolished by vagotomy suggesting, that the ventil-atory response of turtles to increasing levels of F T is primarily co 2 dependent on increased arterial P . It is concluded that both inhibition of pulmonary stretch receptor discharge and a functional increase in central inspiratory volume threshold contribute to tida l volume increases associated with increasing levels of airway CC^. 11. Careful analysis has been made of the correlations between the breath length, the inspiratory interval, the expiratory interval, the length of breath holding and the tidal volume. Comparison is made of the central integration of the vagal volume signal and the control of the breathing pattern in intact turtles with that of mammals following ablation of the pneumotaxic centre. 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