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The effect of carotid sinus pressure on the osmoregulation of arginine vasopressin in anesthetized rabbits Scott, Christine Sharon 1992

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THE EFFECT OF CAROTID SINUS PRESSURE ON THE OSMOREGULATION OF ARGININE VASOPRESSIN IN ANESTHETIZED RABBITS by CHRISTINE SHARON SCOTT B.Sc, McGill University, 1990 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Physiology) We accept t h i s thesis as conforming to rfeJie requirisd standard THE UNIVERSITY OF BRITISH COLUMBIA August, 1992 © Christine Sharon Scott, 1992 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my writ ten permission. Department of The University of British Columbia Vancouver, Canada Date ^epTBA^eeiL JO y /i9z. DE-6 (2/88) ABSTRACT Studies i n r a t , dog and human show that changes i n blood volume influence the r e l a t i o n s h i p between plasma osmolality and plasma arginine vasopressin (AVP) concentration. I t has not been proven whether the change i n blood volume acts through stimulation of the low pressure receptors or a r t e r i a l baroreceptors or both. The purpose of these experiments was to investigate the e f f e c t of changes i n a r t e r i a l baroreceptor stimulation on the r e l a t i o n s h i p between plasma osmolality and plasma AVP i n the anesthetized, a r t i f i c i a l l y v e n t i l a t e d rabbit. Both c a r o t i d sinuses were i s o l a t e d and perfused with blood at servo-controlled pressures of 40, 100, or 140 mmHg. The vagus and a o r t i c depressor nerves were sectioned b i l a t e r a l l y to eliminate input from a t r i a l and a o r t i c arch baroreceptors. Saline (0.3%) was infused i . v . to lower plasma osmolality and 5% sal i n e was infused to r a i s e plasma osmolality. At low (288 + 1 mosm/kg), medium (309 ± 1 mosm/kg) and high (323 + 1 mosm/kg) plasma osmolality, the ca r o t i d sinus pressure was changed from 100 mmHg to 40 mmHg, to 140 mmHg and returned to 100 mmHg. A r t e r i a l blood samples were taken at each caroti d sinus pressure 7 minutes a f t e r the change i n pressure. Plasma immunoreactive AVP (iAVP) was measured by radioimmunoassay. At medium (309 mosm/kg) and high (323 mosm/kg) plasma osmolality, the l e v e l s of plasma AVP were higher at CSP 40 mmHg than at CSP 140 mmHg. The r e l a t i o n s h i p between plasma iAVP and plasma osmolality was expressed as a l i n e a r regression at each of the ca r o t i d sinus pressures. The slope of the regression l i n e at CSP 40 mmHg was greater than i t was at CSP 140 mmHg. The x-intercepts of the regression l i n e s were not s i g n i f i c a n t l y d i f f e r e n t . In the control rabbits, i n which CSP was maintained constant at 100 mmHg, the slope of the AVP/Posm r e l a t i o n s h i p was the same as that i n the experimental animals at the same CSP. These r e s u l t s indicate that a r t e r i a l baroreceptors, acting alone, can change the slope of the AVP/Posm re l a t i o n s h i p . Changes i n c a r o t i d sinus pressure appear to change the s e n s i t i v i t y but not the threshold of the osmotic control of AVP release. TABLE OF CONTENTS PAGE TABLE OF CONTENTS i v LIST OF FIGURES v i ACKNOWLEDGEMENTS v i i i I. INTRODUCTION 1 A. Renal e f f e c t s of AVP 3 B. Cardiovascular e f f e c t s of AVP 3 C. Osmoregulation of AVP 7 D. Baroregulation of AVP 13 1. Relative Contribution of A r t e r i a l and A t r i a l Receptors 17 a. Studies i n Dogs 17 b. Studies i n Rabbits 22 c. Studies i n Primates 25 E. Influence of blood volume on the osmoregulation of AVP 28 II . RATIONALE 34 II I . METHODS 36 A. Carotid Sinus Perfusion 37 B. Changing Plasma Osmolality 41 C. Experimental Protocol 42 D. Hormone Analysis 42 E. S t a t i s t i c a l Analysis 48 IV. RESULTS 50 A. Systemic Variables 50 B. Plasma Osmolality 58 C. Plasma immunoreactive arginine vasopressin 58 D. Control Data 73 V. DISCUSSION 84 A. E f f e c t of CSP on Osmoregulation of AVP 84 B. Osmotic control of AVP 88 C. Experimental Procedure 88 D. Physiological Relevance 90 E. Concluding Comments 93 VI REFERENCES 95 LIST OF FIGURES 1 Relationship between plasma arginine vasopressin (PAVP) and plasma osmolality (POSM) at d i f f e r e n t l e v e l s of l e f t a t r i a l pressure (LAP) i n conscious dogs 30 2 Schematic representation of the preparation used to perfuse the c a r o t i d sinuses 39 3 Experimental protocol 43 4 Mean a r t e r i a l pressure at d i f f e r e n t l e v e l s of car o t i d sinus pressure (CSP) at three l e v e l s of plasma osmolality (Posm) 5 Heart rate at d i f f e r e n t l e v e l s of CSP at three l e v e l s of Posm 6 Right a t r i a l pressure at d i f f e r e n t l e v e l s of CSP at three l e v e l s of Posm 7 Plasma osmolality at d i f f e r e n t l e v e l s of CSP 60 8 Plasma arginine vasopressin (AVP) at d i f f e r e n t l e v e l s of CSP at three l e v e l s of Posm 9 Linear regression of plasma AVP and Posm "^^  °^ 40 mmHg 65 10 Linear regression of plasma AVP and Posm °f 100 mmHg 67 11 Linear regression of plasma AVP and Posm °^ 140 mmHg 69 12 Linear regressions of plasma AVP and Posm 40, 100 and 140 mmHg 71 13 Plasma osmolality with CSP maintained at 100 mmHg...74 14 Mean a r t e r i a l pressure with CSP maintained at 100 mmHg at three l e v e l s of Posm "^ ^ 15 Heart rate with CSP maintained at 100 mmHg.at three l e v e l s of Posm "78 16 Right a t r i a l pressure with CSP maintained at 100 mmHg at three l e v e l s of Posm Plasma AVP with CSP maintained at 100 mmHg at three l e v e l s of Posm ACKNOWLEDGEMENTS I would l i k e to thank Dr. John Ledsome f o r h i s wisdom and guidance and for h i s support that extended beyond my work i n the laboratory. I am gr a t e f u l to him fo r granting me t h i s opportunity. To Jeannie Sharp-Kehl and Carolyn Redekopp I extend my gratitude f o r t h e i r invaluable te c h n i c a l assistance, t h e i r words of encouragement and for creating such a f r i e n d l y working environment. I would l i k e to thank Dr. Lioy and Dr. Vaughan f o r t h e i r contribution as members of my supervisory committee and for providing many hel p f u l suggestions. To Dr. Ray Pederson I extend my appreciation for h i s gracious h o s p i t a l i t y and fo r the special contribution he has made to my time i n the department as a graduate student. I would l i k e to thank Dr. Carol Ann Courneya for her help with the computer and fo r her advice and encouragement i n so many areas. To the graduate students I extend my thanks f o r t h e i r friendship and for many memorable runs i n the endowment lands, r a i n or shine, day or night. And f i n a l l y I would l i k e to express my thanks to my parents for t h e i r encouragement and support. I. INTRODUCTION In 1895, Schafer and Oliv e r showed that extracts of t i s s u e from the p i t u i t a r y gland had vasopressor a c t i v i t y (Oliver and Schafer, 1895). Work by Magnus and Schafer over the next few years established that extracts of the posterior p i t u i t a r y gland also acted on the kidney a f f e c t i n g the rate of urine flow (Magnus and Schafer, 1901). I t was not u n t i l 1954, however, that arginine vasopressin (AVP), the neurohypophysial hormone mediating these e f f e c t s , was synthesized by du Vigneaud (du Vigneaud et a l . , 1954). AVP i s also ref e r r e d to as vasopressin or a n t i d i u r e t i c hormone (ADH). AVP i s a 9-amino acid neurohormone. I t i s synthesized i n the magnocellular neurons of the supraoptic n u c l e i (SON) and the paraventricular nuclei (PVN) of the anterior hypothalamus as part of a larger 166-amino acid precursor molecule which includes a c a r r i e r protein, neurophysin I I . The precursor i s packaged into neurosecretory granules and transported down the axons of the neurohypophysial t r a c t to the terminals i n the posterior p i t u i t a r y . During transport, the precursor undergoes maturation. AVP and i t s neurophysin are released by a process of exocytosis from the posterior p i t u i t a r y i n response to nerve impulses (Bisset and Chowdrey, 1988). The d a i l y plasma AVP concentration i n normal humans and animals, depending on t h e i r state of hydration, ranges from 0.3 to 30 pg/ml (Cowley, 1982). The normal l e v e l of plasma AVP i n overnight fasted healthy adults ranges from 2 to 3 pg/ml corresponding to a plasma osmolality of 287 mosm/kg (Robertson et a l . , 1976; 1986). There i s evidence to suggest that anesthesia and su r g i c a l stress elevate plasma AVP le v e l s (Bonjour and Malvin, 1970). For t h i s reason the v a l i d i t y of using anesthetized animal preparations for the study of the r e f l e x control of AVP has been questioned. However, a study performed by Wehberg et a l . (1991) compared the c a r o t i d baroreflex control of plasma AVP i n conscious and anesthetized dogs. They found that the AVP response to changes i n car o t i d sinus pressure was s i m i l a r i n conscious and anesthetized dogs. Furthermore, McNeill and Pang (1981) found that anesthesia did not cause elevated l e v e l s of plasma AVP i n cats. The h a l f l i f e of AVP appears to vary between species. Values reported i n the rat vary from 0.9 to 8 min, i n the dog vary from 5 to 8 min and i n the human vary from 2 to 21 min (Lauson et a l . , 1974). A study i n v e s t i g a t i n g the disappearance of AVP i n rabbits anesthetized with alpha chloralose found both a rapid and slow component i n the elimination of AVP, with h a l f l i f e values of 0.9 and 5.4 min, respectively (King et a l . , 1989). A. Renal E f f e c t s of AVP The primary action of AVP i s to increase the water permeability of the c o l l e c t i n g tubule i n the kidney and thus promote water reabsorption (Abramow et a l . , 1987). The receptor mediating t h i s e f f e c t i s the V2 receptor. AVP acts by increasing the number of water channels i n the a p i c a l membrane (Abramow et a l . , 1987). The renal e f f e c t s of AVP are very s e n s i t i v e to changes i n AVP concentration. "The f u l l range of renal concentrating and d i l u t i n g capacity i s covered by a ten-fold change i n plasma vasopressin concentration from 0.5 to 5 pg/ml" (Robertson et a l . , 1976). This system enables the organism to maintain body water very constant by r a p i d l y a l t e r i n g urine output. B. Cardiovascular E f f e c t s of AVP AVP i s a potent vasoconstrictor. The pressor e f f e c t s are mediated by V^ receptors on smooth muscle c e l l s . However, the l e v e l s of AVP required to e l i c i t an increase i n blood pressure i n the i n t a c t human or animal are an order of magnitude greater than those required for i t s a n t i - d i u r e t i c e f f e c t s (Cowley, 1982). In dogs, rats and humans the threshold plasma AVP concentration required to produce a r i s e i n a r t e r i a l pressure of at l e a s t 5 mmHg ranged from 30-60 pg/ml. Humans appear to be the least pressor s e n s i t i v e of the three species (Cowley, 1982). The r e l a t i v e l y high l e v e l s of AVP required f o r a pressor response has raised the question of the p h y s i o l o g i c a l s i g n i f i c a n c e of the pressor e f f e c t of AVP. However, i t i s now clear that l e v e l s of AVP lower than those required to r a i s e a r t e r i a l pressure cause a s i g n i f i c a n t increase i n vascular resistance. This increase i n vascular resistance i s accompanied by a slowing of the heart rate and decrease i n cardiac output. I t has been suggested that AVP enhances the responsiveness of the baroreceptor r e f l e x (Cowley, 1982). Studies i n which AVP was t o p i c a l l y applied to blood vessels in vitro indicate that AVP i s the most potent vasoconstrictor i n t h e c i r c u l a t i o n (Altura and Altura, 1977). Topical application of physiological l e v e l s of AVP (1 pg/ml) to r a t mesenteric a r t e r i o l e s caused vasoconstriction. In the same study, the mesenteric a r t e r i o l e s were found to be at l e a s t 3 orders of magnitude more se n s i t i v e to AVP than to angiotensin. Different vessels within the same species exhibited varying s e n s i t i v i t i e s to AVP. Of the r a t mesenteric muscular microvessels, venules are more s e n s i t i v e to AVP than a r t e r i o l e s (Altura, 1973). The vasoconstrictor e f f e c t s of AVP occur predominantly within the splanchnic c i r c u l a t i o n and the vessels supplying s k e l e t a l muscles (Cowley, 1982; Ericsson, 1971). As previously mentioned, p h y s i o l o g i c a l l e v e l s of AVP do not elevate blood pressure i n i n t a c t humans or animals (Cowley, 1982). However, studies by Cowley et al.,(1974) and Montani et a l . (1980), i n the dog, show that, i n the absence of autonomic r e f l e x control, achieved through baroreceptor denervation or ablation of the CNS, ph y s i o l o g i c a l l e v e l s of AVP cause a s i g n i f i c a n t elevation i n blood pressure. Montani et a l . (1980) demonstrated that i n normal conscious dogs ph y s i o l o g i c a l l e v e l s of AVP s i g n i f i c a n t l y increased peripheral vascular resistance, although mean a r t e r i a l pressure remained unchanged. In t h i s study, the AVP-induced increase i n peripheral vascular resistance was accompanied by bradycardia and a dose dependent decrease i n cardiac output. In barodenervated dogs, however, infusion of AVP caused an increase i n peripheral vascular resistance and a s i g n i f i c a n t increase i n mean a r t e r i a l pressure. There was no change i n heart rate or cardiac output i n the absence of baroreflexes. The authors conclude that p h y s i o l o g i c a l le v e l s of plasma AVP do have hemodynamic e f f e c t s but due to an in t e r a c t i o n with the baroreceptor r e f l e x , do not a l t e r a r t e r i a l pressure. I t has been suggested that AVP acts c e n t r a l l y to enhance the responsiveness of the baroreceptor r e f l e x (Cowley et a l . , 1974; Montani et a l . , 1980). The area postrema has been suggested as a c e n t r a l s i t e of action of c i r c u l a t i n g AVP (Undesser et a l . , 1985). The area postrema i s the only area i n the hindbrain without a blood brain b a r r i e r . Infusions of AVP resulted i n a greater decrease i n renal sympathetic nerve a c t i v i t y and heart rate f o r a given increase i n blood pressure than infusions of phenylephrine, suggesting that AVP was enhancing the baroreflex. Following lesions of the area postrema, infusions of AVP and phenylephrine resulted i n s i m i l a r responses. Lesions of the area postrema appeared to abolish the a b i l i t y of AVP to enhance the baroreflex (Undesser et a l . , 1985). Injection of a VI AVP antagonist into the area postrema abolished the a b i l i t y of AVP to enhance the baroreflex (Hasser and Bishop, 1990). AVP contributes to short-term regulation of a r t e r i a l pressure during hypovolemia caused by hemorrhage (Cowley, 1982). In the absence of the baroreflex and the renin-angiotensin system, the other short-term blood pressure regulation systems, a f a l l i n a r t e r i a l pressure i s s t a b i l i z e d by increased lev e l s of AVP. In anesthetized dogs with baroreflex and renin-angiotensin systems blocked the a r t e r i a l pressure was dropped fom 108 to 50 mmHg by hemorrhage (Cowley et a l . , 1980). A r t e r i a l pressure was returned to 90 mitiHg within 3 to 4 minutes as a r e s u l t of AVP secretion. There was a f i v e f o l d increase i n AVP l e v e l s and the response was abolished by the removal of the p i t u i t a r y gland or by i n j e c t i o n of the AVP antagonist, [1-deaminopenicillamine, 4-valine]-8-D-arginine-vasopressin (dPVDAVP). C. osmoregulation of AVP Release In 1947 Verney presented the r e s u l t s of a s e r i e s of experiments which examined the factors involved i n the regulation of a posterior p i t u i t a r y a n t i d i u r e t i c substance (Verney, 1947). These experiments introduced the concept of osmotic control of AVP release. Verney injected a hypertonic solution of sodium chloride, sucrose, sodium sulphate or urea, into a ca r o t i d artery of a conscious dog. He found that sodium chloride, sucrose and sodium sulphate, which do not permeate the c e l l membrane, produced an a n t i d i u r e t i c response, while urea which f r e e l y permeates the c e l l membranes, did not. The a n t i d i u r e t i c response was the same as that produced by the intravenous i n j e c t i o n of extract from the posterior p i t u i t a r y . Verney suggested that •osmoreceptors' located i n the anterior hypothalamus, within the f i e l d of the supraoptic nucleus, were detecting and responding to changes i n the osmolality of the e x t r a c e l l u l a r f l u i d and regulating the release of an a n t i d i u r e t i c hormone (Verney, 1947). Since these early studies by Verney the r e l a t i o n s h i p between plasma osmolality and plasma AVP has been thoroughly examined. At low le v e l s of plasma osmolality, plasma AVP i s suppressed to low or undetectable l e v e l s . Above the 'osmotic threshold', the l e v e l of plasma osmolality at which plasma AVP l e v e l s are detectable, plasma AVP l e v e l s increase with increases i n plasma osmolality. This r e l a t i o n s h i p i s most simply defined i n terms of a l i n e a r regression i n which there i s a highly s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n between plasma AVP and plasma osmolality (Robertson et a l . , 1976). The x-intercept of t h i s l i n e represents the osmotic threshold or set-point of the osmoregulation mechanism. The average threshold for healthy adults i s 280 mosm/kg (Robertson et a l . , 1976). In the conscious dog the threshold was found to be 277 mosm/kg (Quillen and Cowley, 1983) . The slope of t h i s l i n e represents the s e n s i t i v i t y of the osmoregulation mechanism. The average slope f o r healthy adults i s 0.38 pg/ml per mosm/kg (Robertson et a l . , 1976). At t h i s s e n s i t i v i t y , a change i n plasma osmolality of only 1% (2.9 mosm/kg) would change plasma AVP by about 1 pg/ml. Given the s e n s i t i v i t y of the kidney to c i r c u l a t i n g AVP, small changes i n plasma AVP le v e l s w i l l produce r e l a t i v e l y large changes i n urinary concentration (Robertson et a l . . 1976). This enables the organism to respond quickly and e f f e c t i v e l y to changes i n water intake and to maintain body osmolality at a constant l e v e l . There i s considerable v a r i a t i o n between i n d i v i d u a l s i n the s e n s i t i v i t y (slope) and threshold (x-intercept) of t h i s r e l a t i o n s h i p . When the AVP/Posm r e l a t i o n s h i p i s p l o t t e d for a population there i s s i g n i f i c a n t scatter (Robertson et a l . , 1976). However, fo r a given i n d i v i d u a l , the regression l i n e i s reproducible. Plasma AVP secretion depends not only on the absolute l e v e l of plasma osmolality but also on the rate of change of osmolality. I f the rate of change of plasma osmolality i s greater than 2%/hr, the r i s e i n plasma AVP per u n i t change i n plasma osmolality i s exaggerated. The s e n s i t i v i t y of t h i s r e l a t i o n s h i p i s rate-dependent (Robertson et a l . , 1976). Verney o r i g i n a l l y postulated the existence of osmoreceptors i n the region of the anterior hypothalamus (Verney, 1947). Although the existence of c e n t r a l osmoreceptors i s generally accepted (Ramsay 1985), t h e i r precise location remains controversial. There i s s t i l l some question as to whether these osmoreceptors are within the neurosecretory c e l l s of the SON and PVN or whether they are a d i s t i n c t set of c e l l s with projections to these neurosecretory c e l l s (Bisset and Chowdrey, 1988). Some investigators maintain that the neurosecretory c e l l s i n the SON act as the 'true' osmoreceptors (Leng et a l . , 1982; 1985). Leng (1980) measured e x t r a c e l l u l a r action potentials of antidromically i d e n t i f i e d neurosecretory c e l l s i n the SON in vivo i n response to ap p l i c a t i o n of hypertonic s a l i n e . AVP-secreting c e l l s i n the SON responded to a d i r e c t a p p l i c a t i o n of hypertonic s a l i n e (1-4M:5xlO~Sml) with an increase i n t h e i r f i r i n g rate. Mason et a l . (1980, 1983) studied i n t r a c e l l u l a r recordings from the SON i n is o l a t e d perfused s l i c e s of r a t hypothalamus in vitro and found that when ph y s i o l o g i c a l l e v e l s of NaCl or mannitol were added to the perfusion medium, there was depolarization of the membrane p o t e n t i a l and an increase i n frequency of excitatory post-synaptic potentials (EPSPs). They concluded that the neurosecretory c e l l s of the SON and PVN are d i r e c t l y osmosensitive but require presynaptic or afferent input f o r t h e i r f u l l expression (Mason, 1983). The strongest evidence against the notion of the SON as the cerebral osmoreceptor i s that an i n t r a c a r o t i d perfusion of hypertonic urea f a i l s to stimulate the release of AVP (Thrasher et a l . , 1980a; 1980b). The blood-brain b a r r i e r i s impermeable to urea. Urea i n the blood w i l l therefore withdraw water from the CSF causing a r i s e i n NaCl concentration of the CSF. This p r i n c i p l e i s used i n the treatment of cerebral edema i n which a hyperosmotic solu t i o n of urea i s used to reduce i n t r a c r a n i a l pressure. I f the osmoreceptors were within the blood-brain b a r r i e r , they would detect t h i s change i n the osmolality of CSF and stimulate the secretion of AVP. The f a i l u r e of hypertonic urea to stimulate AVP secretion suggests that the osmoreceptors l i e outside of the blood-brain b a r r i e r where they are i n contact with blood-borne solutes. Urea f r e e l y crosses the c e l l membrane (Schloerb, 1960), and so does not exert osmotic pressure on the osmoreceptor. There i s much evidence to suggest that the osmoreceptors l i e i n the circumventricular organs (Thrasher, 1985; Bisset and Chowdrey, 1988). The circumventricular organs surround the cerebral v e n t r i c l e s and l i e outside the blood-brain b a r r i e r . In t h i s l o c ation they can detect changes i n the osmolality of both the cerebral s p i n a l f l u i d (CSF) and the c i r c u l a t i n g blood. The circumventricular organs include: the antero-ventral region of the t h i r d v e n t r i c l e (AV3V), a complex which includes the organum vasculosum of the laminae terminalis (OVLT) and the nucleus medianus; the subfornical organ (SFO); and the area postrema (Bisset and Chowdrey, 1988). The r e l a t i v e importance of the circumventricular organs i n the osmotic control of AVP may vary between species. In the dog and r a t , the osmoreceptors appear to be i n the AV3V region. Ablation of the OVLT i n the dog and r a t attenuates the osmotic control of AVP release (Johnson, 1985; Ramsay, 1985; Thrasher, 1985). The SFO i s involved i n the control of drinking and i n the release of AVP by angiotensin, but there i s no evidence that i t i s involved i n the osmotic control of AVP release (Knepel et a l . , 1982). The area postrema has been suggested as the s i t e of the osmoreceptor i n the cat (Bisset et a l . , 1985). There are neural interconnections and efferent projections to the SON and PVN from a l l of the circumventricular organs. However, i t i s unknown whether these projections s e l e c t i v e l y innervate the AVP-secreting c e l l s (Bisset and Chowdrey, 1988). Peripheral osmoreceptors are located i n the area of the p o r t a l vein and the mesenteric vessels which drain the upper small i n t e s t i n e , regions innervated by mesenteric side branches of the major splanchnic nerves. The p h y s i o l o g i c a l s i g n i f i c a n c e of these splanchnic osmoreceptors has been questioned. However, recent studies by Choi-Kwon et a l . (1990; 1991) show that these osmoreceptors do play a s i g n i f i c a n t r o l e i n regulating AVP secretion during g a s t r i c s a l t and water administration i n conscious r a t s . In these studies, changes i n plasma AVP l e v e l s were correlated with the osmolality of the g a s t r i c infusion i n the absence of s i g n i f i c a n t changes i n systemic osmolality. The r e s u l t s suggest that c e n t r a l osmoreceptors are mainly involved i n s i g n a l l i n g slow changes of systemic osmolality, f o r example, during dehydration, whereas splanchnic receptors sense immediate changes i n g a s t r o i n t e s t i n a l solute concentration, for example, following food or water intake (Choi-Kwon et a l . , 1990). Splanchnic osmoreceptors are s i m i l a r to c e n t r a l osmoreceptors i n t h e i r s p e c i f i c i t y . They respond to g a s t r i c infusions of L i C l , NaCl, sodium isethionate, mannitol and sucrose but not urea (Choi-Kwon and Baertschi, 1991). While e a r l i e r work by V a l l e t and Baertschi (1982) indicated a s p i n a l and vagal afferent pathway from the splanchnic osmoreceptors, the more recent studies by Choi-Kwon and Baertschi (1991) suggest that the afferent f i b r e s t r a v e l i n the s p i n a l cord . D. Barorecmlation of AVP In 1938 Rydin and Verney found that hemorrhage i n h i b i t e d a water d i u r e s i s i n the conscious dog (Rydin and Verney, 1938). This was the f i r s t i n d i c a t i o n that a decrease i n blood volume would stimulate the release of AVP. I t has since been demonstrated i n a number of animal species that a decrease of 10 to 15% of blood volume i s a potent Stimulus for the release of vasopressin (Clark and Rocha E S i l v a , 1967; Share, 1968; Szcepanska-Sadowska, 1972). Conversely, an increase i n blood volume leads to a decrease i n AVP l e v e l s i n a number of species (Shade and Share, 1975; Ledsome et a l . , 1985). This suggests that receptors s e n s i t i v e to changes i n blood volume may be t o n i c a l l y active. There are two groups of stretch receptors that respond to changes i n blood volume and pressure; low pressure or a t r i a l receptors and high pressure or a r t e r i a l receptors. The low pressure receptors are located i n the a t r i a of the heart and respond to changes i n a t r i a l volume which correspond to changes i n blood volume. These receptors are also referred to as a t r i a l receptors and have afférents that run i n the vagus. Evidence of these receptors involved i n the regulation of AVP was f i r s t provided by Henry, Gauer and Reeves (1956) who showed that expansion of a balloon i n the l e f t atrium resulted i n d i u r e s i s i n anesthetized dogs. B i l a t e r a l vagotomy eliminated the d i u r e t i c response to l e f t a t r i a l distension i n d i c a t i n g that the afférents of these receptors were ca r r i e d i n the vagus (Ledsome and Linden, 1968). Using a bioassay to measure plasma AVP, Johnson et a l . (1969) demonstrated that distension of the l e f t atrium resulted i n a decrease i n plasma AVP l e v e l s and an increase i n urine volume. These r e s u l t s have been confirmed i n conscious dogs by Fater et a l . (1982) using radioimmunoassay to measure plasma AVP l e v e l s . The same study confirmed the view that receptors i n the pulmonary vasculature and r i g h t heart do not contribute to t h i s response (Fater et a l . , 1982) . The high pressure receptors respond to changes i n a r t e r i a l pressure. They are the a r t e r i a l baroreceptors located i n the arch of the aorta and i n the c a r o t i d sinus. In 1962 Share and Levy provided evidence of the existence of receptors mediating AVP secretion located i n the ca r o t i d sinus by showing that occlusion of both common c a r o t i d a r t e r i e s i n vagotomized anesthetized dogs resulted i n an increase i n plasma AVP l e v e l s . When the c a r o t i d sinuses were denervated occlusion of the common ca r o t i d a r t e r i e s did not produce an increase i n plasma AVP l e v e l s . In rabbits b i l a t e r a l section of the a o r t i c depressor nerves resulted i n elevated plasma AVP le v e l s (Bond et a l . 1972). The receptors i n the arch of the aorta have been more d i f f i c u l t to study than the ca r o t i d sinus receptors. Their l o c a t i o n i n the arch of the aorta means they cannot be perfused e a s i l y . In addition, i n a l l species but the rabbit, the a o r t i c depressor nerve runs i n the vagus such that the a o r t i c receptors are d i f f i c u l t to s e l e c t i v e l y denervate. However, i t i s assumed that these receptors respond s i m i l a r i l y to the ca r o t i d sinus receptors. Work on anesthetized rabbits allows s e l e c t i v e denervation of the a o r t i c arch receptors and demonstrates that the two sets of a r t e r i a l receptors show considerable redundancy i n t h e i r regulation of AVP. Unless the a o r t i c arch receptors are denervated, changes i n c a r o t i d sinus perfusion pressure w i l l have no e f f e c t on AVP l e v e l s (Courneya et a l . 1988). A r t e r i a l receptors also appear to respond to changes i n pulse pressure. In the anesthetized dog, a reduction i n c a r o t i d sinus pulse pressure resulted i n decreased i n h i b i t i o n of AVP release and elevated plasma AVP l e v e l s (Share and Levy, 1966) . In addition to low and high pressure receptors, chemoreceptors, v e n t r i c u l a r receptors and receptors i n the pulmonary vascular system have been implicated i n the control of AVP release. Under conditions of severe hemorrhage and during c a r o t i d occlusion, a c t i v a t i o n of the c a r o t i d chemoreceptors may stimulate AVP release (Share, 1988). Share and Levy (1966) found that perfusion of an i s o l a t e d c a r o t i d sinus with deoxygenated blood i n vagotomized, open-chest, a r t i f i c i a l l y v e n t i l a t e d dogs resulted i n an increase i n plasma AVP l e v e l s . They concluded that the release of AVP was due to the stimulation of the c a r o t i d chemoreceptors. The contribution of l e f t v e n t r i c u l a r receptors i s controversial (Thames et a l . , 1980; Wang et a l . , 1988; Ledsome, 1985a). Despite reference by a number of investigators to cardiopulmonary receptors (Thames and Schmid, 1979; 1981), there i s no evidence that receptors i n the pulmonary system a c t u a l l y contribute to the regulation of AVP secretion (Schultz et a l . , 1982; Ledsome, 1985a). 1. Relative Contribution of A r t e r i a l and A t r i a l Receptors While i t i s accepted that high and low pressure receptors contribute to the regulation of AVP i n response to changes i n blood pressure and volume, the r e l a t i v e contribution of these two sets of receptors to the regulation of AVP secretion remains c o n t r o v e r s i a l . There appear to be differences between species i n the r e l a t i v e importance of the two sets of receptors (Share, 1988). a. Studies i n Dogs I t i s generally accepted that i n the dog a t r i a l receptors play a dominant r o l e i n the stimulation of AVP release following a small to moderate reduction i n blood volume, while a r t e r i a l receptors only contribute following a more severe hemorrhage when a r t e r i a l pressure begins to f a l l (Share 1988). This i s consistent with a number of studies i n which i t was found that a nonhypotensive reduction i n blood volume (10 - 15% of blood volume) stimulated AVP release while a greater reduction i n blood volume (30% of blood volume) was required to cause a decrease i n a r t e r i a l pressure and thus to stimulate a r t e r i a l receptors (Share, 1968; Szczepanska-Sadowska, 1972). I t has been suggested that i n dogs a decrease of 3% of blood volume i s enough to elevate plasma AVP l e v e l s (Claybaugh and Share, 1973). The predominance of a t r i a l receptors i s supported by Wang et a l . (1983) i n t h e i r study of the AVP response to hemorrhage i n conscious dogs. A small, non-hypotensive, reduction i n blood volume (10 ml/kg) caused a s i g n i f i c a n t increase i n plasma AVP i n in t a c t but not cardiac denervated dogs. A larger reduction i n blood volume (20 - 30 ml/kg), which resulted i n a f a l l i n mean a r t e r i a l pressure, caused a s i g n i f i c a n t increase i n plasma AVP i n i n t a c t dogs and a much smaller response (only 10 - 15% of t h i s réponse) i n cardiac denervated dogs. Likewise, Share (1968) found that b i l a t e r a l vagotomy, which denervated the a t r i a l receptors, reduced the hemorrhage-induced increase i n AVP i n anesthetized dogs. Wang et a l . (1983) suggest that while a r t e r i a l baroreceptors may contribute to the response to hemorrhage, a t r i a l volume receptors play a dominant ro l e i n the release of AVP i n response to continuous hemorrhage i n the conscious dog. The predominance of a t r i a l receptors i n the regulation of AVP during continuous hemorrhage i n dogs i s not un i v e r s a l l y accepted (Ledsome, 1985a; Shen et a l . , 1991). An invest i g a t i o n of plasma AVP le v e l s during continuous hemorrhage i n anesthetized dogs found that the greatest changes i n plasma AVP l e v e l s , as blood volume was changed, were associated with a small range of a t r i a l pressures and small changes i n a t r i a l receptor a c t i v i t y (Ledsome et a l . , 1985b). These r e s u l t s suggested that decreased input from the a t r i a l receptors may not be the major cause of elevated l e v e l s of AVP seen i n hypovolemia. Ledsome et a l . also found a high degree of c o r r e l a t i o n between the mean a r t e r i a l pressure and the natural log of plasma AVP l e v e l s . These findings do not support the view of a t r i a l receptors as predominant i n the regulation of AVP release i n response to hypovolemia. A more recent study by Shen et a l . (1991) supports the view that a t r i a l receptors are not the major regulators of AVP release during continuous hemorrhage i n conscious dogs. Shen et a l . looked at plasma AVP l e v e l s i n response to hemorrhage which reduced mean a r t e r i a l pressure by the same amount (20 mmHg) over the same time period. Intact and cardiac denervated animals had s i m i l a r increases i n AVP i n response to hemorrhage while s i n o a o r t i c denervated animals had depressed AVP release i n response to hemorrhage. It has been suggested that the a t r i a l receptors are involved i n the release of AVP during the non-hypotensive period of hemorrhage (Claybaugh and Share, 1973), but i n t h i s study the increase i n plasma AVP l e v e l s during the f i r s t 10 ml/kg of blood loss was s l i g h t l y greater i n the cardiac denervated dogs than i n the i n t a c t dogs. Shen states that, contrary to the generally accepted view, a t r i a l receptors are not the major regulators of AVP release during hemorrhage i n conscious dogs. Rather, a r t e r i a l receptors are the major regulators of AVP release i n response to hemorrhage with a t r i a l receptors making a minor contribution. Shen also suggests that the r o l e of a t r i a l receptors may be d i f f e r e n t i n response to an increase i n a t r i a l pressure and volume than to a decrease i n a t r i a l pressure and volume. Ledsome (1985a) supports t h i s notion suggesting that " a t r i a l receptors may have more importance i n responses to increases i n blood volume than decreases i n blood volume". Studies by Thames and Schmid (1979, 1981) looked at the i n t e r a c t i o n between 'cardiopulmonary receptors' with afférents i n the vagus and a r t e r i a l receptors i n the control of AVP release. The term 'cardiopulmonary receptors' i s used by these investigators to r e f e r to receptors i n the a t r i a and pulmonary vascular bed with afférents i n the vagus nerve. However, the s i g n i f i c a n c e of receptors i n the pulmonary bed involved i n the regulation of AVP i s controversial (Ledsome, 1985a). In anesthetized and conscious dogs, 'cardiopulmonary receptors' with afférents i n the vagus exert t o n i c i n h i b i t i o n of the release of AVP (Thames and Schmid, 1979; Bishop et a l . , 1984). When t h i s influence was removed by vagal cold block or vagotomy, there was an increase i n AVP. Unloading of a r t e r i a l baroreceptors alone by s i n o a o r t i c denervation f a i l e d to release AVP. Unloading of both 'cardiopulmonary' and a r t e r i a l receptors resulted i n higher l e v e l s of AVP than vagotomy or vagal cold block alone. In further studies i n anesthetized dogs, Thames and Schmid (1981) found that when carotid sinus pressure was held constant at 50 mmHg, vagal cold block, which removed tonic vagal i n h i b i t o r y input, resulted i n large increases i n plasma AVP. Raising c a r o t i d sinus pressure to 135 mmHg during vagal cold block resulted i n an increase i n plasma AVP l e v e l s , while r a i s i n g c a r o t i d sinus pressure to 200 mmHg during vagal cold block resulted i n a decrease i n plasma AVP. These r e s u l t s suggest a dominant r o l e f o r c a r o t i d baroreceptors over 'cardiopulmonary receptors' i n the control of AVP with s u f f i c i e n t loading (200 mmHg) of ca r o t i d sinus receptors (Thames and Schmid, 1981). In summary, evidence i n conscious and anesthetized dogs supports a predominant r o l e for a t r i a l receptors i n the regulation of AVP i n response to small changes i n blood volume (10%). A r t e r i a l receptors appear to contribute to AVP regulation following larger changes i n blood volume (20 - 30%). A t r i a l receptors with afférents i n the vagus exert a tonic i n h i b i t o r y influence on the a r t e r i a l baroreceptors. b. Studies i n Rabbits The r e l a t i v e contribution of the two sets of receptors i n the regulation of plasma AVP i n anesthetized rabbits has also been investigated. In rabbits, a r t e r i a l baroreceptors appear to play a dominant r o l e i n the regulation of AVP. Plasma AVP has been measured i n response to stepwise changes i n c a r o t i d sinus pressure i n the presence and absence of input from either the a o r t i c arch baroreceptors with afférents i n the a o r t i c depressor nerve or a t r i a l receptors with afférents i n the vagus nerve (Courneya et a l . , 1988). Plasma AVP l e v e l s increased as c a r o t i d sinus pressures decreased only once the a o r t i c depressor nerves had been sectioned. Subsequent b i l a t e r a l vagotomy did not a l t e r t h i s response. These r e s u l t s i l l u s t r a t e the redundant control of AVP secretion by the two sets of baroreceptors and suggest that receptors with vagal afférents have l i t t l e t onic influence on the baroreceptor control of AVP release. In subsequent studies, Courneya et a l . (1989a) measured plasma AVP l e v e l s i n response to blood volume changes (±10%, ±20%) before and a f t e r b i l a t e r a l vagotomy. Carotid sinus pressure was maintained at 100 mmHg. The hormone response was measured i n anesthetized rabbits with a o r t i c depressor nerves eith e r i n t a c t or sectioned. Plasma AVP l e v e l s were increased i n response to hemorrhage i n animals with i n t a c t a o r t i c depressor nerves but not i n animals with a o r t i c depressor nerve sectioned. Vagotomy did not have a s i g n i f i c a n t e f f e c t on t h i s response. These r e s u l t s indicate that the AVP response to hemorrhage was due to withdrawal of afferent input from a o r t i c baroreceptors not a t r i a l receptors. In rabbits with i n t a c t a o r t i c depressor and vagus nerves, volume expansion did not a l t e r plasma AVP l e v e l s . Following vagotomy however, volume expansion resulted i n a s i g n i f i c a n t reduction i n plasma AVP l e v e l s . In animals with a o r t i c depressor nerve sectioned, volume expansion d i d not a l t e r plasma AVP l e v e l s either before or a f t e r vagotomy confirming the conclusion that a r t e r i a l baroreceptors and not a t r i a l receptors mediate the changes i n plasma AVP i n response to changes i n blood volume i n t h i s preparation (Courneya et a l . , 1989a). In a further study by t h i s group the influence of c a r o t i d sinus pressure on changes i n plasma AVP i n response to changes i n blood volume (±10% and ±20% of blood volume) in a o r t i c baroreceptor denervated anesthetized rabbits was investigated (Courneya et a l . , 1989b). In the hemorrhage experiments, reduction of blood volume by 20% increased the response of plasma AVP to changes i n c a r o t i d sinus pressure, although there did not appear to be a s i g n i f i c a n t i n t e r a c t i o n between the two s t i m u l i . This i s consistent with the hypothesis that large decreases i n blood volume are needed to stimulate s i g n i f i c a n t release of AVP through cardiac receptors acting alone. Volume expansion, however, did not s i g n i f i c a n t l y a l t e r plasma AVP l e v e l s i n response to high or low c a r o t i d sinus pressures. After volume expansion i n f a c t , plasma AVP was unaffected by changes i n c a r o t i d sinus pressure. I t i s possible that there was already maximum i n h i b i t i o n of AVP release. These experiments demonstrate the r o l e of blood volume i n the regulation of AVP i n the anesthetized rabbit. Changes i n blood volume appear to contribute to the baroreceptor stimulation of AVP release. While Courneya et a l . (1988) found a r t e r i a l baroreceptors to be predominant i n the regulation of AVP i n anesthetized rabbits. Quail et a l . (1987) reached d i f f e r e n t conclusions as to the r o l e of a r t e r i a l and a t r i a l baroreceptors on the release of AVP i n conscious r a b b i t s . Their studies involved hemorrhage (35% of blood volume) i n conscious rabbits. They studied response to hemorrhage i n four groups: a r t e r i a l and a t r i a l receptors i n t a c t ; a t r i a l receptors blocked; a r t e r i a l receptors blocked; and both cardiac and a r t e r i a l receptors blocked. They concluded that the stimulus for AVP release comes e n t i r e l y from a t r i a l baroreceptors. In t h e i r studies they found no increase i n plasma AVP i n response to hemorrhage following cardiac denervation. I t has been suggested by Courneya et a l , (1988) that the hemorrhage was terminated too early and not allowed to proceed to a point at which there may have been a r t e r i a l receptor-mediated release of AVP. In summary, the evidence from studies i n rabbits suggests that i n t h i s species a r t e r i a l receptors are predominant i n the regulation of AVP. Receptors with vagal afférents have l i t t l e t o n i c influence on the a r t e r i a l baroreceptor control of AVP release. c. Studies i n Primates The r e l a t i v e r o l e s of a t r i a l and a r t e r i a l receptors i n the control of AVP i n humans and nonhuman primates i s c o n t r o v e r s i a l . In monkeys, however, a t r i a l or low pressure receptors do not appear to be the predominant regulators of AVP release (Share, 1988). The r e l a t i o n s h i p between a r t e r i a l pressure and plasma AVP l e v e l s and l e f t v e n t r i c u l a r end d i a s t o l i c pressure (LVEDP) and plasma AVP l e v e l s was studied i n anesthetized monkeys. Plasma AVP l e v e l s were measured following two hemorrhages of 10% of blood volume (Gilmore et a l . , 1982). There was a s i g n i f i c a n t inverse r e l a t i o n s h i p between a r t e r i a l pressure and plasma AVP but not between LVEDP and plasma AVP l e v e l s , i n d i c a t i n g that i n the primate high pressure receptors are predominant i n the blood pressure-volume control of AVP release. The e f f e c t of non-hypertensive volume expansion on AVP l e v e l s i n anesthetized monkeys has also been investigated (Gilmore et a l . , 1980). Two increases of 15% of blood volume f a i l e d to influence plasma AVP l e v e l s despite a s i g n i f i c a n t increase i n l e f t v e n t r i c u l a r end d i a s t o l i c pressure suggesting that i n the primate, low pressure receptors do not play a s i g n i f i c a n t r o l e i n regulating plasma AVP l e v e l s i n response to changes i n blood volume. These r e s u l t s are confirmed i n studies using conscious monkeys. Plasma AVP lev e l s were measured i n response to hemorrhage of 10%, 15% and 20% of blood volume i n conscious monkeys (Arnauld et a l . , 1977). Plasma AVP l e v e l s only increased once there was a decrease i n a r t e r i a l pressure. Given that a change i n a r t e r i a l pressure i s required f o r a change i n plasma AVP release, high pressure or a r t e r i a l receptors appear to have the dominant r o l e i n the blood pressure-volume regulation of AVP. Studies i n humans y i e l d discrepant r e s u l t s as to the r e l a t i v e contribution of the two sets of receptors. The e f f e c t of moderate (non-hypotensive) isoosmotic loading of a t r i a l and a r t e r i a l baroreceptors on plasma AVP i n humans using volume expansion, lower body p o s i t i v e pressure and head-down t i l t has been investigated (Goldsmith et a l . , 1984) . These procedures had no e f f e c t on plasma AVP suggesting that i n humans, plasma AVP i s unresponsive to moderate changes i n tone of eithe r cardiac or a r t e r i a l baroreceptor r e f l e x and that the s e n s i t i v i t y of the blood volume-pressure mediated stimulation of AVP release i s much lower than i t i s i n the animal models studied. Other investigators have found that a t r i a l or low pressure receptors have l i t t l e or no influence on the regulation of AVP release during isoosmotic changes i n blood volume i n humans, but that a r t e r i a l or high pressure receptors are the predominant mediators of AVP release (Norsk, 1989). A change i n pulse pressure appears to be an important determinant of plasma AVP l e v e l s i n humans (Norsk, 1989). To summarize these findings, AVP regulation i n humans and nonhuman primates i s less s e n s i t i v e to changes i n blood volume and pressure than i t i s i n dogs. Low pressure or a t r i a l receptors appear to have a minimal r o l e i n the control of plasma AVP l e v e l s unless hypotension i s produced. These r e s u l t s suggest an important r o l e for the high pressure or a r t e r i a l receptors i n AVP regulation (Share, 1988; Menninger, 1985). E. Influence of Blood Volume on the Osmoregulation of AVP Studies i n the human, r a t and dog indicate that osmotic and hemodynamic factors do not act independently of each other i n t h e i r control of AVP secretion. Changes i n blood volume influence the r e l a t i o n s h i p between plasma osmolality and plasma AVP (Robertson and Athar, 1975; Dunn et a l . , 1973; Wang et a l , 1984; Quillen and Cowley, 1983). The r e l a t i o n s h i p between plasma osmolality and plasma AVP at low, normal and high l e v e l s of l e f t a t r i a l pressure (LAP) produced by either hemorrhage or autoinfusion of whole blood i n conscious dogs was studied (Quillen and Cowley, 1983). A r t e r i a l pressure was maintained at a constant l e v e l suggesting that changes i n blood volume were detected by a t r i a l and not a r t e r i a l receptors. Hypervolemia and hypovolemia changed both the osmotic threshold f o r AVP release and the s e n s i t i v i t y of the system. This was indicated by a change i n the slope and x-intercept of the regression l i n e which rel a t e s plasma osmolality and AVP. Hypervolemia s h i f t e d the regression l i n e to the r i g h t with a decreased slope (reduced s e n s i t i v i t y ) and an increase i n threshold. Hypovolemia s h i f t e d the regression l i n e to the l e f t with an increased slope (increased s e n s i t i v i t y ) and a decrease i n threshold (figure 1). While previous studies have shown that the isoosmotic changes i n blood volume required to influence plasma AVP l e v e l s are r e l a t i v e l y high, i n t h i s study, the changes i n blood volume required to influence plasma AVP l e v e l s at high plasma osmolality were r e l a t i v e l y small. In f a c t , the l e v e l s of LAP used i n t h i s study represent the l e v e l s that would occur during normal d a i l y a c t i v i t y . These r e s u l t s indicate that volume receptors may play an important r o l e i n the d a i l y control of f l u i d and e l e c t r o l y t e balance i n dogs (Quillen and Cowley, 1983) . In a s i m i l a r study the s p e c i f i c influence of a t r i a l receptors on the osmoregulation of AVP was investigated (Wang et a l . , 1983). The e f f e c t of blood volume changes, r e s u l t i n g from infusion of s a l i n e or of dehydration, on the r e l a t i o n s h i p between plasma AVP and plasma osmolality i n conscious cardiac denervated and sham-operated co n t r o l dogs was compared. Hypovolemia resulted i n an increase i n the s e n s i t i v i t y of the system i n the control dogs but had no e f f e c t i n the cardiac-denervated dogs suggesting that the influence of volume on osmoregulation of AVP i s mediated by a t r i a l receptors. The threshold of the r e l a t i o n s h i p was not affected (Wang et a l . , 1984). Relationship between plasma arginine vasopressin (PAVP) and plasma osmolality (POSM) at d i f f e r e n t l e v e l s of l e f t a t r i a l pressure (LAP) i n conscious dogs. Hypovolemia (Hypo) LAP = -0.9 cm H 2 O ; Normovolemia (Normo) LAP = 1.8 cm H 2 O ; Hypervolemia (Hyper) LAP = 7 . 7 cm H 2 O . Taken from Q u i l l e n and Cowley, 1983. In conscious r a t s , the osmoregulation of AVP was influenced by moderate changes i n blood volume (Dunn et a l . , 1973). Modest hypovolemia reduced the osmotic threshold for AVP release and increased the s e n s i t i v i t y of the osmolality-AVP r e l a t i o n s h i p . Human studies have yielded s i m i l a r r e s u l t s . Lower body negative pressure of -40 mmHg which s i g n i f i c a n t l y lowered central venous pressure but not mean a r t e r i a l pressure, resulted i n elevated plasma AVP l e v e l s i n subjects with high plasma osmolality l e v e l s (>294 mosm/kg) but not i n subjects with plasma osmolality l e v e l s below 294 mosm/kg (Leimbach et a l . , 1984). The authors concluded that nonhypotensive unloading of a t r i a l and a r t e r i a l receptors can influence plasma AVP l e v e l s when plasma osmolality i s elevated. The e f f e c t of blood volume on the osmoregulation of AVP was studied using f l u i d deprivation, orthostasis, and hypertonic s a l i n e infusion to change plasma osmolality and/or blood volume (Robertson and Athar, 1975). Changes i n blood volume influenced the osmotic threshold f o r AVP release but not the s e n s i t i v i t y of the system (Robertson et a l . , 1976). Other investigators do not f i n d s i g n i f i c a n t baroreceptor e f f e c t s on the osmoregulation of AVP i n humans (Goldsmith et a l . , 1985; 1987). These studies suggest that changes i n blood volume which would not independently a f f e c t plasma AVP l e v e l s influence the secretion of AVP through an i n t e r a c t i o n with the osmotic control of AVP. Receptors s e n s i t i v e to changes i n blood volume may play an important r o l e i n the d a i l y c ontrol of f l u i d and e l e c t r o l y t e balance through t h e i r i n t e r a c t i o n with the osmoregulatory system (Quillen and Cowley, 1983). How the osmoregulatory and baroregulatory systems in t e r a c t i n the control of AVP secretion from the neurohypophysis i s not known. I t i s possible that neurons i n the SON and PVN receive input from both the baroregulatory and osmoregulatory systems. The inputs from the two receptor systems converge and are integrated at the l e v e l of the neurohypophysis (Robertson et a l . , 1977; Robertson and Ganguly, 1986). While the e f f e c t s of hypovolemia on the osmoregulation of AVP have been studied, i t i s not cl e a r whether the e f f e c t s are mediated by the high or low pressure receptors or both. The influence of changes i n a r t e r i a l pressure alone on the osmotic stimulation of AVP has not yet been investigated. I I . RATIONALE The release of AVP i s regulated by plasma osmolality and by blood volume and pressure (Share, 1988). Osmotic and hemodynamic input do not act independently i n t h e i r regulation of AVP (Robertson, 1987; Baylis, 1987). In studies i n r a t , dog and human the e f f e c t of changes i n blood volume on the r e l a t i o n s h i p between osmolality and AVP has been examined (Robertson and Athar, 1975; Dunn et a l . , 1973; Wang et a l , 1984; Quillen and Cowley, 1983). These studies show that moderate blood volume changes influence the plasma osmolality-AVP r e l a t i o n s h i p . This influence i s expressed i n terms of a change i n the s e n s i t i v i t y and/or threshold of the osmotic control of AVP (Robertson, 1987). I t i s not known whether the e f f e c t s of the changes i n blood volume are mediated by low pressure receptors or a r t e r i a l receptors or both. This study was designed to investigate the e f f e c t of changes i n ca r o t i d sinus pressure on the r e l a t i o n s h i p between plasma osmolality and AVP i n the anesthetized rabbit. More s p e c i f i c a l l y , t h i s study w i l l investigate the e f f e c t of three l e v e l s of car o t i d sinus pressure (40 mmHg, 100 mmHg and 140 mmHg) on plasma AVP at low, medium and high l e v e l s of plasma osmolality and how these changes a f f e c t the s e n s i t i v i t y and threshold of the r e l a t i o n s h i p between plasma osmolality and plasma AVP. The n u l l hypothesis i s that changing c a r o t i d sinus pressure w i l l have no e f f e c t on the r e l a t i o n s h i p between plasma osmolality and plasma AVP. The influence of changes i n a r t e r i a l pressure alone on the osmoregulation of AVP has not yet been described. In t h i s preparation, section of the vagus nerves and a o r t i c depressor nerves w i l l eliminate input from a t r i a l receptors and a o r t i c baroreceptors respectively. m . METHODS Research was ca r r i e d out on 14 male New Zealand white rabbits (3.3 ± 0.1 kg). Animals were anesthetized with a mixture of alpha chloralose (100 mg/kg, Sigma Chemicals, St Louis, MO, USA) and urethane (1 g/kg, BDH Inc., Poole, England) injected into an ear vein. A cannula was inserted into the trachea and the animals were a r t i f i c i a l l y v e n t i l a t e d with room a i r supplemented with 100% 02-A r t e r i a l pH, PCO2 and PO2 were measured using a blood gas analyzer (Corning Glass Works, Medfield, MA, USA, Model 165/2). To ensure that a r t e r i a l PCO2 was maintained between 27 and 33 mmHg, and pH within the range of 7.35 and 7.55, the respiratory rate was adjusted and aliquots of IM NaHC03 were administered intravenously. Cannulae (PE 190) were placed i n the femoral artery and in the r i g h t atrium v i a the l e f t external jugular vein and were connected to pressure transducers (Statham Instruments Co., Puerto Rico, Model P23Db). The r i g h t a t r i a l cannula was marked 5 cm from the t i p to indicate the length required to ensure that i t was co r r e c t l y positioned i n the r i g h t atrium. Mean a r t e r i a l pressure (MAP) and r i g h t a t r i a l pressure (RAP) were recorded on a thermal array recorder (Astro-Med, Inc., West Warwick, RI, USA, Model MT 95000). Leads were attached to the r i g h t leg and chest to monitor the electrocardiogram from which the heart rate was obtained. A cannula (PE 190) was placed i n the r i g h t external jugular vein for the infusion of s a l i n e by a constant flow infusion pump (Cole-Parmer, Chicago, IL, USA, Masterflex Model 173-20-25). A cannula (PE 240) was placed i n the bladder to drain the urine. Body temperature was maintained between 37 and 39°C by a heated s u r g i c a l table. Blood was removed from an anesthetized donor ra b b i t by cardiac puncture. This blood was heparinized, f i l t e r e d and kept at 3 7 O C . Donor blood was used for blood sample replacement. Rabbits do not have antigens on t h e i r red blood c e l l s which allows the exchange of blood between rabbi t s . A. Carotid Sinus Perfusion The c a r o t i d sinuses were i s o l a t e d b i l a t e r a l l y . The in t e r n a l c a r o t i d artery and three major branches of the external c a r o t i d artery were t i e d o f f on both sides. Smaller branches of the external c a r o t i d artery were l e f t i n t a c t to allow flow through the ca r o t i d sinus at a l l times. A cannula (PE 205) was inserted i n the l e f t common c a r o t i d artery proximally. Cannulae (PE 190) were inserted i n both common ca r o t i d a r t e r i e s just below the c a r o t i d sinus. Blood was withdrawn from the l e f t common ca r o t i d artery and passed through a r o l l e r pump (Watson-Marlow, Marlow, UK, Model MHRE), a heated damping chamber, a f i l t e r and f i n a l l y into the d i s t a l end of the r i g h t and l e f t c a r o t i d a r t e r i e s , thus perfusing the car o t i d sinuses. A cannula (PE 50) was placed i n one branch of the l e f t i n t e r n a l maxillary artery and connected to a pressure transducer. The sig n a l was fed into a servo-control unit which adjusted the speed of the r o l l e r pump to ensure that a fixe d pressure was maintained i n the sinus to within + 1 mmHg (figure 2). Despite the damping chamber, the r o l l e r pump imposed on the mean pressure a pulse of 20 mmHg. Before carot i d sinus perfusion was started, heparin (lithium heparin, Sigma Chemicals, St Louis, MO, USA) was administered (1000 U/kg) followed by 1000 U every 30 min thereafter. The a o r t i c depressor nerves and vagus nerves were sectioned b i l a t e r a l l y by cut t i n g the vagus nerve proximal to i t s junction with the superior laryngeal nerve. This eliminated input from a o r t i c arch baroreceptors and cardiac receptors. The ca r o t i d sinus baroreflex was tested by increasing to 140 mmHg and decreasing to 40 mmHg car o t i d sinus pressure and measuring the r e f l e x change i n MAP. Three hours a f t e r the i n i t i a l dose, a supplemental dose of anesthetic (10% of i n i t i a l ) was administered. This was repeated every hour thereafter. An infusion of donor blood equivalent to 10% of the blood volume (6 ml/kg) was administered to replace that l o s t during surgery. When Ficmre 2 surgery was complete, there was a 30 - 60 min e q u i l i b r a t i o n period before the experimental protocol began. Carotid sinus pressure was maintained at 100 mmHg during t h i s period. B. Changing Plasma Osmolality NaCl (0.45%) was infused into the r i g h t external jugular vein at a rate of 1 ml/min i n order to maintain plasma osmolality at basal l e v e l . To lower plasma osmolality, 0.3% NaCl was infused at a rate of 3 ml/min f o r 30-45 min. This lowered osmolality to 288 + 1 mosm/kg. I t should be noted that 288 mosm/kg corresponds to a normal l e v e l of plasma osmolality for the rabbit. However, since i t i s the lowest l e v e l used i n t h i s study, i t i s described as 'low'. Plasma osmolality was maintained at t h i s l e v e l by infusion of 0.45% NaCl at 1 ml/min. Osmolality was held at t h i s l e v e l f or 15-30 min before c a r o t i d sinus pressure was changed and samples were taken. To r a i s e plasma osmolality, 5% NaCl was infused at a rate of 0.5 ml/min f o r 30 min. Plasma osmolality rose to 309 ± 1 mosm/kg and in f u s i o n was stopped. Once the infusion pump was turned o f f , there was a 5 min e q u i l i b r a t i o n period to ensure that osmolality was uniform throughout the c i r c u l a t i o n before c a r o t i d sinus pressure was changed and blood samples were taken. The infusion of 5% NaCl at 0.5 ml/min was repeated to r a i s e plasma osmolality to 323 + 1 mosm/kg. At the end of the e q u i l i b r a t i o n period, plasma osmolality had reached 2 8 8 + 1 mosm/kg. With c a r o t i d sinus pressure at 100 mmHg, cardiovascular variables were measured and a blood sample was taken. Samples were immediately replaced with an equivalent volume of donor blood. CSP was then decreased to 40 mmHg. Seven minutes l a t e r cardiovascular variables were measured and a blood sample was taken. This sequence was repeated with CSP at 140 mmHg and then at 100 mmHg. Plasma osmolality was raised to 309 ± 1 mosm/kg and the sequence of car o t i d sinus pressure changes was performed. This was repeated a t h i r d time at plasma osmolality 323 + 1 mosm/kg (figure 3). In 7 control animals the same experimental protocol was car r i e d out with c a r o t i d sinus pressure maintained at 100 mmHg. D. Hormone Analysis Samples of blood (5 ml) were drawn into c h i l l e d syringes. 3.5 ml were transferred into EDTA tubes. (Vacutainer, Becton Dickinson, Canada, Inc., Mississauga, Ont.) for hormone analysis. 1.5 ml were transferred into heparinized p l a s t i c t e s t tubes for osmolality measurements. Experimental Protocol. CSP, Carotid Sinus Pressure. C S P ( m m H g ) S A M P L E T I M E 100 10 min I N F U S I O N 0.3% N a C l O S M O L A L I T Y (mosm/Kg) 40 140100 " I " SI S2 S3 S4 i i i i h i l l . 40 140100 I 1 u 85 S6 S7 S8 J L h i l l 0.45% 5% STOP 5% 288 309 40 140 100 t t t S9 S10S11 S12 I I I I . I . I t STOP 323 Samples were centrifuged at 1500 x g for 15 min i n a r e f r i g e r a t e d centrifuge (Beckman Instruments Inc., Palo Alto, CA, USA, Model TJ-6). Plasma osmolality was measured by freezing point depression (Advanced Instruments, Needham Heights, MA, USA, Advanced Digimatic Osmometer, Model 3D2). Plasma was stored at -20°C u n t i l extraction f o r radioimmuoassay (RIA). EDTA plasma samples were thawed and a c i d i f i e d with 100 u l of IN HCL per m i l l i l i t r e of plasma (Fisher S c i e n t i f i c Ltd, Ottawa, Ont.). AVP was extracted from plasma by adsorption to s i l i c a gel columns (EM Science, Cherry H i l l , NJ, USA, Adsorbex RP-18 extraction columns). Extraction columns were washed with 5 ml 100% methanol (Fisher S c i e n t i f i c Ltd, Ottawa, Ont.), followed by 5 ml d i s t i l l e d H 2 O . A measured volume of thawed plasma was applied to the column and allowed to pass through. The columns were then rinsed with 5 ml d i s t i l l e d H 2 O . The hormone was eluted with 3.5 ml 90% methanol-0.5% t r i f l u o r o a c e t i c acid (TFA, Sigma Chemicals, St Louis, MO, USA). Extracts were dried with N2 and stored at -20°C u n t i l radioimmunoassay (RIA). A high s e n s i t i v i t y RIA for AVP was developed using synthetic AVP (#V-0377, Sigma Chemicals, St Louis, MO) as antigen and standard. Antiserum L004 was raised i n a New Zealand white rabbit i n response to immunization with an AVP-bovine thyroglobulin (bovine thyroglobulin, Sigma Chemicals. St Louis, MO, USA) conjugate. The AVP-bovine thyroglobulin conjugate was produced using coupling agent ethyl carbodiimide (Sigma Chemicals, St Louis, MO, USA) according to methods developed by Skowsky and Fisher (1972). The s p e c i f i c i t y of t h i s antiserum was assessed by cross-r e a c t i v i t y which i s expressed as the mass r a t i o of AVP to the t e s t substance to produce 50% i n h i b i t i o n of iodinated AVP:antibody. C r o s s - r e a c t i v i t y with a number of t e s t substances were as follows: l y s i n e vasopressin, 49.6%; oxytocin, 1.28%; vasotocin, 0.003%; and angiotensin I I , <0.001%. The antiserum was used i n the assay at a f i n a l d i l u t i o n of 1:70000. The value of 20% depression of maximum binding (upper l i m i t of s e n s i t i v i t y ) was 1.13 ± 0.05 pg/tube (n=33). The value of 50 % of maximum binding was 2.9 + 0.06 pg/tube (n=33). The interassay v a r i a b i l i t y , expressed as % c o e f f i c i e n t of v a r i a t i o n , was 11% for one sample (n=30), 10% for a second sample (n=29), and 12% for a t h i r d sample (n=29). The average (n=30) intra-assay v a r i a b i l i t y , expressed as % c o e f f i c i e n t of v a r i a t i o n , was + 4.34%. AVP iodination was performed using a modification of the method of Greenwood, Hunter and Glover (1963). The following reagents were added to 3 ug (10 ul) AVP: 10 u l 0.5M phosphate buffer (pH 7.4); 5 u l (0.5mCi) Na 125i (Amersham Radiochemicals, Oakville, Ont.); and 5 u l (5 ug) c a r r i e r - f r e e chloramine T (Sigma Chemicals, St. Louis, MO). The reaction proceeded for 30 sec and was terminated with 50 u l (12.5 mg) bovine serum albumin (BSA, ICN Biomedicals, Mississauga, Ont.). This mixture was then applied to a prepared CM-Sephadex C-25 column (Pharmacia, Upsala, Sweden) equi l i b r a t e d with 0.6M acetate buffer (pH 4.8). S p e c i f i c a c t i v i t y of the iodinated peptide was calculated using the self-displacement method of Morris (1976) and was found to be approximately 1600 uCi/ug. Aliquots of the f r a c t i o n s of the eluted AVP-125i peak were stored at -20^0 and used i n RIA f o r up to 6 weeks. For each assay, a standard curve was generated. AVP standards ranging i n concentration from 0.1 to 50 pg/ml were prepared from the AVP stock solution (#V-0377 Sigma Chemicals, St Louis, MO, USA) d i l u t e d with phosphate buffer (O.IM phosphate, 0.1% BSA, pH 7.4). Maximum binding tubes received 400 u l buffer and 50 u l antibody (Ab). Non s p e c i f i c binding tubes received 450 u l buffer. Standard curve tubes received 100 u l AVP standard, 300 u l buffer and 50 u l Ab. Plasma sample s p e c i f i c binding tubes received 100 u l plasma sample, 300 u l buffer and 50 u l Ab. Plasma sample non s p e c i f i c binding tubes received 350 u l buffer and 100 u l plasma sample. A l l RIA procedures were ca r r i e d out i n p l a s t i c t e s t tubes kept i n i c e . Maximum binding, non s p e c i f i c binding, standard curve and plasma sample s p e c i f i c binding tubes were a l l performed i n t r i p l i c a t e . Plasma sample non s p e c i f i c binding tubes were performed i n duplicate. Tubes were mixed and incubated at 4°C for 20 hours. 50 u l AVP-125i (approximately 2000 cpm) was then added to each tube. Tubes were mixed and incubated at 4°C for 40 hours. To separate hormone bound to Ab from free hormone, lOOul (8 mg/ml) bovine gamma globulin (ICN Biomedicals, Mississauga, Ont.) and 600 u l polyethylene g l y c o l (PEG, Carbowax 8000, Fisher S c i e n t i f i c Ltd., Ottawa, Ont.) were added to each tube. Tubes were mixed and centrifuged i n a r e f r i g e r a t e d centrifuge at 1500 x g for 45 min at 4°C. The supernatant (free hormone) was discarded leaving a p r e c i p i t a t e (bound hormone) i n the tube. Each tube was counted f o r 10 min using an automatic gamma counter (LKB-Wallac, Wallac Oy, Finland, RiaGamma 1274). E. STATISTICAL ANALYSIS Systemic variables (mean a r t e r i a l pressure, r i g h t a t r i a l pressure and heart rate) at d i f f e r e n t plasma osmolalities and carotid sinus pressures were analyzed by ANOVA. S t a t i s t i c a l s i g n i f i c a n c e was tested using the post hoc Newman-Keuls t e s t . Values are expressed as the mean ± standard error of the mean (SEM). Plasma immunoreactive AVP at d i f f e r e n t plasma osmolalities and ca r o t i d sinus pressures were analyzed by ANOVA. S t a t i s t i c a l s i g n i f i c a n c e was tested using the Wilcoxon Signed Ranks t e s t , the nonparametric analogue of the paired t - t e s t . AVP le v e l s do not follow a standard d i s t r i b u t i o n necessitating a non-parametric t e s t for si g n i f i c a n c e . A l i n e a r regression of plasma AVP and plasma osmolality at CSP 40 mmHg, 100 mmHg and 140 mmHg was performed using the pooled data. The equations of the l i n e a r regressions were used to p l o t three l i n e s , with associated standard errors, representing the r e l a t i o n s h i p between plasma osmolality and plasma AVP at each of the three c a r o t i d sinus pressures. The sequence of car o t i d sinus pressure changes was as follows: from 100 mmHg to 40 mmHg to 140 mmHg to 100 mmHg. For s i m p l i c i t y , the systemic, plasma osmolality and plasma iAVP values f o r CSP 100 mmHg were presented as an average of the two sets of values at CSP 100 mmHg. IV. RESULTS Following the e q u i l i b r a t i o n period, during which time CSP was maintained at 100 mmHg, baseline values were as follows: plasma osmolality, 288 + 1 mosm/kg; plasma immunoreactive arginine vasopressin (iAVP), 2.4 + 0.6 pg/ml; mean a r t e r i a l pressure, 106 ± 5 mmHg; r i g h t a t r i a l pressure, 3.5 ± 0.6 cmH20; and heart rate, 272 + 9 beats/min. A. Systemic Variables Changing CSP from 100 mmHg to 40 mmHg s i g n i f i c a n t l y increased MAP (p<0.001). Changing CSP from 40 mmHg to 140 mmHg s i g n i f i c a n t l y decreased MAP (p<0.001) (figure 4). Changing CSP from 100 mmHg to 40 mmHg produced a small but not s i g n i f i c a n t increase i n heart rate. Changing CSP from 40 mmHg to 140 mmHg produced a small but not s i g n i f i c a n t decrease i n heart rate (figure 5). Absence of a s i g n i f i c a n t cardiac baroreflex i s due to b i l a t e r a l section of the vagus nerves. The r e f l e x change i n MAP was maintained through the course of the experiment i n d i c a t i n g that the c a r o t i d sinus r e f l e x remained i n t a c t and functional. This response was not affected by changes i n plasma osmolality nor were there s i g n i f i c a n t changes over time. There were no s i g n i f i c a n t changes i n RAP i n response to changes i n CSP, nor did RAP change s i g n i f i c a n t l y during the course of the experiment as a r e s u l t of the in f u s i o n of s a l i n e (figure 6). Mean a r t e r i a l pressure (MAP) at d i f f e r e n t l e v e l s of car o t i d sinus pressure (CSP) at three l e v e l s of plasma osmolality, n=14. * S t a t i s t i c a l l y d i f f e r e n t from CSP = 40 mmHg. + S t a t i s t i c a l l y d i f f e r e n t from CSP = 100 mmHg. 150r s 100 + 50 -I o o o O mm T 1 i l l i i i i o o o O T^H 288 mosm/kg 2 309 mosm/kg 323 mosm/kg o o o O CSP (mmHg) Ficmre 5 350 288 mosm/kg 309 mosm/kg 323 mosm/kg 300 a; a; 250 1 200 150 1 o o o o Ti< T V 1 III i l l Hi Io o o O Tj< o o o Tj< O CSP (mmHg) Right a t r i a l pressure (RAP) at d i f f e r e n t l e v e l s of c a r o t i d sinus pressure (CSP) at three l e v e l s of plasma osmolality, n=14 ^ 288 mosm/kg 309 mosm/kg I 323 mosm/kg C S P (mmHg) B. Plasma Osmolality I n i t i a l l y , plasma osmolality was lowered to 288 + 1 mosm/kg. I t should be noted that 288 mosm/kg represents a 'normal' l e v e l of plasma osmolality but i n t h i s study represents the lowest l e v e l used. Plasma osmolality was then raised to 309 + 1 mosm/kg and f i n a l l y to 323 + l mosm/kg. Plasma osmolality was succes s f u l l y maintained at each l e v e l for the run of CSP changes (see figure 7). C. Plasma immunoreactive arginine vasopressin (iAVP) At 'low' plasma osmolality (288 + 1 mosm/kg), l e v e l s of plasma iAVP were 1.9 + 0.4 pg/ml at CSP 140 mmHg, 2.4 ± 0.6 pg/ml at CSP 100 mmHg, and 2.5 ± 0.7 pg/ml at CSP 40 mmHg. The values of plasma iAVP were not s i g n i f i c a n t l y d i f f e r e n t at the d i f f e r e n t l e v e l s of carotid sinus pressure. At 'medium' plasma osmolality (309 + 1 mosm/kg), plasma iAVP was 3.8 + 0.8 pg/ml at CSP 140 mmHg, 4.6 + 0.8 pg/ml at CSP 100 mmHg and 4.9 + 0.6 pg/ml at CSP 40 mmHg. The value of plasma iAVP at CSP 40 mmHg was s i g n i f i c a n t l y higher than the value of plasma iAVP at CSP 140 mmHg (p<0.05). The value of plasma iAVP at CSP 100 mmHg was not s i g n i f i c a n t l y d i f f e r e n t from the value of plasma iAVP at CSP 140 mmHg or 40 mmHg. At 'high' plasma osmolality (323 + 1 mosm/kg), plasma iAVP was 7.0 + 1.5 pg/ml at CSP 140 mmHg, 8.3 ± 1.2 pg/ml at CSP 100 mmHg and 9.7 + 2.0 pg/ml at CSP 40 mmHg. The value of plasma iAVP at CSP 40 mmHg was s i g n i f i c a n t l y higher than the value of plasma iAVP at CSP 140 mmHg (p<0.05). The value of plasma iAVP at CSP 100 mmHg was not s i g n i f i c a n t l y d i f f e r e n t from the value of plasma iAVP at CSP 140 mmHg or 40 mmHg (figure 8).. Plasma osmolality at d i f f e r e n t l e v e l s of c a r o t i d sinus pressure (CSP), n=14. Plasma Osmolality (mosm/kg) O CD O 140 W 40 1 1 CO o o -r-a 40 ^^^^^^^ Cû O —T— CO o —1 Plasma arginine vasopressin (AVP) at d i f f e r e n t l e v e l s of c a r o t i d sinus pressure (CSP) at three l e v e l s of plasma osmolality, n=14. * S t a t i s t i c a l l y d i f f e r e n t from CSP = 140 mmHg. Plasma AVP (pg/ml) 01 n Ul 3 B 140 100 40 140^^^ 40 "I—I—I—I—I—I—I—I—I—I—I—I—I—I—I H • The r e l a t i o n s h i p between plasma osmolality and plasma iAVP was expressed as a l i n e a r regression at each of the caro t i d sinus pressures (figures 9 through 11). At each CSP, plasma AVP concentration and plasma osmolality show a po s i t i v e l i n e a r c o r r e l a t i o n (p<0.001). At ca r o t i d sinus pressure 40 mmHg, the slope of the l i n e i s 0.19 pg/ml per mosm/kg, at 100 mmHg the slope i s 0.17 pg/ml per mosm/kg and at 140 mmHg the slope i s 0.13 pg/ml per mosm/kg. Figure 12 shows the three regressions (+ standard error of predicted values) plotted on the same graph. The x-intercepts for the three regression l i n e s are as follows: at CSP 40 mmHg, 276 + 3 mosm/kg; at CSP 100 mmHg, 278 + 3 mosm/kg; and at CSP 140 mmHg, 275 + 3 mosm/kg. The x-intercepts for the three regressions are not s i g n i f i c a n t l y d i f f e r e n t . Given that the x-intercepts of these l i n e s are not s i g n i f i c a n t l y d i f f e r e n t and the l e v e l s of iAVP at medium and high plasma osmolality are s i g n i f i c a n t l y d i f f e r e n t at high and low CSP, then i t i s reasonable to state that the slope of the r e l a t i o n s h i p between plasma osmolality and plasma AVP i s altered by changes i n carotid sinus pressure. P l a s m a Osmola l i ty (mosm/kg ) Relationship between plasma arginine vasopressin (AVP) and plasma osmolality at d i f f e r e n t l e v e l s of c a r o t i d sinus pressure (CSP), n=14. Lines generated from l i n e a r regressions with standard errors of the predicted values. D. Control Data In the control animals, the three l e v e l s of plasma osmolality were 283 + 2 mosm/kg, 304 + 1 mosm/kg and 322 ± 1 mosm/kg (figure 13). Plasma osmolality was maintained at a constant l e v e l . There was no change i n MAP, HR or RAP over time (figures 14 through 16). There was no s i g n i f i c a n t difference i n l e v e l s of plasma iAVP i n the four blood samples taken at each l e v e l of plasma osmolality when car o t i d sinus pressure was maintained at 100 mmHg (figure 17). This indicates that the differences i n plasma iAVP seen i n the experimental group are not a function of the time course of the experiment but can be rel a t e d to changes i n ca r o t i d sinus pressure. Plasma iAVP concentration and plasma osmolality show a po s i t i v e l i n e a r c o r r e l a t i o n (p<0.01). The slope of the regression l i n e for the control data i s 0.17 pg/ml per mosm/kg and the x-intercept i s 274 ± 2 mosm/kg. Plasma osmolality with c a r o t i d sinus pressure (CSP) maintained at 100 mmHg, n=7. Plasma Osmolality (mosm/kg) 01 05 O O ro to *j œ o o CO o 03 O O h-» O CO o CO CO o 100 100 100 n ? S 100 100 100 CO o CO o 1 T T T T T T T I Ficmre 14 1 5 0 r 283 m o s m / k g 304 m o s m / k g 322 m o s m / k g 100 50 o o o o o o i I I I i l l I o o o o o o o o o o o o CSP (mmHg) Heart rate with c a r o t i d sinus pressure (CSP) maintained at 100 mmHg at three l e v e l s of plasma osmolality, n=7. Heart Rate D^ Ul o oi o o o 1 1 1 i-100 100 100 100 ^^^^^^^ 100 100 100 3 100^^^^^^^ 0 100 ^^^^^^^ Ficmre 16 283 m o s m / k g 304 m o s m / k g 322 m o s m / k g i i i i O O o O O o o o o o o o o o o o o o CSP (mmHg) Plasma arginine vasopressin (AVP) with c a r o t i d sinus pressure (CSP) maintained at 100 mmHg at three l e v e l s of plasma osmolality, n=7. Plasma AVP (pg/ml) 100 100 100 n Ul 100^^ 05 00 5 100^^^^^ -F— ^ lO 05 00 o T •I V. DISCUSSION A. E f f e c t of CSP on the Osmorecmlation of AVP The purpose of t h i s study was to examine the e f f e c t of car o t i d sinus pressure on the re l a t i o n s h i p between plasma osmolality and plasma AVP i n anesthetized r a b b i t s . The re s u l t s indicate that changes i n ca r o t i d sinus pressure (CSP) do influence the osmotic control of AVP release. At medium and high plasma osmolality, the l e v e l of plasma AVP was s i g n i f i c a n t l y higher at a CSP of 40 mmHg than at a CSP of 140 mmHg. The r e l a t i o n s h i p between plasma osmolality and AVP at each CSP i s expressed as three l i n e a r regressions (figure 12). At low ca r o t i d sinus pressure (40 mmHg) the slope of the l i n e i s 0.19 pg/ml per mosm/kg. At normal CSP (100 mmHg) the slope i s 0.17 pg/ml per mosm/kg. At high CSP (140 mmHg) the slope i s 0.13 pg/ml per mosm/kg. Given that the x-intercepts of these l i n e s are not s i g n i f i c a n t l y d i f f e r e n t and the l e v e l s of iAVP at medium and high plasma osmolality are s i g n i f i c a n t l y d i f f e r e n t at high and low CSP, then i t i s reasonable to state that the slope of the r e l a t i o n s h i p between plasma osmolality and plasma AVP i s alte r e d by changes i n ca r o t i d sinus pressure. Changes i n a r t e r i a l pressure acting alone do influence the AVP/Pog^ re l a t i o n s h i p . The slope of these l i n e s i s a measure of s e n s i t i v i t y (Robertson et a l . , 1976; 1987). Low CSP appears to increase the s e n s i t i v i t y of the osmotic control of AVP whereas high CSP appears to decrease the s e n s i t i v i t y . The x-intercept of these l i n e s i s a measure of the osmotic threshold or the set-point of the system (Robertson et a l . , 1976). A change i n CSP does not appear to a f f e c t the osmotic threshold f o r AVP release. In previous studies, the influence of changes i n blood volume on the osmoregulation of AVP has been studied. However, i t i s not known whether the changes i n blood volume are acting through input to high or low pressure receptors. In f a c t , many of these studies have assumed changes i n blood volume without d i r e c t measurement of changes i n a t r i a l pressure or mean a r t e r i a l pressure (Robertson and Athar, 1975; Dunn et a l . , 1973). The present study provides an opportunity to look s p e c i f i c a l l y at the e f f e c t of input to the a r t e r i a l receptors on the AVP/Posm r e l a t i o n s h i p . How does the influence of CSP on the AVP/Posm re l a t i o n s h i p i n anesthetized rabbits i n the present study compare to the influence of blood volume on t h i s r e l a t i o n s h i p i n conscious dogs (Quillen and Cowley, 1983), humans (Robertson and Athar, 1976) and rats (Dunn et a l . , 1973) i n terms of changes i n s e n s i t i v i t y and/or threshold? Threshold i s consistently affected by changes i n blood volume but s e n s i t i v i t y i s not. In the study by Qui l l e n and Cowley (1983) on conscious dogs, hypo and hypervolemia appeared to have a small e f f e c t on the slope and a greater e f f e c t on the osmotic threshold fo r AVP release. A decrease i n LAP of 2.7 cmH20 from 1.8 cmH20 increased the slope s l i g h t l y and s h i f t e d the threshold from 277 to 270 mosm/kg, while an increase i n LAP of 5.9 cmH20 decreased the slope s l i g h t l y and s h i f t e d the threshold from 277 to 281 mosm/kg. The authors state that the changes i n blood volume are mediated by the low pressure receptors alone, since a r t e r i a l pressure does not change. However, they cannot be certai n that the high pressure receptors are not affected perhaps through a change i n pulse pressure. Robertson and Athar looked at the e f f e c t of blood volume on the osmotic control of AVP i n humans (1975). In t h i s study, plasma osmolality was changed by dehydration or by infusion of hypertonic s a l i n e . These procedures d i d not change plasma osmolality over a wide range making i t d i f f i c u l t to e f f e c t i v e l y describe the AVP response. Changes i n posture and infusion of sa l i n e were used to change blood volume. However, there were no measurements of cen t r a l venous pressure, r i g h t a t r i a l pressure or mean a r t e r i a l pressure to indicate how these procedures were a c t u a l l y a f f e c t i n g volume and pressure receptors. Changes i n blood volume were estimated from changes i n hematocrit. Robertson and Athar found that changes i n blood volume d i d not a l t e r the slope of the r e l a t i o n s h i p between osmolality and AVP but did s h i f t the osmotic threshold for AVP release. An increase i n blood volume raised the osmotic threshold while a decrease i n blood volume lowered the osmotic threshold. Dunn et a l . (1973) found that i n conscious r a t s , hypovolemia increased the s e n s i t i v i t y and lowered the threshold of osmotic regulation of AVP. Hematocrit was used as a measure of blood volume. Once again there were no measurements of r i g h t a t r i a l pressure or mean a r t e r i a l pressure. Rats were s a c r i f i c e d during the course of each procedure to provide enough blood for radioimmumoassay and so could not be used for each of the procedures. Although the present study shows that CSP does influence the r e l a t i o n s h i p between plasma osmolality and plasma AVP, the magnitude of t h i s e f f e c t i s not as great as might be expected. Courneya et a l . (1988) measured plasma AVP l e v e l s i n response to changes i n CSP i n anesthetized rabbits with vagus and a o r t i c depressor nerves sectioned while plasma osmolality was maintained at 300 mosm/kg. They found much larger increases i n plasma iAVP at CSP 40 mmHg than were found at the same CSP i n the current study even at high plasma osmolality. However, the large v a r i a b i l i t y i n the response between animals does not allow the conclusion that these differences are s i g n i f i c a n t . B. Osmotic Control of AVP To our knowledge, there has been no previous des c r i p t i o n of the re l a t i o n s h i p between plasma osmolality and plasma AVP for the rabbit. In the present study the re l a t i o n s h i p between plasma osmolality and AVP was investigated i n the anesthetized rabbit with vagus and a o r t i c depressor nerves sectioned and ca r o t i d sinus pressure maintained close to a normal value. The osmotic control of AVP i n t h i s model i s described by a slope of 0.17 pg/ml per mosm/kg. In the conscious dog, at a normal volume state, the slope of t h i s r e l a t i o n s h i p i s 0.21 pg/ml per mosm/kg (Quillen and Cowley, 1983). The accepted value f o r humans i s 0.38 pg/ml per mosm/kg (Robertson et a l . , 1976). The osmotic threshold for AVP release i n the anesthetized rabbit i s 278 mosm/kg. In the conscious dog the threshold i s 277 mosm/kg (Quillen and Cowley, 1983) and i n humans, 280 mosm/kg (Robertson et a l . , 1976). C. Experimental Procedure Plasma AVP le v e l s ranged from 0 to 15 pg/ml during the course of the experiment. Anesthetized animal preparations have been c r i t i c i z e d for producing elevated AVP l e v e l s . However, i n a recent study, Wehberg et a l . found that there was no difference i n the car o t i d baroreflex c o n t r o l of AVP between conscious and anesthetized dogs (Wehberg et a l . , 1991). The l e v e l s of AVP found i n t h i s study are well within the normal phy s i o l o g i c a l range. The baseline plasma AVP l e v e l i n conscious New Zealand white rabbits i s 2.5 pg/ml at plasma osmolality 287 mosm/kg (Courneya, personal communication). In t h i s study, at c a r o t i d sinus pressure 100 mmHg, plasma osmolality of 288 mosm/kg corresponded to plasma iAVP of 2.4 pg/ml. During the course of the experiment, approximately 200 ml of sal i n e was infused. There was some water excretion (approximately 50 ml) and hematocrit decreased. These observations suggest that blood volume was increased. However, there was no s i g n i f i c a n t change i n RAP. To eliminate input from the a t r i a l receptors, the vagus nerves were sectioned. The osmotic regulation of AVP secretion depends on the absolute l e v e l of plasma osmolality as well as on the rate of change of plasma osmolality (Robertson et a l . , 1976). Robertson suggests that i f the change i n plasma osmolality i s greater than 2% per hour, the AVP response i s exaggerated. In the present study the change i n plasma osmolality was approximately 7% per hour, a value much greater than the recommended 2% per hour. The experimental protocol required t h i s rate of change i n order to cover a wide range of osmolalities and complete the run of CSP changes at each l e v e l of osmolality. The influence of t h i s factor on the r e s u l t s cannot be assessed from these experiments. When the r e l a t i o n s h i p between plasma osmolality and AVP i n t h i s study i s compared to that found for the dog or human, the r e l a t i o n s h i p i s very s i m i l a r and does not appear to be exaggerated. D. Ph y s i o l o g i c a l Relevance Pressure and volume baroreceptors modulate the osmotic control of AVP so that changes i n blood volume and osmolality are regulated more e f f e c t i v e l y than would occur by osmoreceptors alone. In conscious rats, hypovolemia alone only stimulated AVP release when the f a l l i n blood volume was 10%. However, a much smaller decline i n blood volume (2%) d i d influence AVP l e v e l s through the osmotic control (Dunn et a l . , 1973). In the study performed by Q u i l l e n and Cowley (1983), the changes i n LAP during hypo and hypervolemia represent l e v e l s seen during normal d a i l y a c t i v i t y . These changes i n blood volume are smaller than those observed i n studies looking at the independent e f f e c t of blood volume which required changes of 10 - 15% (Szczepanska-Sadowska, 1972; Clark and Rocha E S i l v a , 1967). Small changes i n blood volume which would not have an e f f e c t on AVP release independently w i l l exert an e f f e c t through t h e i r modulation of the osmotic regulation of AVP release. Therefore, blood volume control of AVP release i s not only relevant during severe hemorrhage but appears to have a r o l e i n the d a i l y c ontrol of blood volume and osmolality. S i m i l a r i l y , i n humans (Robertson and Athar, 1975), changes i n blood volume which independently would have no e f f e c t on plasma AVP l e v e l s exert an e f f e c t by regulating the osmotic control of AVP. Q u i l l e n and Cowley (1983) give the examples of hemorrhage, dehydration, ingestion of tap water (hypotonic volume load), and ingestion of a s a l t y meal (hypertonic volume load) as events i n which input from volume receptors modulate osmotic control of AVP secretion. In dehydration, plasma osmolality increases and stimulates AVP secretion. There i s a concomitant decrease i n blood volume which increases the s e n s i t i v i t y of t h i s osmotic control and r e s u l t s i n higher plasma AVP l e v e l s . In the case of a hypertonic volume load, increased plasma osmolality stimulates AVP secretion. However, the increase i n blood volume decreases the s e n s i t i v i t y of t h i s response so that plasma AVP l e v e l s are lower than i f the system was under osmotic control alone. This guards against futher volume loading. I t i s conceivable that small changes i n a r t e r i a l pressure w i l l have the same modulating r o l e i n the rabbit as changes i n blood volume have i n the dog (Quillen and Cowley, 1983) or r a t (Dunn et a l . , 1973). To see the e f f e c t s of small changes i n a r t e r i a l pressure, such as those that would be seen during normal d a i l y a c t i v i t y , i t would be necessary to repeat t h i s study using smaller changes i n CSP. In the present study, the stimulus of a change i n a r t e r i a l pressure was lim i t e d to the carotid sinus baroreceptor. I t i s usually assumed, although not proven, that stimulation of a o r t i c baroreceptors w i l l have s i m i l a r e f f e c t s and add to the e f f e c t s of the car o t i d sinus baroreceptor. Thus when a r t e r i a l pressure changes i n the inta c t animal there may be a change i n vasopressin i n response to much smaller changes i n a r t e r i a l pressure. In rabbits and humans the blood pressure-volume control of AVP secretion appears to be mediated p r i m a r i l y by a r t e r i a l baroreceptors (Courneya et a l . , 1988; Leimbach et a l . , 1984; Norsk, 1989). Therefore, determining the int e r a c t i o n of a r t e r i a l pressure and osmolality i n the regulation of AVP i n the rabbit may have important implications as to how the system functions i n the human. E. C o n c l u d i n g ffrmiments 1. At medium (309 mosm/kg) and high (323 mosm/kg) le v e l s of plasma osmolality, changing the c a r o t i d sinus pressure (CSP) caused s i g n i f i c a n t changes i n plasma arginine vasopressin (AVP). 2. There i s a l i n e a r r e l a t i o n s h i p between plasma osmolality (Posm) ^^ '^  plasma AVP. Changing the CSP, changed the slope of t h i s l i n e . The slope was greater at CSP of 40 mmHg than i t was at CSP of 140 mmHg. Changing the CSP did not a f f e c t the x-intercept of t h i s l i n e . Low CSP appears to increase the s e n s i t i v i t y of the AVP/Posm r e l a t i o n s h i p whereas high CSP appears to decrease i t . CSP does not appear to influence the threshold of the AVP/Posm r e l a t i o n s h i p . 3. Previous studies have shown that changes i n blood volume influence the AVP/Posm re l a t i o n s h i p . I t i s not known whether these e f f e c t s are mediated by low pressure receptors or a r t e r i a l baroreceptors. The present study suggests that a r t e r i a l baroreceptors acting alone, change the slope of the plasma AVP/ plasma osmolality r e l a t i o n s h i p . 4. The mechanism by which changes i n blood volume and pressure influence the osmotic control of AVP release i s not known. I t appears that changes i n blood volume and pressure may have a r o l e i n the d a i l y control of AVP release through the modulation of the AVP/Posm r e l a t i o n s h i p . VI. REFERENCES 2. Altura, B.M. Selective microvascular c o n s t r i c t o r actions of some neurohypophyseal peptides. Eur.J.Pharmacol. 24:49-60, 1973. 3. Altura, B.M. and Altura, B.T. Vascular smooth muscle and neurohypophyseal hormones. Federation Proc. 36:1853-1860, 1977. 4. Arnauld, E., Czernichow, P., Fumoux, F. and Vincent, J . The e f f e c t s of hypotension and hypovolaemia on the l i b e r a t i o n of vasopressin during haemorrhage i n the unanaesthetized monkey {Macaca Mulatta). Pflugers Arch. 371:193-200, 1977. 5. Bayl i s , P.H. Osmoregulation and control of vasopressin secretion i n healthy humans. Am.J.Physiol. 253:R671-R678, 1987. 6. Bie, P. Osmoreceptors, vasopressin and control of renal water excretion. Physiological Reviews 60(4):961-1048, 1980. 7. Bishop, V.S., Thames, M.D. and Schmid, P.G. E f f e c t s of b i l a t e r a l vagal cold block on vasopressin i n conscious dogs. Am.J.Physiol. 246:R566-R569, 1984. 8. Bisset, G.W. and Chowdrey, H.S. Control of release of vasopressin by neuroendocrine re f l e x e s . Journal of Experimental Physiology 73:811-872, 1988. 9. Bisset, G.W., Chowdrey, H.S. and Feldberg, W. An osmosensitive zone on the dorsal surface of the medulla i n the cat. J.Physiol. 365:30P, 1985. 10. Bond, G.C. and Trank, J.W. Plasma a n t i d i u r e t i c hormone concentration a f t e r b i l a t e r a l a o r t i c nerve section. Am.J.Physiol. 222(3): 595-598, 1972. 11. Bonjour, J.P. and Malvin, R.L. Plasma concentrations of ADH i n conscious and anesthetized dogs. Am.J.Physiol. 218(4): 1128-1132, 1970. 12. Choi-Kwon, S. and Baertschi, A.J. Splanchnic osmosensation and vasopressin: mechanisms and neural pathways. Am.J.Physiol. 261:E19-E25, 1991. 13. Choi-Kwon, S., McCarty, R. and Baertschi, A.J. Splanchnic control of vasopressin secretion i n conscious r a t s . Am.J.Physiol. 259:E19-E26, 1990. 14. Clark, B.J. and Rocha E S i l v a , M. An afferent pathway for the s e l e c t i v e release of vasopressin i n response to ca r o t i d occlusion and haemorrhage i n the cat. J.Physiol. 191:529-542, 1967. 15. Claybaugh, J.R. and Share, L. Vasopressin, renin, and cardiovascular responses to continuous slow hemorrhage. Am.J.Physiol. 224(3): 519-523, 1973. 16. Courneya, C.A., Rankin, A.J., Wilson, N. and Ledsome, J.R. Carotid sinus pressure and plasma vasopressin i n anesthetized rabbits. Am.J.Physiol. 255:H1199-H1205, 1988. 17. Courneya, C.A., Wilson, N. and Ledsome, J.R. Carotid sinus pressure, blood volume, and vasopressin i n the anesthetized rabbit. Canadian Journal of Physiology and Pharmacology 67:1386-1390, 1989b. 18. Courneya, C.A., Wilson, N. and Ledsome, J.R. Plasma vasopressin and a t r i a l n a t r i u r e t i c factor i n response to blood volume changes i n the anesthetized rabbit. Canadian Journal of Physiology and Pharmacology 67:344-352, 1989a. 19. Cowley, A.W. Vasopressin and cardiovascular regulation. In: Cardiovascular Physiology IV; International Review of Physiology, Volume 26, edited by Guyton, A.C. and H a l l , J.E. Baltimore: University Park Press, 1982, p. 189-242. 20. Cowley, A.W., Monos, E. and Guyton, A.C. Interaction of vasopressin and the baroreceptor r e f l e x system i n the regulation of a r t e r i a l blood pressure i n the dog. Circulation Research 34:505-514, 1974. 21. Cowley, A.W., Switzer, S.J. and Guinn, M.M. Evidence and qu a n t i f i c a t i o n of the vasopressin a r t e r i a l pressure control system i n the dog. Circulation Research 46:58-67, 1980. 22. Dunn, F.L., Brennan, T.J., Nelson, A.E. and Robertson, G.L. The r o l e of blood osmolality and volume i n regulating vasopressin secretion i n the r a t . The Journal of Clinical Investigation 52:3212-3219, 1973. 23. DuVigneaud, V., Gish, D.T. and Katsoyannis, P.G. A synthetic preparation possessing b i o l o g i c a l properties associated with arginine-vasopressin. J.Am.Chem.Soc. 76:4751-4752, 1954. 24. Ericsson, B.F. E f f e c t of vasopressin on the d i s t r i b u t i o n of cardiac output and organ blood flow i n the anesthetized dog. Acta Chir.Scand. 137:729-738, 1971. 25. Fater, D.C., Schultz, H.D., Sundet, W.D., Mapes, J.S. and Goetz, K.L. E f f e c t s of l e f t a t r i a l s t r e t c h i n cardiac-denervated and i n t a c t conscious dogs. Am.J.Physiol. 242:H1056-H1064, 1982. 26. Gilmore, J.P., Peterson, T.V., Wesley, C.R. and Share, L. High versus low pressure receptors i n modulating the volumetric control of a n t i d i u r e t i c hormone secretion i n the monkey. Basic Res.Cardiol. 77:250-254, 1982. 27. Gilmore, J.P., Zucker, I.H., E l l i n g t o n , M.J., Richards, M. and Share, L. F a i l u r e of acute intravascular volume expansion to a l t e r plasma vasopressin i n the nonhuman primate, Macaca fascicularis. Endocrinology 106:979-982, 1980. 28. Goldsmith, S.R., Cowley, A., Francis, G.S. and Cohn, J.N. E f f e c t of increased intracardiac and a r t e r i a l pressure on plasma vasopressin i n humans. Am.J.Physiol. 246:H647-H651, 1984. 29. Goldsmith, S.R., Cowley, A., Francis, G.S. and Cohn, J.N. Reflex control of osmotically stimulated vasopressin i n normal humans. Am.J.Physiol. 248:R660-R663, 1985. 30. Goldsmith, S.R., Dodge, D. and Cowley, A. Nonosmotic influences on osmotic stimulation of vasopressin i n humans. Am.J.Physiol. 252:H85-H88, 1987. 31. Greenwood, F.C, Hunter, W.M. and Glover, J.S. The preparation of l ^ ^ I - l a b e l l e d human growth hormone of a high s p e c i f i c r a d i o a c t i v i t y , Biochem. J. 89:114, 1963. 32. Hasser, E.M. and Bishop, V.S. Reflex e f f e c t of vasopressin a f t e r blockade of VI receptors i n the area postrema. Circulation Research 67:265-271, 1990. 33. Henry, J.P., Gauer, O.H. and Reeves, J.L. Evidence of the a t r i a l l o c ation of receptors influencing urine flow. Circulation Research 4:85-90, 1956. 34. Johnson, A.K. Role of the p e r i v e n t r i c u l a r t i s s u e surrounding the anteroventral t h i r d v e n t r i c l e (AV3V) i n the regulation of body f l u i d homeostasis. In: Vasopessin, edited by Schrier, R.W. New York: Raven Press, 1985, p. 319-331. 35. Johnson, J.A., Moore, W.W. and Segar, W.E. Small changes i n l e f t a t r i a l pressure and plasma a n t i d i u r e t i c hormone t i t e r s i n dogs. Am.J.Physiol. 217(1):210-214, 1969. 36. King, K.A., Courneya, C.A., Tang, C., Wilson, N. and Ledsome, J.R. Pharmacokinetics of vasopressin and a t r i a l n a t r i u r e t i c peptide i n anesthetized rabbits. Endocrinology 124:77-83, 1989. 37. Knepel, W., Nutto, D. and Meyer, D.K. E f f e c t of transection of subfornical organ efferent projections on vasopressin release induced by angiotensin or isoprenaline i n the r a t . Brain Research 248:180-184, 1982. 38. Lauson, H.D. Metabolism of the neurohypophysial hormones. In: Handbook of Physiology Endocrinology, edited by Creep, R.O. and Astwood, E.B. Washington DC: American Physiological Society, 1974, p. 287. 39. Ledsome, J.R. A t r i a l receptors, vasopressin and blood volume i n the dog. Life Sciences 36:1315-1330, 1985a. 40. Ledsome, J.R. and Linden, R.J. The r o l e of l e f t a t r i a l receptors i n the d i u r e t i c response to l e f t a t r i a l distension. J.Physiol. 198:487-503, 1968. 41. Ledsome, J.R., Ngsee, J . and Wilson, N. Plasma vasopressin concentration i n the anesthetized dog before and af t e r a t r i a l distension. J.Physiol. 338:413-421, 1983. 42. Ledsome, J.R., Wilson, N. and Courneya, CA. Plasma vasopressin during increases and decreases i n blood volume i n anesthetized dogs. Canadian Journal of Physiology and Pharmacology 63:224-229, 1985b. 43. Leimbach, W.N., Schmid, P.G. and Mark, A.L. Baroreflex control of plasma arginine vasopressin i n humans. Am.J.Physiol. 247:H638-H644, 1984. 44. Leng, G. Rat supraoptic neurons: the e f f e c t s of l o c a l l y applied hypertonic s a l i n e . J.Physiol. 304:405-414, 1980. 45. Leng, G., Dyball, R.E.J, and Mason, W.T. Electrophysiology of osmoreceptors. In: Vasopressin, edited by Schrier, R.W. New York: Raven Press, 1985, p. 333-342. 46. Leng, G., Mason, W.T. and Dyer, R.G. The supraoptic nucleus as an osmoreceptor. Neuroendocrinology 34:75-82, 1982. 47. Magnus, R. and Schafer, E.A. The action of p i t u i t a r y extracts upon the kidney. J.Physiol. 27:ix-x, 1901. 48. Mason, W.T. Supraoptic neurones of r a t hypothalamus are osmoreceptive. Nature 287:154-157, 1980. 49. Mason, W.T. E l e c t r i c a l properties of neurones recorded from the rat supraoptic nucleus i n v i t r o . Proceedings of the Royal Society B217, 141-161, 1983. 50. McNeill, J.R. and Pang, C.C.Y. E f f e c t of pentobarbital anesthesia and surgery on the control of a r t e r i a l pressure and mesenteric resistance i n cats: r o l e of vasopressin and angiotensin. Can.J.Physiol.Pharmacol. 60:363-368, 1981. 51. Menninger, R.P. Current concepts of volume receptor regulation of vasopressin release. Federation Proc. 44:55-58, 1985. 52. Montani, J . , L i a r d , J . , Schoun, J . and Mohring, J . Hemodynamic e f f e c t s of exogenous and endogenous vasopressin at low plasma concentrations i n conscious dogs. Circulation Research 47:346-355, 1980. 53. Morris, S.J. S p e c i f i c r a d i o a c t i v i t y of radioimmunoassay tracer determined by self-displacement; a re-evaluation. Clinicia Chimica Acta 73:213-216, 1976. 54. Norsk, P. Influences of low and high-pressure baroreflexes on vasopressin release i n humans. Acta Endocrinologica 121:1-121, 1989. 55. Oliver, G. and Schafer, E.A. On the p h y s i o l o g i c a l actions of p i t u i t a r y body and c e r t a i n other glandular organs. Am.J.Physiol. 18:211-219, 1895. 56. Quail, A.W., Woods, R.L. and Korner, P.I. Cardiac and a r t e r i a l baroreceptor influences i n release of vasopressin and renin during hemorrhage. Am.J.Physiol. 252:H1120-H1126, 1987. 57. Q u i l l e n , E.W. and Cowley, A.W. Influence of volume changes on osmolality-vasopressin r e l a t i o n s h i p s i n conscious dogs. Am.J.Physiol. 244:H73-H79, 1983. 58. Ramsay, D.J. Osmoreceptors subserving vasopressin secretion - an overview. In: Vasopressin, edit ad by Schrier, R.W. New York: Raven Press, 1985, p. 291-298. 59. Robertson, G.L. Physiology of ADH secretion. Kidney Int. 32:S20-S26, 1987. 60. Robertson, G.L. and Athar, S. The i n t e r a c t i o n of blood osmolality and blood volume i n regulating plasma vasopressin i n man. J.Clin.Endocrinol.Metab. 42:613-620, 1975. 61. Robertson, G.L., Athar, S. and Shelton, R.L. Osmotic control of vasopressin function. In: Disturbances in body fluid osmolality, edited by Andreoli, T.E., Grantham, J . J . and Rector, F.C. Baltimore: Waverly Press, 1977, p. 125-148. 62. Robertson, G.L. and Ganguly, A. Osmoregulation and baroregulation of plasma vasopressin i n e s s e n t i a l hypertension. Journal of Cardiovascular Pharmacology 8(S7):S87-S91, 1986. 63. Robertson, G.L., Shelton, R.L. and Athar, S. The osmoregulation of vasopressin. Kidney Int. 10:25-37, 1976. 64. Rydin, H. and Verney, E.B. The i n h i b i t i o n of water-d i u r e s i s by emotional stress and by muscular exercise. Q.J.Exp.Physiol. 27:343-374, 1938. 65. Schloerb, P.R. Total body water d i s t r i b u t i o n of creatinine and urea i n nephrectomized dogs. Am.J.Physiol. 199(4):661-665, 1960. 66. Schultz, H.D., Fater, D.C., Sundet, W.D., Geer, P.G. and Goetz, K.L. Reflexes e l i c i t e d by acute str e t c h of a t r i a l vs. pulmonary receptors i n conscious dogs. Am.J.Physiol. 242:H1065-H1076, 1982. 67. Shade, R.E. and Share, L. Volume control of plasma a n t i d i u r e t i c hormone concentration following acute blood volume expansion i n the anesthetized dog. Endocrinology 97:1048-1057, 1975. 68. Share, L. Ef f e c t s of caroti d occlusion and l e f t a t r i a l distension on plasma vasopressin t i t e r . Am.J.Physiol. 208(2):219-223, 1965. 69. Share, L. Control of plasma ADH t i t e r i n hemorrhage: r o l e of a t r i a l and a r t e r i a l receptors. Am.J.Physiol. 215(6):1384-1389, 1968. 70. Share, L. Role of vasopressin i n cardiovascular regulation. Physiol.Rev. 68:1248, 1988. 71. Share, L. and Levy, M.N. Cardiovascular receptors and blood t i t e r of a n t i d i u r e t i c hormone. Am.J.Physiol. 203(3):425-428, 1962. 72. Share, L. and Levy, M.N. Carotid sinus pulse pressure, a determinant of plasma a n t i d i u r e t i c hormone concentration. Am.J.Physiol. 211(3): 721-724, 1966. 73. Share, L. and Levy, M.N. E f f e c t of c a r o t i d chemoreceptor stimulation on plasma a n t i d i u r e t i c hormone t i t e r . Am.J.Physiol. 210(1): 157-161, 1966. 74. Shen, Y., Cowley, A. and Vatner, S.F. Relative r o l e s of cardiac and a r t e r i a l baroreceptors i n vasopressin regulation during hemorrhage i n conscious dogs. Circulation Research 68:1422-1436, 1991. 75. Skowsky, W.R. and Fisher, D.A. The use of thyroglobulin to induce a n t i g e n i c i t y to small moleciles. J.Lab.Clin.Med. 80:134-144, 1972. 76. Szczepanska-Sadowska, E. The a c t i v i t y of the hypothalamo-hypophysial a n t i d i u r e t i c system i n conscious dogs. Pflugers Arch. 335:139-146, 1972. 77. Thames, M.D., Peterson, M.G. and Schmid, P.G. Stimulation of cardiac receptors with veratrum a l k a l o i d s i n h i b i t s ADH secretion. Am.J.Physiol. 239:H784-H788, 1980. 78. Thames, M.D. and Schmid, P.G. Cardiopulmonary receptors with vagal afférents t o n i c a l l y i n h i b i t ADH release i n the dog. Am.J.Physiol. 237(3):H299-H304, 1979. 79. Thames, M.D. and Schmid, P.G. Interaction between c a r o t i d and cardiopulmonary baroraflexes i n control of plasma ADH. Am.J.Physiol. 241:H431-H434, 1981. 80. Thrasher, T.N. Circumventricular organs, t h i r s t , and vasopressin secretion. In: Vasopressin, edited by Schrier, R.W. New York: Raven Press , 1915, p. 311-318. 81. Thrasher, T.N., Brown, C.J., K e i l , L.C. and Ramsay, D.J. T h i r s t and vasopressin release i n the dog: the osmoreceptor or sodium receptor mechanism? Am.J.Physiol. 238:R333-R339, 1980a. 82. Thrasher, T.N., Jones, R.G., K e i l , L . C , Brown, C.J. and Ramsay, D.J. Drinking and vasopressin release during v e n t r i c u l a r infusions of hypertonic solutions. Am.J.Physiol. 238:R340-R345, 1980b. 83. Undesser, K.P., Hasser, E.M., Haywood, J.R., Johnson, A.K. and Bishop, V.S. Interactions of vasopressin with the area postrema i n a r t e r i a l baroreflex function i n conscious rabbits. Circulation Research 56:410-417, 1985. 84. V a l l e t , P.G. and Baertschi, A.J. Spinal afférents f o r peripheral osmoreceptors i n the r a t . Brain Research 239:271-274, 1982. 85. Verney, E.B. The a n t i d i u r e t i c hormone and the factors which determine i t s release. Proceedings of the Royal Society B135:25-106, 1947. 86. Wang, B.C., Flora-Ginter, G., Leadley, R.J. and Goetz, K.L. Vent r i c u l a r receptors stimulate vasopressin release during hemorrhage. Am.J.Physiol. 254:R204-R211, 1988. 87. Wang, B.C., Sundet, W.D., Hakumaki, M.O.K., Geer, P.G. and Goetz, K.L. Cardiac receptor influences on the plasma osmolality-plasma vasopressin r e l a t i o n s h i p . Am.J.Physiol. 246:H360-H368, 1984. 88. Wang, B.C., Sundet, W.D., Hakumaki, M.O.K. and Goetz, K.L. Vasopressin and renin responses to hemorrhage i n conscious, cardiac-denervated dogs. Am.J.Physiol. 245:H399-H405, 1983. 89. Wehberg, K.E., Gala, G.J. and Brunner, M.J. Comparison of c a r o t i d baroreflex control of plasma AVP concentration i n conscious and anesthetized dogs. Am.J.Physiol. 261:R950-R956, 1991. 

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