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Vasodilators and venous tone D'Oyley, Heather M. 1988

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VASODILATORS AND VENOUS TONE By HEATHER M. D'OYLEY B. Sc. (Honours), University of Brit ish Columbia, 1987 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER'S OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of Pharmacology & Therapeutics, Faculty of Medicine We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA SEPTEMBER 1988 ©HEATHER M. D'OYLEY, 1988 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 written permission. Department of Phl/m2mlfrjLj fl/lH Thftftpft/fcc; The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date ft SeptirYbe^ 38m DE-6(3/81) i i ABSTRACT The objective of these experiments was to investigate the effects of various membrane receptor-mediated and receptor-independent vasodilators on the resistance and capacitance vessels of conscious, unrestrained rats by measuring mean arterial pressure (MAP) and mean circulatory f i l l i n g pressure (MCFP), an index of total body venous tone. l n the f i r s t set of experiments the dose-response effects of the directly-acting vasodilators nitroglycerin, sodium nitroprusside and hydralazine were determined in intact rats as well as in rats treated with the ganglionic blocker, hexamethonium.- The effects of these drugs were compared with those of the vehicle, normal saline, in control rats. In intact rats, iv infusion of nitroglycerin did not alter MAP while iv infusions of nitroprusside and hydralazine caused dose-dependent decreases in MAP. In intact rats, nitroglycerin and sodium nitroprusside did. not affect MCFP while hydralazine increased MCFP. After treatment with hexamethonium al l three drugs decreased MCFP, though the decreases in MCFP caused by hydralazine were not s ignif icantly different from the corresponding changes in saline-treated rats. Therefore, sodium nitroprusside and hydralazine but not nitroglycerin were effective arteriolar dilators in intact rats; a l l three drugs dilated arterioles in ganglionic-blocked rats, l n intact rats, the direct venodilator actions of nitroprusside and nitroglycerin were masked by endogenous sympathetic tone. When sympathetic nerve act iv ity was attenuated, both drugs had venodilatory effects. Hydralazine, on the other hand, hao insignificant venodilatory effects both in the presence and absence of the sympathetic reflexes. i i i In the second set of experiments we determined the dose-response e f f e c t s of hexamethonium, phentolamine, p razos in and rauwolsc ine — the l a t t e r being non - s e l e c t i v e a , o -^ - se lec t i ve , ana o ^ " 5 6 ! 6 0 ^ 6 adrenoceptor an tagon i s t s , r e s p e c t i v e l y — in i n t a c t r a t s . P razos in and rauwolsc ine were a l so admin is tered to r a t s w i th r e f l e x l y increased venous tone induced by the i n f u s i o n of h yd r a l a z i ne . In i n t a c t r a t s i v i n f u s i on s of p r a zo s i n , phentolamine and rauwolsc ine a l l caused dose-dependent decreases in MAP; only rauwolsc ine reduced MCFP to l e v e l s s l i g h t l y below c o n t r o l . Hexametho-nium caused a aecrease in MAP as we l l as a markea reduct ion in MCFP. A f t e r venous tone was r a i s ed by the i n f u s i on of h yd r a l a z i ne , both prazos in and rauwolsc ine dose-dependently decreased MCFP. Therefore, the r e s i s t ance and capac i tance vesse l s conta in both a ^ - and ©^-adrenoceptors. in the i n t a c t r a t , however, the capac i tance vesse l s are somewhat r e s i s t a n t to the e f f e c t s of p o s t j u n c t i o n a l l y a c t i ng a -antagon i s t s i n con t ra s t to the e f f e c t s of hexamethonium which acts at the l e v e l of the gang l i on . iv TABLE OF CONTENTS CHAPTER Page 1. INTRODUCTION 1 1.1 Ref lex Contro l of the Capacitance Vessels 1 1.1.1 The sp lanchn ic bed 3 1.2 The Sympathetic Nervous System 4 1.2.1 C l a s s i f i c a t i o n of pe r i phe ra l adrenoceptors 5 1.3 D i r e c t l y Ac t i ng Smooth Muscle Relaxants 8 1.3.1 N i t r o g l y c e r i n 8 1.3.2 Soaium n i t r o p r u s s i a e 9 1.3.3 Hydra laz ine 10 1.4 Methoas of Determining Venous Capacitance 11 1.4.1 Mean c i r c u l a t o r y f i l l i n g pressure 12 1.5 The Nature of the Problem 15 1.5.1 D i r e c t l y a c t i n g smooth muscle r e l axan t s 15 1.5.2 a-aarenoceptor antagonis ts and hexamethonium 16 2. METHOD 18 2.1 Su rg i ca l P repara t ion 18 2.2 Measurement of ACFP 18 2.3 Experimental P ro toco l 18 2.3.1 D i r e c t l y a c t i n g smooth muscle r e l axan t s 18 2.3.2 a-aarenoceptor antagonis ts and hexamethonium 19 2.4 Drugs 2u 2.5 C a l c u l a t i o n s 21 2.6 S t a t i s t i c a l ana l y s i s 21 3. RESULTS 21 3.1 D i r e c t l y Ac t i ng Smooth Muscle Relaxants 21 3.1.1 Contro l values 21 3.1.2 N i t r o g l y c e r i n 24 3.1.3 N i t r op r u s s i de 24 3.1.4 Hydra laz ine 31 V CHAPTER Page 3.2 a - a d r e n o c e p t o r A n t a g o n i s t s and Hexamethonium 31 3.2.1 S e l e c t i v i t y o f p r a z o s i n and r a u w o l s c i n e 31 3.2.2 C o n t r o l v a l u e s 31 3.2.3 In t h e a b s e n c e o f h y d r a l a z i n e 36 3.2.4 In t h e p r e s e n c e o f h y d r a l a z i n e 36 4. DISCUSSION 45 4.1 D i r e c t l y A c t i n g Smooth M u s c l e R e l a x a n t s 45 4.2 a - a d r e n o c e p t o r A n t a g o n i s t s and Hexamethonium 47 4.3 Summary 52 5. REFERENCES 54 v i L I S T O F T A B L E S T A B L E P a g e 1 C o n t r o l v a l u e s o f m e a n a r t e r i a l p r e s s u r e , h e a r t r a t e 22 a n d mean c i r c u l a t o r y f i l l i n g p r e s s u r e f o r t h e d i r e c t l y a c t i n g v a s o d i l a t o r s . 2 C o n t r o l v a l u e s o f m e a n a r t e r i a l p r e s s u r e , h e a r t r a t e  23 a n d m e a n c i r c u l a t o r y f i l l i n g p r e s s u r e  f o r t h e d i r e c t l y a c t i n g v a s o d i l a t o r s i n i n t a c t a n d h e x a m e t h o n i u m t r e a t e d r a t s . 3 C o n t r o l v a l u e s o f m e a n a r t e r i a l p r e s s u r e , h e a r t r a t e  34 a n d m e a n c i r c u l a t o r y f i l l i n g p r e s s u r e f o r a - a n t a g o n i s t s a n d h e x a m e t h o n i u m . 4 C o n t r o l v a l u e s o f m e a n a r t e r i a l p r e s s u r e , h e a r t r a t e 35 a n a m e a n c i r c u l a t o r y f i l l i n g p r e s s u r e  f o r p r a z o s i n a n d r a u w o l s c i n e i n i n t a c t a n d h y d r a l a z i n e t r e a t e a r a t s . v i i LIST Or FIGURES FIGURE Page 1 Effect of saline on mean arterial pressure, heart 25 rate and mean circulatory f i l l i n g pressure in intact and hexamethonium treated rats. 2 Effect of nitroglycerin on mean arterial pressure, 27 heart rate ana mean circulatory f i l l i n g pressure in intact and hexamethonium treated rats. 3 Effect of sodium nitroprusside on mean arterial 29 pressure, heart rate and mean circulatory f i l l i n g pressure in intact and hexamethonium treated rats. 4 Effect of hydralazine on mean arterial pressure, 32 heart rate and mean circulatory f i l l i n g pressure in intact and hexamethonium treated rats. 5 Effect of the glucose vehicle on mean arterial 37 pressure, heart rate and mean circulatory f i l l i n g pressure in intact and hydralazine treated rats. 6 Effect of hexamethonium on mean arterial pressure, 39 heart rate and mean circulatory f i l l i n g pressure in intact rats. 7 Effects of phentolamine, prazosin, and rauwolscine 41 on mean arterial pressure, heart rate ana mean circulatory f i l l i n g pressure in intact rats. 8 Effects of prazosin, rauwolscine and the glucose 43 vehicle on mean arterial pressure, heart rate and mean circulatory f i l l i n g pressure in hydralazine treated rats. vi i i ACKNOWLEDGEMENTS The author wishes to thank Catherine Cheuk Ying Pang for her excellent advice, supervision ana guidance. Her contributions are gratefully acknowledged. The author also extends thanks to a l l of the other members of the Department of Pharmacology & Therapeutics who offered their advice and guidance as well as to Ms. Elaine L. Jan and Mrs. Margaret Wong for their secretarial assistance. While this work was carried out the author was a recipient of a British Columbia Heart Foundation Research Traineeship ana would like to thank B. C. Heart Foundation for their financial contribution. D'UYLEY, H. M. 1 1. INTRODUCTION Vascular resistance, cardiac function, total blooo volunie as well as vascular compliance are the primary constituents in the maintenance of cardiovascular homeostasis. Caraiac output which is synonymous with venous return under steady state conditions, represents the product of summating these complex and integrated components. Techniques have long been avai l -able to accurately monitor alterations in heart rate (HR), contract i l i ty, peripheral resistance, blood volume, ana cardiac output. Consequently the effects of vasoactive substances, whether of endogenous or exogenous origin, on these parameters are easily and often aocumentea. The veins, also referred to as the capacitance vessels, are known to contain up to 70% of the total blood volume ana therefore constitute the 'reservoir' of the c i rcu la - t ion. Blooa may be mobilizea from this store of blood under conditions of reaucea blooa pressure or blooa volume, such as postural hypotension or haemorrhage, in order to maintain venous return ana cardiac output. Due to technical d i f f i cu l t i e s , however, there is limited information regarding the j_n vi vo effects of vasoactive agents on total body venous tone. 1.1 Reflex Control of the Capacitance Vessels Reflex venoconstriction is defined as a neurogenically induced contrac-tion of the venous smooth muscle that results in a rightward shift of the pressure-volume curve when venous pressure, on the x-axis, is plotted against blood volume on the y-axis. Capacitance, the total blood volume at a given pressure, is the sum of the 'stressed' and 'unstressed' blooo volumes (Shoukas and Sagawa, 1973; Rothe, 1983; breenway and Lautt, 1986). The slope of a given capacitance curve is callea the vascular compliance, a D'UYLEY, H. fe. 2 measure of venous d i s tens ib i l i ty . At zero transmural pressure, extrapolated from the capacitance curve, capacitance vessels usually contain a s i gn i f i -cant fraction of their normal blood volume (kothe, 1983). This reservoir, the unstressed blood volume, is haemodynamically inactive. Thus only the remainder of the total blood volume, the stressed volume, is active in the circulat ion. . Venous pressure, which together with the resistance to flow determines venous return ana cardiac output, is dependent on the stressed volume as well as the venous compliance. Venoconstriction can therfore shift the capacitance curve by changing either the vascular compliance or the unstressed volume (thereby mobilizing the stressed volume) or both (Shoukas and Sagawa, 1973; Rothe, 1983). Decreasing carotia sinus (bartelston, 1960) or aortic arch (Karim et a l . , 1978) pressure by various means reduces baroreceptor discharge which induces vasoconstriction, venoconstriction, increased myocardial contract i l -ity ana increased hk. The increase in vena cava! pressure during carotia artery occlusion can be blockea by the ganglionic blocker tetraethylammonium as well as the ergot alkaloias (Bartelstone, 1960), proviaing evidence that i t is a sympathetically meaiated reflex. Shoukas and Sagawa (1973) were able to demonstrate that in response to carotia sinus hypotension the maximal decrease in compliance occurred at the same intra-sinus pressure as the maximal increase in peripheral resistance. Though greater when inducea by changes in carotid sinus rather than aortic arch pressure (Karim et a l . , 1978), these alterations in cardiac status by the baroreceptor reflexes serve to minimize arterial hypotension. In fact i t has been shown that by altering total systemic capacity, the carotia sinus baroreceptor reflex can modify cardiac output by as much as 40% per 25 mmhg change in intrasinus pressure (Shoukas and Sagawa, 1973). Compensatory reflex mechanisms in D'OYLEY, H. M. 3 response to changes in blooa pressure ana volume may be more aependent on changes in the unstressed blood volume than alterations in compliance; this is reflected by parallel shifts in the capacitance curves after the infusion of noradrenaline and hexamethonium — indicative of enhanced ana attenuated sympathetic nerve act iv ity, respectively (Drees and Kothe, 1974; 6reenway ana Lautt, 1986). It is imperative, however, to realize that the venous response to baroreceptor reflex activation is not homogeneous throughout the various vascular beds. As early as 1921 i t was recognized that the canine superfi-c ia l veins, though sympathetically innervated do not participate in the 'pressor' reflexes but rather are unaer local thermoregulatory control (Donegan, 1921). The same study found that the veins of the skeletal muscle ana vena cava show no reflex activity ana are only weakly responsive to exogenously appliea aarenaline; in contrast, the mesenteric veins are densely innervated by sympathetic nerves ana participate actively in the reflex response. 1.1.1 The splanchnic bed. The size and vascularity of the splanchnic bea make i t an important factor in regulating the volume and the pressure of the blood in general circulation. Early stuaies, such as that by Gr i f f i th and Emery (1930), found that in cats there was an active decrease in l iver volume in response to stimulation of the baroreceptor reflex by arterial hypotension or haemorrhage. In cats and dogs the pressor response to carotid occlusion has been shown to be reducea by approximately 50% after elimination of either the cardiac sympathetics or the peripheral innervation (Wang et a l . , 1970). Therefore, the reflex in response to common carotid occlusion is meaiated by both cardiac and peripheral factors. The same study suggested that splanchnic innervation is the major contributor to the D'UYLEY, H. M. 4 pe r i phe ra l component; the pressor response was reduced by almost 40% a f t e r sp lanchn ic nerve s e c t i o n . Upon c u t t i n g the sp lanchn ic nerve, ca rd i ac output was s i g n i f i c a n t l y reduced with an accompanying decrease in the s t roke volume and no change in HR suggest ing a c t i v e v enocon s t r i c t i on (Wang et a l . , 1970). Thus, the sp lanchn ic c i r c u l a t i o n f unc t i on s as an important blood r e s e r v o i r t ha t can be p r e f e r e n t i a l l y enhanced or depleted by capac i tance changes (Wang et a l . , 1970; Hoka et a l . , 1988). In man, i t has been shown that s p l anch -n i c blood volume i s reduced by 40% f o l l o w i n g a 1 l i t r e haemmorrhage wh i le mean a r t e r i a l pressure (MAP), HR, ca rd iac output and sp lanchnic vascu la r r e s i s t a n c e are unchanged ( P r i ce et a l . , 196b). M o b i l i z a t i o n of the sp l anch -n i c blood volume may the re fo re be s u f f i c i e n t to su s ta in a r t e r i a l pressure in the case of m i ld haemorrhage. Although massive dev i a t i on s i n the blood volume cannot be compensated f o r by changes in venous capac i tance , the r e f l e x c o n t r o l of venous as we l l as a r t e r i a l smooth muscle prov ides a temporary homeostatic mechanism whose r o l e i t i s to minimize changes in ca rd i ac output ana a r t e r i a l pressure. Transcapi11ary s h i f t s , rena l water r e t e n t i o n and a l t e r a t i o n s in water in take prov ide long term c o r r e c t i o n f o r l a r ge f l u c t u a t i o n s in blood volume. 1.2 The Sympathetic Nervous System The sympathetic nervous system plays an important r o l e i n the mainten-ance of c a rd i a c output by i n f l u e n c i n g both ca rd i ac and pe r i phe ra l f a c t o r s . A pressor e f f e c t by the i n j e c t i o n of adrenal e x t r a c t s was f i r s t reported by O l i v e r and Schafer (1895) ana the a c t i v e component, ad rena l i ne , was soon i s o l a t e d and i d e n t i f i e d by Abel i n 1899. The resemblance between the responses to adrenal e x t r a c t i n j e c t i o n and nerve s t i m u l a t i o n led to the suggest ion that adrena l ine was the substance re leased from sympathetic nerve te rm ina l s ( E l l i o t , 19U4). The sympathomimetic response, however, was not D'OYLEY, H. M. 5 always identical to that e l ic i ted by aarenaline; in fact, Barger and Dale (1910) found that noradrenaline mimicked the effects of sympathetic nerve stimulation more closely. Von Euler (1946) found only small amounts of adrenaline present in sympathetically innervated tissues. Rather, he established that the major substance released from the sympathetic nerve terminals was noradrenaline. Acetylcholine released from the preganglionic fibres induces the release of noradrenaline from the nerve terminal as well as adrenaline and noradrenaline from the adrenal medulla (Burn and Rand, 1959). The catecholamines, once released, interact with receptors to e l i c i t a response in the effector organ. Since enzymatic catabolism of the catecholamines by catechol-O-methyltransferase and monoamine oxidase is located intrace l lu lar ly , the response is terminated largely by the rapid reuptake of the catecholamines in the axons (uptake 1; noradrenaline) or the effector cel ls (uptake 2; adrenaline ana noradrenaline). 1.2.1 Classif ication of peripheral adrenoceptors. The concept of receptor mediated responses was f i r s t introduced at the turn of the century in order to explain the effects of curare upon skeletal muscle (Langley, 1905). however, it was Dale (1906) who f i r s t made use of receptor theory in connection with the sympathetic nervous system; the ergot alkaloias pre-vented only the 'excitatory' or motor responses while having l i t t l e effect on the ' inhibitory' responses e l ic i ted by adrenaline. The currently accepted classif icat ion of adrenoceptors was proposed by Alquist (1948) who found that there were two dist inct receptor types, a- and e-adrenoceptors, in the sympathetic nervous system. These receptors were class i f ied not simply as 'excitatory' or ' inhibi tory' , but rather on the basis of their relative responsiveness in various tissues to a series of sympathomimetic amines. The a-adrenoceptors, highly sensitive to noradrena-u'UYLEY, H. M. 6 l i n e , a d r e n a l i n e ana r e l a t i v e l y i n s e n s i t i v e t o i s o p r o p y l n o r a d r e n a l i n e , a r e f o u n a p r i m a r i l y i n t h e v a s c u l a t u r e . On t h e o t h e r n a n a , e - a d r e n o c e p t o r s , e x h i b i t i n g h i g h s e n s i t i v i t y t o i s o p r o p y l n o r a d r e n a l i n e , a d r e n a l i n e and n o r a d r e n a l i n e , a r e f o u n d i n t h e h e a r t , v a s c u l a t u r e and o t h e r smooth m u s c l e s . In s u b s e q u e n t y e a r s , h o w e v e r , i t became a p p a r e n t t h a t n e i t h e r t h e 6- n o r t h e a - r e c e p t o r p o p u l a t i o n was homogeneous. L a n a s e t a l . ( 1 9 6 7 ) , t e s t i n g a s e r i e s o f e - a g o n i s t s i n v a r i o u s t i s s u e s , was a b l e t o d e m o n s t r a t e t h e e x i s t -e n c e o f d i s c r e t e r e c e p t o r s w h i c h he c l a s s i f i e d as e ^ - a n a B 2 ~ a d r e n o c e p ~ t o r s . The e ^ - a d r e n o c e p t o r m e d i a t e s l i p o l y s i s and c a r d i a c s t i m u l a t i o n w h e r e a s t h e e 2 " a d r e n o c e P t o r i n d u c e s t h e r e l a x a t i o n o f smooth m u s c l e i n t h e b r o n c h i o l e s and t h e v a s c u l a t u r e ( L a n a s e t a l . , 1 9 6 7 ) . S i m i l a r l y , c l a s s i f i c a t i o n o f t h e a - a d r e n o c e p t o r was m o d i f i e d as new e v i d e n c e was p r e s e n t e d . As e a r l y as 1956 i t was r e p o r t e d t h a t n o r a d r e n a l i n e o v e r f l o w f r o m t h e c a t s p l e e n was i n c r e a s e d i n t h e p r e s e n c e o f t h e o - a n t a g o n -i s t d i b e n a m i n e (brown and G i l l e s p i e , 1 9 5 b ) . I t was s u b s e q u e n t l y shown t h a t a number o f o t h e r a - a n t a g o n i s t s i n c l u d i n g p h e n t o l a m i n e ( L a n g e r , 1970; F a r n e b o ano H amberger, 1970) and p h e n o x y b e n z a m i n e ( S t a r k e e t a l . , 1971; L a n g e r , 1970; F a r n e b o and h a m b e r g e r , 1970; L a n g e r ana V o g t , 1971) were a l s o c a p a b l e o f i n c r e a s i n g n o r a a r e n a l i n e o v e r f l o w i n a u c e a by n e r v e s t i m u l a -t i o n . A number o f t h e o r i e s were p u t f o w a r a t o a c c o u n t f o r t h i s p r o p e r t y o f a - a a r e n o c e p t o r a n t a g o n i s t s . I t was g e n e r a l l y b e l i e v e d t h a t t h e a n t a g o n i s t s i n c r e a s e d o v e r f l o w by i n h i b i t i n g t h e m e t a b o l i s m and n e u r o n a l u p t a k e o f n o r a d r e n a l i n e ( L a n g e r , 1970) r a t h e r t h a n a f f e c t i n g t h e amount o f t h e t r a n s -m i t t e r r e l e a s e d . However, i t was s o o n f o u n d t h a t p h e n o x y b e n z a m i n e was a b l e t o i n c r e a s e n o r a d r e n a l i n e r e l e a s e i n a d d i t i o n t o i n h i b i t i n g n e u r o n a l and e x t r a n e u r o n a l u p t a k e ( L a n g e r , 1 9 7 1 ) . In f a c t t h e i n c r e a s e i n n o r a d r e n a l i n e o u t p u t o c c u r r e d a t l o w e r d o s e s t h a n t h a t s u f f i c i e n t t o c a u s e u p t a k e b l o c k a d e ( S t a r k e e t a l . , 1 9 7 1 ) . D'OYLEY, H. M. 7 Starke et a l . (1971) proposed that an increase in the stimulat ion- in-duced release of noradrenaline may be a property common to a-receptor block-ers. Yet, a causal relationship between the blockade of the post-junctional response and the increase in transmitter release was excluded. An increase in transmitter release was observed in the perfused rabbit heart where the receptors of the effector organ are primarly of the e^-subtype (Starke et a l . , 1971). As a result, the hypothesis of o-meaiateo, presynaptic regula-tion of noradrenaline release was developea. According to this hypothesis, noradrenaline released by nerve stimulation activates presynaptic receptors and thereby regulates its own release by a negative feedback mechanism (Starke, 1972). Substantiating the hypothesis was the finding that a-recep-tor agonists such as phenylephrine, oxymetazoline ana naphazoline dose-dependently decreased the stimulation-induced overflow of noradrenaline from perfusea rabbit heart (Starke, 1972). Langer (1974) proposed that the a-aarenoceptor be c lass i f iea on the basis of anatomical location with the postsynaptic receptor being designated a^ ana the regulatory presynaptic receptor, a^. By classifying a-receptors on the basis of anatomical location, Langer (1974) assumed homogeneity in the postsynaptic a-receptor s ites. Bently et a l . (1977) found that the pressor response in pithed rats and anaesthetized cats to exogenous noradrenaline and sympathetic nerve stimula- tion was resistant to prazosin, an a^-antagonist, suggesting the existance of two dist inct postsynaptic receptor types. Drew and Whiting (1979) found that the vasopressor response to exogenous noradrenaline was part ia l ly susceptible to both prazosin ana yohimbine. Administered concurrently, the pressor response to adrenaline was fu l l y blocked by these a-receptor antag-onists though neither could abolish the response when administered separately D'OYLEY, H. M. 8 (Flavahan ana Mcbrath, 1980). These observations lea to the c las s i f i ca -tion of the postsynaptic receptor into o^- as well as the a^-subtypes, both of which induce constriction of the vascular smooth muscle (Timmermans et a l . , 1979; Docherty ana McGrath, 1980; Timmermans ana van Zwieten, 1980). The aj-adrenoceptor exhibits high sensit ivity to phenylephrine ana methoxamine ana the responses are inhibited by prazosin. The postsynaptic o^-adrenoceptor, similar in character to those found presynaptically, is stimulated by agonists such as clonidine, BHT-920 ana BHT-933 and is antag-onizea by yohimbine and rauwolscine. 1.3 Directly Acting Smooth Muscle Relaxants Dilation of the peripheral vasculature may occur in a receptor-mediatea or a receptor-independent manner. by interacting with specific receptor sites on the smooth muscle membrane, for example, e-adrenoceptor agonists ana a-aarenoceptor antagonists will lower blooa pressure when aaministerea intravenously. Compounas that exert their effects at the level of the smooth muscle without interacting with plasma membrane receptors, may also incuce dilation of the peripheral vasculature. 1.3.1 Nitroglycerin. Nitroglycerin was f i r s t introauced into c l in i ca l practice by fourrel (1897) for use in the treatment of angina pectoris, however, neither the haemoaynamic nor the intracel lular mechanisms by which the compound produces its therapeutic effects have been fu l ly elucidated. It has been suggested that nitroglycerin's c l in ica l effectiveness is the result of: di lation of large coronary arteries resulting in an increase of driving pressure and blood flow to myocardial ischaemic areas, the resistance vessels of which are already maximally dilated as a result of hypoxia (Winbury et a l . , 1969); arterial di lation producing a decrease in blood pressure without a concomitant increase in cardiac output, thereby D'OYLEY, H. M. 9 decreasing cardiac work (Eldridge et a l . , 1955; Ito ana Hirakawa, 1984); ana/or, venodilation producing a decrease in venous return and caraiac output (Mason and Braunwald, 1965; Mil ler et a l . , 1976). Intracel lularly, the generally accepted scheme for smooth muscle relaxa-tion by the nitrovasodi1ators was proposed by Ignarro in 1981 (citea in Kreye, 1984) in which the key step is the biotransformation of the organic nitrate, nitroglycerin, to glycerol d in i t r i te and inorganic n i t r i te , N0~ — a step which occurs in the presence of cysteine. Nitrite is then converted to n i t r i c oxiae, NO", which has been shown to stimulate soluble guanylyl cyclase and increase cbMP levels (Arnola et a l . , 1977). Relaxation is believed to be mediated through cbMP-depenoent dephosphoryla-tion of the myosin light chain (Rapoport et a l . , 1983). Proviaing further inairect eviaence for this hypothesis is the fact that that the generation of radiolabeled glycerol d in i t r i te and cbMP by n i tro- glycerin precedes smooth muscle relaxation in the rabbit aorta ano bovine pulmonary artery and vein (Kawamoto et a l . , 1987; Brien et a l . , 1986). 1.3.2 Soaium nitroprussiae. Johnson (1929) was the f i r s t to aissociate the powerful low-aose arterial actions of sodium nitroprusside from its toxic effects. Since then the drug has been c l i n i ca l l y useful in the treatment of hypertensive emergencies (Page et a l . , 1955; Palmer and Lasseter, 1975). From whole animal studies Johnson (1929) and Page et a l . (1955) concluded that sooium nitroprusside exerts its hypotensive action by direct relaxation of the peripheral vasculature rather than by inducing cardiac depression, central vasomotor depression, or affecting the sympathetic nervous system. The substance is selective for vascular smooth muscle; there is no effect on uterine or duooenal smooth muscle at doses that are markedly hypotensive (Page et a l . , 1955). By using a series of D'OYLEY, H. M. 10 nitrogen-containing compounds, Johnson (1929) attributed the hypotensive effect of sodium nitroprusside to the nitroso group rather than the cyanogen. This was subsequently confirmed when i t was found that the liberation of the cyanogen did not occur until after the onset of vascular relaxation (Page et a l . , 1955). These early findings correlate well with Ignarro's proposed scheme (cited in Kreye, 1984) for the relaxation mediated by the nitrovasodilators. Since nitroprusside spontaneously liberates n i t r i te in aqueous solution the compound bypasses the cysteine-dependent biotransformation described above for nitroglycerin. 1.3.3 Hydralazine. Hydralazine was introduced in the early 1950's and was soon found to be effective in reducing arterial pressure in both animals (Craver et a l . , 1951; Moyer et a l . , 1951) and man (Freis et a l . , 1953). The arterial hypotension is accompanied by a marked decrease in total peripheral resistance and an increase in HR and cardiac output (Freis et a l . , 1953; Ablad, 1963). These cardiac effects make this compound unsuitable for the treatment of hypertensive patients who have angina pectoris (Oudson et a l . , 1956). The origin of the increase in HR and cardiac output is not apparent. It has been suggested that it may be part ia l ly due to: a direct stimulatory effect on the heart which appears to be mediated by stimulation of the e-adrenoceptor (Moyer et a l . , 1951; Khatri et a l . , 1977; Riggs et a l . , 1978); a ref lexly mediated stimulation of the heart secondary to the arterial hypotension (Khatri et a l . , 1977); or, an increase in venous return (Freis et a l . , 1953). Early studies suggested that the compound acted centrally to produce its effects since it induced minimal hypotension in spinal animals (Craver et a l . , 1956). It was also suggested that the compound exerted its hypotensive action by producing peripheral adrenergic blockade (Moyer et a l . , 1951). The work of Stunkard D'OYLEY, H. M. 11 et a l . (1954) in spinalis transected and sympathectomized hypertensive patients, and that of Ablad and Mellander (1963) in the sympathectomized cat, refuted the above theories and established that hydralazine exerts its hypotensive effects by acting directly on the vascular smooth muscle. The intracel lular mechanism by which hydralazine relaxes smooth muscle is not yet unaerstooa. It has been proposed that hydralazine relaxes arterial and venous smooth muscle by interfering with the release o f . in t ra -2+ cel lu lar Ca which is needed to support vascular contraction (Lipe and Moulds, 1961). The vasodilator has been shown to aecrease phosphorylation of the myosin light chain in a Ca^+-depenoent manner indicating that hyaralazine may influence the contractile proteins by an action on the regu-latory Ca^ +-binding protein calmodulin (Jacobs, 1984). Khayyal et a l . (1981) were unable to relax KCl-induced tone in the rabbit renal artery even though hydralazine at the same concentration coulo effectively relax noradrenaline-inducea tone. These authors suggested that the vasoailatory effects of hydralazine may be partial ly dependent on the pharmacological nature of the induced tone rather than a ubiquitous action on electromechan-ical coupling. However, i t should be notea that Lipe and Moulds (1981) were able to shift the dose-response curves to a number of agonists, including KC1, when tested in vitro on human arteries and veins. 1.4 Methods of Determining Venous Capacitance The majority of in vivo studies assess constrictor or di lator effects in the capacitance vessels indirect ly. Venous constriction or di lation in response to various stimuli is inferred either by changes in cardiac output, changes in the volume of an extracorporeal reservoir at constant cardiac output as measured by the long c i rcu i t technique, or by changes in the venous tone of a single vascular bed as measured by plethysmography. As D'OYLEY, h. M. 12 discussed above, however, cardiac output is controlled by a number of fac-tors only one of which is vascular compliance. While i t is true that an increase in venous tone can raise cardiac output (Guyton, 1955), an increase in cardiac output does not necessarily indicate venoconstriction (Greenway, 1982). The long c i rcu i t technique involves routing total venous return through an extracorporeal reservoir and back into the right atrium, the pulmonary artery, or a systemic artery via a pump which holds the cardiac output con-stant. A change in reservoir volume is interpreted to be due to a change in the unstressed blooo volume of the experimental subject. The method, however, requires the use of anaesthesia and extensive surgery both of which are known to affect the cardiovascular reflexes. Also Greenway (1982) found that i f cardiac output was altered by any means, the maintenance of an a r t i -f i c i a l l y constant value, in i t se l f , results in modification of the reservoir volume. Thus, fluctuations in reservoir volume indicate alterations in cardiovascular status without revealing the source of the change. Similar-ly, the use of plethysmography has its l imitations. Often plethysmography is used as a means to monitor the forearm venous tone of human subjects (Miller et a l . , 197b; Coll ier et a l . , 1978; Imhof et a l . , 1980). human cutaneous veins, however, are unoer thermoregulatory control and are only minimally influencea by the baroreceptor reflex that affects the deep veins (Donegan, 1921; kothe, 1983). Indeed it has been shown that changes in the venous tone of the human forearm do not accurately reflect the alterations in total body venous tone (Gerson et a l . , 1982). 1.4.1 Mean circulatory f i l l i n g pressure. The objective of my research was to investigate the effects of various vasodilators in conscious unrestrained rats using mean circulatory f i l l i n g pressure (MCFP) as an index D'OYLEY, H. h. 13 of total body venous tone (Guyton et a l . , 1954; Guyton, 1955; Grodins, 1959). MCFP, the pressure that would exist i f there were instantaneous equilibration of blood throughout the circulation is the function of a number of factors including blood volume and the arterial and venous compliances. MCFP is a concept based on the following equations formulated by Grodins (1959). U = (P a - P v)/K (a) P f l = BV /Ca (b) d a P y = BVy/Cv (C) BV = BV + BVW (d) a V where 0 = cardiac output during steady state; P f l and P v = arterial and venous pressures, respectively; R = systemic vascular resistance; C ano C = arterial and venous compliances, respectively; BV d v o and bVy = arterial ana venous blood volumes. By rearranging these equations, equations (e) ano (f) are obtained: P. = BV/(C f l + C v) c v f c/ (c a + C y ) (e) P v = B V / ( C a + C v ) - C a R Q / ( C a + C v ) (f) It is apparent that at 0 = 0 , P f l = P v = BV/(C& + C y ) . Therefore when the circulation is stopped (Q = 0), an equilibrium pressure can theore-D ' O Y L E Y , H. M. 14 t i c a l l y be obtained throughout the c irculat ion, Guyton (1955) callea this equilibrium pressure MCFP. It is apparent from equations (e) and (f) that MCFP is proportional to total blood volume ana inversely proportional to the overall compliance of the systemic c irculat ion. Since the compliance of the veins is much greater than that of the arteries (Guyton et a l . , 1973, Samar ana Coleman, 1978; Yamamoto et a l . , 1980), changes in MCFP ref lect predomin-antly alterations in total body venous tone provided that blood volume remains constant. An increase or decrease in MCFP represents a shift in the capacitance c u r v e — venoconstriction or venodilation respectively, unless measurements of MCFP are made over a range of blood volumes, one cannot determine whether the shift is due to a change in unstressed blood volume, or vascular compliance or both. MCFP ooes not ref lect alterations in the venous tone of a single vascular bed. Local effects may redistribute blood from one venous bed to another ano may not affect total systemic compliance or the total unstressed volume. Rather, MCFP reflects a generalizea veno-constriction or venoailation response throughout the circulat ion. A number of techniques have been aeveloped to measure MCFP in experi-mental animals. In al l cases blood flow is temporarily arrested ano the arterial and venous pressures brought to equilibrium, MCFP. In the anaes-thetized, open-chest dog where the technique was original ly aevelopea (Guyton et a l . , 1954), circulation was stoppea by the injection of acetyl-choline or by ventricular f i b r i l l a t i o n . Equilibrium was rapialy established with an arteriovenous pump so that the sympathetic reflexes, init iated 6 to 8 seconds after the cessation of flow, could not alter venous tone; central venous pressure was measured via a cannula inserted into the vena cava 4-5 seconds following circulatory arrest (Guyton et a l . , 1954; Guyton et a l . , 1973; Drees and Rothe, 1974: Ito and Hirakawa, 1984). Guyton's method for D'OYLEY, h. Ni. 15 stopping c irculat ion, however, was founa to be ineffective in the rat. Consequently, Samar and Coleman (1978) oevelopea a method for measuring MCFP in conscious rats in which the heart was stopped by means of an externally operated hydraulic occluaer about the pulmonary artery. Although effective, this procedure required open-chest surgery and a 10 to 14 day recovery period before experiments could be performed. In 1980 Yamamoto et a l . introauced a method for measuring MCFP in conscious, unrestrained rats requiring only minor surgery ana short recovery periods. We have verif ied that reproducible MCFP measurements can be obtainea in conscious rats ana this method has previously been usea in our laboratory to monitor the changes in total boay venous tone induced by the administration of calcium antagonists (Waite et a l . , 1988) ana various pressor agents (Pang ana Tabrizchi, 1986; Tabrizchi ana Pang, 1987). 1.5 The Nature of the Problem The aim of the investigation was to examine the haemoaynamic effects of various receptor ana non-receptor meaiatea vasoailators in conscious unrestrainea rats using MCFP as an inaex of total boay venous tone. 1.5.1 Directly acting smooth muscle relaxants. Vasodilator arugs such as hyaralazine (Ablaa, 1963; Leenen and keeves, 1987), sodium nitro-prusside (Ross and Cole, 1973; Pagani et a l . , 1978), e-receptor agonists (Leenen and Reeves, 1967) and calcium antagonists (Taira et a l . , 1980; 0gilvie,1985) have been shown to increase venous return or cardiac output. The mechanisms responsible for the increase in venous return may be a reauc-tion of the flow resistance (arteriolar dilation) and/or venoconstriction due to a reflex increase in venous tone (Ito ana Hirakawa, 1984). lnaeea i t has been shown that in intact conscious rats calcium antagonists causea ai lation of the resistance vessels that was accompanied by venoconstriction b'UYLEY, h. M. 16 (Waite et a l . , 1988). When sympathetic nerve activity in these rats was attenuated with the ganglionic blocker, hexamethonium, the calcium antagon-ists exerted a venodilator effect. In these experiments we assessed the direct and reflex effects of the vasodilators nitroglycerin, sodium nitroprusside and hydralazine on MAP, HR and MCFP. Using Guyton's method, Ito and Hirakawa (1984) were able to demonstrate that nitroglycerin decreases MCFP in anaesthetized open-chest dogs. However, the effect of this compound on MCFP has not previously been determined in conscious animals. Yamamoto et a l . (1980) was able to show that a single dose of hydralazine decreasea MAP but aid not change MCFP. Since a decrease in MAP should produce activation of the sympathetic nervous system via the baroreceptor reflex i t is possible that hydralazine may have had a venodilator effect that was obscured by reflex venoconstriction. Therefore, we usea Yamamoto's method to compare the dose-response effects of nitroglycerin, hydralazine ana sodium nitroprussiae in intact rats as well as rats subjectea to ganglionic blockaae by continuous infusion of hexameth-onium. 1.5.2 a-aarenoceptor antagonists ana hexamethonium. This set of experiments was designed to investigate the role of o-adrenoceptors in the venous bea using selective antagonists of the a^- and o^,-receptors. Two classes of a-adrenoceptors have been shown to exist in the peripheral vascu-lature at the post-junctional level (Drew ana Whiting, 1979). Using j_n vitro techniques the two subtypes, both of which mediate contraction of vascular smooth muscle, have been identified in the arteries and veins of various species (De May and Vanhoutte, 1980; De May and Vanhoutte, 1981; Shoji ana Shigei, 1983; Alabaster et a l . , 1985). With respect to the resistance vessels i t has been shown in vivo that the two a-adrenoceptors D'OYLEY, h. M. 17 contribute relat ively equally to the maintenance of b l o o a pressure in cons-cious ana pithed rats (Kobinger ana Pichler, 198 ; Pang and Tabrizchi, 1986). _ln vitro studies suggest that the two a-aorenoceptors are not distributed equally throughout the venous system; the c^-receptor is found predominantly in veins (Docherty and Starke, 1982; Steen et a l . , 1984; Tornebrant et a l . , 1985). However, the results from in vivo studies measuring MCFP have yielded confl icting results. In sedated dogs i t has been found, using equipressor doses, that c^-receptor stimulation increased MCFP to a greater extent than c^-receptor stimulation ( A p p l e t o n et a l . , 1986). In contrast, oose-response curves for o^ - and ^ - s e l e c -tive agonists constructed in this laboratory using Yamamoto's method show that while stimulation of both o-receptors increased MAP, MCFP was increased by stimulation of the ^-adrenoceptor alone (Pang and Tabrizchi, 198b). To further examine the relative roles of c^-adrenoceptors on venous tone, we examined the haemodynamic effects of phentolamine, prazosin and rauwols-cine — nonselective, o^-selecti ve, ana o^-selective adrenoceptor antag-onists — on basal total body venous tone. For a comparison with these agents which al l act at the postjunctional level, we also determined the dose-response relationship o f the ganglionic blocker, hexamethonium. A second set of experiments was conducted in order to determine the effects of prazosin and rauwolscine on MCFP when total body venous tone was raised as a result of reflex venoconstriction induced by the concurrent administration of hydralazine. D'OYLEY, H. M. 18 2. METHOD 2.1 Surgical Preparation MCFP was determined using the method of Yamamoto et a l . (1980). Male Sprague-Dawley rats (320 - 440g) were anaesthetized with halothane. Cannulae were inserted into the i l i a c artery to record arterial pressure ana hk, into the i l i a c vein for the infusion of drugs and into the inferior vena cava (via another i l i a c vein) for the measurement of central venous pressure by a pressure transducer (P23DB, Gould Statham, CA, u. S. A.). A sal ine-f i l l e o balloon-tippeo cannula was insertea into the right atrium via the right external jugular vein, as previously aescribea (Pang et a l . , ly8b). The cannulae were f i l l e a with heparinizea normal saline (25 l.u./ml), tun-nelled subcutaneously to the back of the neck, exteriorized ana securea. The rats were recoverec for at least lb hours before further use. 2.2 Measurement of MCFP MCFP was aeterminea in conscious, unrestrained rats by inflating the balloon in the right atrium in order to temporarily stop the circulat ion, within 5 seconas after circulatory arrest, the central venous pressure increased to a plateau value, referred to as the venous plateau pressure (VPP), while MAP decreased to a f inal value, referred to as the final arterial pressure ( F A P ) . 2.3 Experimental Protocol 2.3.1 Directly acting smooth muscle relaxants. Rats were randomly assigned to eight groups (n = 6 per group). In the f i r s t study individual dose-response curves (MAP, HR, and MCFP) were constructed for nitroglycerin (1.7 x 10~9 to 5.6 x 10" 8 mol/kg/min), hyaralazine(2.5 x 10~8 to -7 8 8.U x 10 mol/kg/min), ana soaium nitroprusside(1.3 x 10 to D'OYLEY, H. to. 19 4.3.x 1U~7 mol/kg/min) or normal saline (0.9 to 27 x 10"^ ml/min). The vascular parameters were measured following a 10 min infusion of the appropriate substance or vehicle. A recovery period of 4 min, during which infusion was stopped, was allowed between doses. ln the secona study, the effects of the vasodilators and vehicle on vascular parameters were examined again in the presence of a continous infu-sion of hexamethonium (2.4 to 5.8 x 10~7 mol/kg/min). The hexamethonium infusion, through the cannulae in the inferior vena cavae, was started 10 min prior to commencing the dosing schedule for the test substances to allow the system to equilibrate. The oose of hexamethonium infused was adjusted at the beginning of infusion by monitoring the reduction in the tachycardia proaucec by iv injection of 1-2 pg acetylcholine. Experiments in which the acetylcholine-induceo tachycardia was reduced by less than 50% during the course of the experiment were rejected. The vascular parameters were meas-ured prior to hexamethonium infusion, after 10 min hexamethonium infusion, ano then after each subsequent 10 min infusion of various ooses of the test substance. Infusion of hexamethonium was temporarily halted during each measurement of M C F P . 2.3.2 a-adrenoceptor antagonists ana hexamethonium. The relative se lect iv i t ies of the a^- and a2~adrenoceptor antagonists were determined using male Sprague-Dawley rats (390 - 450g) anaesthetized with pentobarbital. Cannulae were insertea into the i l i a c vein for the infusion of drugs ana into the i l i a c artery to recora arterial pressure and HR with a pressure transducer (P23DB, Gould Statham, CA, U. S. A.). Prazosin (1.8 x 10" 7 mol/kg/min) or rauwolscine (1.3 x 10~7 mol/kg/min) was infused intravenously at a rate of 0.0265 ml/min. The pressor responses to bolus injections of methoxamine (0.25 mg/kg) ana bHT-933 (1 mg/kg) were examined D'OYLEY, H . M. 20 prior to and following a 10 minute infusion of the antagonist and at 5 minute intervals thereafter. Hats were then randomly assigned to eight groups. In the f i r s t stuay (n=6 per group) individual dose-response curves (MAP, HR, and MCFP) were G _7 constructed for phentolamine (9.4 x 10 to 9.4 x 10 mol/kg/min), 9 —7 —9 prazosin (7.5 x 10 to 7.5 x 10 mol/kg/min), rauwolscine (7.3 x 10 to 7.3 x 10" 7 mol/kg/min), hexamethonium (3.2 x 10~b to 7.1 x 10" b mol/kg/min), and the vehicle which was aciait iec (0.25% acetic acid) 5% glucose (0.9 to 27 x 10'^ ml/min). The vascular parameters were meas-ured following a 7 min infusion of the appropriate aose of drug or vehicle. The substances were administered continuously with no recovery period between doses. In a second part of this study, the effects of prazosin, rauwolscine (n=7 per group) and the vehicle (n=6) on vascular parameters were examined again in the presence of a continuous infusion of hydralazine (1.0 x 10 ^ mol/kg/min). The hydralazine infusion, through the cannulae in the inferior vena cavae, was started 15 min prior to commencing the dosing schedule for the test substances to allow the system to equilibrate. 2.4 Drugs M i l drugs were made fresh daily. Nitroglycerin (Roussel Canaoa Inc.), sodium nitroprusside (Fisher Scient i f ic) anc hydralazine (Sigma Chemical Co. MO, U.S.A.) were dissolved in normal saline. Phentolamine hydrochloride (C1BA Pharmaceutical Co., Summit, NJ), prazosin hydrochloride (Pfzier Central Research, Sandwich, England), and rauwolscine hydrochloride (Carl Roth Gmb H and Co., NY) were dissolved in acidif ied (0.25% acetic acic) 5% glucose. When studying the dose-response characteristics of hexamethon-ium bromioe (K and K Laboratories CA, U.S.A.) the compound was also dissolved in acidif ied glucose. Hexamethonium was dissolved in heparinizec (5 lU/ml) normal saline when a single dose was administered continuously. D'uYLEV, H. M. 21 2.5 Calculations MCFP was calculateo from the equation of Samar ana Coleman (1978) u s i n g a value of 1/60 for the arterial to venous compliance ratio (Yamamoto et a l . , 1980). MCFP = VPP + 1/60 (FAP - VPP) 2.6 Stat ist ical analysis A l l results were analyzea by use of analysis of variance; complete ran-dom design for comparisons between groups, and block design, for comparisons within the same group. For multiple comparisons of data, Duncan's multiple range test was usea to compare group means. In a l l cases, a probability of error of less than 0.05 was selectea as the criterion for stat i s t ica l significance. 3. ktSULTS 3.1 D i r e c t l y A c t i n g Smooth Muscle Relaxants 3.1.1 Control values. Table 1 shows the control values of MAP, hk ana MCFP for groups 1-4. There were no aifferences in control values for e i t h e r MP or MCFP among the groups. With r e s p e c t to hk, there were no aifferences among the dilator groups though they al l differed signif icantly from the vehicle control. Table 2 shows the control values before ana after hexamethonium infusion for groups 5-8. Prior to hexamethonium infusion there were no aifferences among groups for control values of MAP, hk, ana MCFP. Hexamethonium decreasea MAP in al l groups. The decrease was significant only for the hydralazine group, however. HR was not s ignif icantly affected. MCFP was lowered in a l l groups though only the decreases observed in the nitroglycer-in and vehicle groups were signif icant. D'uYLEY, H. M. 22 TAbLE 1. Control values of MAP, hk and MCFP In Intact kats MP (mniHg) Hk (beats/min) MCFP (mmHg) Saline 113 ± 2 410 ± 5 b.2 0.1 Nitroglycerin 116 ± 3 358 ± 1U 6.2 ± U.2 Hydralazine 111 1 352 ± 10 5.8 ± 0.1 Sodium nitroprussiae 114 ± 5 37b ± 9 6.2 ± 0.2 Each value represents the mean * SEM; n = 6. U'OYLEY, H. M. 23 TABLE 2. Contro l Values of MAP, Hk Ana MCFP in Intact ana Hexamethonium I rea tea Rats . MAP (mmHg) HR (beats/min) MCFP (mmHg) Sa l i ne No hexamethonium hexamethonium N i t r o g l y c e r i n No hexamethonium hexamethoni um Hydra laz ine No hexamethonium Hexamethonium Sooium n i t r o p r u s s i d e No hexamethonium Hexamethonium 103 * 4 97 ± 5 lUb ± 3 1U3 * 3 111 * 2 94 * 3 113 * 7 105 * 4 400 * 29 370 ± 22 352 * 12 350 * 10 37b * 16 373 * 13 345 * 10 345 * 10 6.U * 0.1 5.3 ± 0.3 6.3 * 0.1 5.6 * 0.2 6.3 * 0.1 5.9 * 0 .3 6.1 * 0.2 5.8 * 0.4 Each value represents the mean ± StM; n = 6. D'OYLEY, H. M. 24 The infusion of the vehicle had no significant effect on MAP or HK (Fig. 1). MCFP, however, aeclinea graaually with time ana the decrease was founa to be significant at 120 min. ln the presence of hexamethonium, HR was not altered but both MAP and MCFP gradually decreased with time. Neither one of these effects was significant when compared to the corresponding post- hexa-methonium control value (Table 2). 3.1.2 Nitroglycerin. In intact rats nitroglycerin s l ightly but not s ignif icantly decreased MAP and i t altered neither HR nor MCFP (Fig. 2). In the presence of hexamethonium, nitroglycerin aid not alter hk though it causea a small but significant decrease in MAP at doses four ana six. MCFP was cose-aepenaently aecreasea ana the decrease was significant at the highest three doses. The decreases in MCFP by the fourth ana f i f t h dose of nitroglycerin in the presence of hexamethonium were also significantly greater than the corresponding values in the vehicle time-control group (Fig. 1). 3.1.3 Nitroprussiae. Nitroprussiae aose-aependently decreased MAP ana increased Hk but aia not s ignif icantly alter MCFP in intact rats (Fig. 3). The respective aecreases in MAP ana increases in Hk were significant at the highest three doses. After treatment with hexamethonium, nitroprussiae causea a significant decrease in MAP at a l l doses except the f i r s t , the decreases were signif icantly greater than the corresponding aecreases in intact rats, hexamethonium abolishea the nitroprussiae-inauced tachycaraia. MCFP was dose-dependently decreased by nitroprusside and, at the highest dose, MCFP was s ignif icantly lower than the post-hexamethonium MCFP control value (Table 2). This decrease in MCFP was also signif icantly greater than the corresponaing aecrease in the time-control group (Fig. 1). D'OYLEY, H. M. 25 5 i 10 • -1.0 J 0 40 80 120 Time (min) Figure 1 D'OYLEY, H. Nl. 26 F i g . 1. Dose-response curves f o r the e f f e c t s of normal s a l i n e on mean a r t e r i a l pressure (MAP), heart r a te (Hk) ana mean c i r c u l a t o r y f i l l i n g p re s -sure (MCFP) in consc ious , i n t a c t ( • ) or hexamethonium-treatea ( T ) r a t s , represented as change from con t ro l va lues . Each po int represents the mean * SEM; n = 6. * denotes s i g n i f i c a n t d i f f e r e n c e from c o n t r o l va lues . D'OYLEY, H. Figure 2 D'OYLEY, H. M. 28 F i g . 2. Dose-response curves f o r the e f f e c t s of n i t r o g l y c e r i n on mean a r t e r i a l pressure (MAP), heart r a te (HR) and mean c i r c u l a t o r y f i l l i n g p re s -sure (MCFP) in consc ious , i n t a c t ( • ) or hexamethonium-treateo ( • ) r a t s , represented as change from con t ro l va lues . Each po int represents the mean ± SEM; n = b. * denotes s i g n i f i c a n t d i f f e r e n c e from con t r o l va lues . D'OYLEY, H. h. 30 F i g . 3. D o s e - r e s p o n s e c u r v e s f o r t h e e f f e c t s o f s o d i u m n i t r o p r u s s i a e on mean a r t e r i a l p r e s s u r e (MAP), h e a r t r a t e (HRJ ana mean c i r c u l a t o r y f i l l i n g p r e s s u r e (MCFP) i n c o n s c i o u s , i n t a c t ( • ) o r h e x a m e t h o n i u m - t r e a t e a ( • ) r a t s , r e p r e s e n t e e as c h a n g e f r o m c o n t r o l v a l u e s . E a c h p o i n t r e p r e s e n t s t h e mean * SEM; n = b. * d e n o t e s s i g n i f i c a n t a i f f e r e n c e f r o m c o n t r o l v a l u e s . b'UYLEY, H. to. 31 3.1.4 Hydralazine. In intact rats, hydralazine dose-dependently decreased MhP and increased toCFP (Fig. 4). The decrease in MAP was s i gn i f i -cant at the highest four doses whereas the increase in toCFP was significant at the highest three doses. HK was s l ightly but not s ignif icantly increased. After hexamethonium treatment, hydralazine caused a dose-oependent decrease in toAP that was significant at the last three ooses. HK was not affectea whi.le toCFP was signif icantly decreased ,at doses two, three, f ive and six. However, these decreases were not s ta t i s t i ca l ly significant when compared to the corresponding post-hexamethonium time-control values (Fig. 1). 3.2 a-adrenoceptor Antagonists and Hexamethoniuni 3.^.1 Selectivity of prazosin ano rauwolscine. It was found that at ooses up to ano inducing 1.8 x 10 _ b mol/kg, prazosin selectively reduced the pressor response to methoxamine without decreasing the pressor response to BHT 933. For rauwolscine, tne pressor response to methoxamine was not reduced until a dose greater than 1.9 x 1U fa mol/kg was administered. Therefore, except for the f inal cose, a l l of the doses of prazosin and rauwolscine used were selective for the a^- ano the a^-adrenoceptor, respectively. 3.2.2 Control values. Table 3 shows tne control values of MAP, HK anc MCFP for groups 1-5. There were no significant differences among groups for the MAP control values. With respect to HR, only the phentolamine control differed signif icantly from the vehicle group. The MCFP control values for the phentolamine ano rauwolscine groups were signif icantly lower than the corresponding vehicle control. Table 4 shows the control values before anc after hydralazine infusion for groups b-6. Prior to hyaralazine infusion there were no differences among groups for control values of MMP or HK. The MCFP control for rauwols-Change In MCFP (nmflg) Change in HR (beats/min) Change In MAP (nmHg) • i o -< CO rv D'UYLEY, H. M. 33 F i g . 4 . Dose-response curves f o r the e f f e c t s of hydra laz ine on mean a r t e r i a l pressure (Map), heart r a te (hk) ana mean c i r c u l a t o r y f i l l i n g pressure (MCFP) in consc ious , i n t a c t ( • ) or hexamethonium-treated ( • ) r a t s , represented as change from con t r o l va lues . Each po int represents the mean ± SEM; n = 6. * denotes s i g n i f i c a n t d i f f e r e n c e from con t r o l va lues . b'OYLEY, H. M. 34 TABLE 3. Contro l Values of MAP, Hk ana MCFP in Intact Rats. MAP (mmhg) Hk (beats/min) MCFP (nimhg) Veh i c l e 1U4 ± 5 368 ± 10 b.l ± 0.2 Phentolami ne 1U2 ± b 397 ± 7 5.7 ± 0.1 Prazos in 99 ± 3 385 ± b 6.2 ± 0.2 kauwolsc ine 103 ± 3 38b ± 5 5.6 ± U.2 hexamethoni um 97 ± 3 394 ± 21 6.1 ± 0.2 Each value represents the mean * SEM; n = 6. D'OYLEY, h. M. 35 TABLE 4. Control Values of MAP, hk and MCFP in Intact and Hydralazine Treated kats. MAP (mmHg) Hk (beats/min) MCFP (mmHg) Vehicle No Hydralazine 97 * 2 378 * 14 5.5 ± 0.1 hydralazine 73 * 4* 487 ± 33* 0.4 ± 0.3* Prazosin No hydralazine 101 * 4 40b» ± i i 5.8 * U.l Hydralazine 72 * 4* 501 * 10* 6.6 * 0.2* kauwolscine No Hydralazine 103 » 7 397 * 22 6.0 ± 0.1 Hydralazine 77 * 4* 494 * 23* 6.8 * 0.2* Each value represents the mean ± SEM; n = 7 for each group except the vehicle where n = 6. * denotes significant difference from the correspond-ing pre-hydralazine value. U'UYLtY, H. M. 36 cine was signif icantly higher than that of the vehicle group. Infusion of hydralazine signif icantly decreased MAP ana signif icantly increasea hk ana MCFP in a l l groups. After hyaralazine infusion there were no significant differences among groups for values of MP, Hk or MCFP. The infusion of the vehicle haa no significant effect on MP, Hk, or MCFP when administered alone or in the presence of hyaralazine (Fig. 5). 3.2.3 In the absence of hydralazine. Infusion of hexamethonium causea a dose-dependent decrease in MP ana MCFP with a small but not significant dose-dependent decrease in Hk. The decrease in MP was significant at a l l doses while the aecrease in MCFP was significant at a l l but the f i r s t aose (Fig. 6). Phentolamine ana prazosin caused dose-depenaent aecreases in MAP out haa no s ta t i s t i ca l l y significant effect on hk or MCFP. The aecrease in MP by phentolamine was significant at a l l but the f i r s t aose while that inaucea by the infusion of prazosin was significant at a l l doses (Fig. 7). kauwolscine dose-depenaently decreased MP and had no effect on HR. The decrease in MAP was significant at the two highest doses. MCFP was unchanged until the last dose at which time it was signif icantly decreased (Fig. 7). 3.2.4 Prazosin ano rauwolscine in the presence of hydralazine. In the presence of hydralazine the infusion of prazosin caused a slight decrease in MP and hk (Fig. 8). Although MP values after the infusion of prazosin were not s ignif icantly different from the post-hyaralazine control (Table 4), they were signif icantly different from the corresponding vehicle time-control values (Fig. 5, Fig. 6). MCFP, on the other hand, was dose-de-pendently decreased by prazosin and, at the highest two aoses, was s i g n i f i -cantly lower than the post-hydralazine MCFP control value (Table 4). D'OYLEY, h. M. 37 Figure 5 . D'OYLEY, H. M. 38 F i g . b. Dose-response curves f o r the e f f e c t s of the v e h i c l e , a c i d i f i e a g lucose, on mean a r t e r i a l pressure (MAP), heart r a te (hk) ana mean c i r c u l a -t o r y f i l l i n g pressure (MCFP) in consc ious , i n t a c t ( • ) or h y d r a l a z i n e -t r e a t e a ( • ) r a t s , representee as change from con t r o l va lues . Each po int represents the mean ± SEM; n = b. * denotes s i g n i f i c a n t d i f f e r e n c e from con t r o l va lues . Change i n MCFP (mmHg) Change i n HR (beats/min) Change i n MAP (mmHg) Co D'OYLEY, h. M. 40 F i g . b. Dose-response curves f o r the e f f e c t s of hexamethonium on mean a r t e r i a l pressure (MAP), heart r a te (Hk) ana mean c i r c u l a t o r y f i l l i n g pressure (MCFP) in consc ious , i n t a c t ( • ) r a t s , represented as change from con t r o l v a l ue s . Each po int represents the mean * SEM; n = b. * aenotes s i g n i f i c a n t d i f f e r e n c e from con t r o l va lues . Change in MCFP (mmHg) Change in HR (beats/min) Change in MAP (mmHg) D'OYLEY, H. h. 42 F i g . 7. Dose-response curves f o r the e f f e c t s of phentolamine ( • ), p razos in ( • ), ana rauwolsc ine ( • ), on mean a r t e r i a l pressure (MAP), heart ra te (HR) ana mean c i r c u l a t o r y f i l l i n g pressure (NiCFP) in consc ious , i n t a c t r a t s , representea as change from con t r o l va lues . Each po int represents the mean ± SENi; n = 6 . * aenotes s i g n i f i c a n t a i f f e r e n c e from con t r o l va lues . D'OYLEY, h. M. 43 20 r -40 L ~ 60 • I 40 . S 20 \ Log (mols/kg/min x 10 ) Figure 8. b'UYLEY, H. to. 44 F i g . 8. bose-response curves f o r the e f f e c t s of the glucose veh i c l e ( A ), prazos in [ • ), ana rauwolsc ine ( • ), on mean a r t e r i a l pressure ItonP), heart r a t e (HR) ana mean c i r c u l a t o r y f i l l i n g pressure (toCFP) in consc ious , h y a r a l a z i n e - t r e a t e a r a t s , represented as change from con t r o l va lues . Each po in t represents the mean ± SEM; n = 7. * aenotes s i g n i f i c a n t d i f f e r e n c e from con t r o l va lues . D'OYLtY, h. M. 4b kauwolscine, when administered concurrently with hydralazine, had no significant effect on Hk although it aose-depenaently decreased MAP. The decrease in M P was significant at the f inal dose when comparea to the corresponoing post-hydralazine control (Table 4) and, when comparea to the appropriate time-control (Fig. 5, Fig.8), was significant at the last three doses. MCFP was dose-dependently aecreasea by rauwolscine and, at the high-est three doses, MCFP was significantly lower than the post-hydralazine MCFP control value (Table 4). At a l l aoses but the secona, this decrease in MCFP was also s ignif icantly greater than the corresponding aecrease in the prazo-sin group (Fig. 8). 4. DISCUSSION 4.1 Directly Acting Smooth Muscle kelaxants In the intact rat i t was found that nitroglycerin had l i t t l e effect on M P , MCFP and Hk. After impairment of the sympathetic reflexes, however, nitroglycerin induced a small aecrease in MAP ana a marked decrease in MCFP suggesting that nitroglycerin is more effective in di lating the venous than the arteriolar vasculature. lhis finaing is in agreement with _in_ vitro stuaies which have shown that in isolatea vessels the magnituae ana duration of nitroglycerin's relaxant effects is greater in the veins than the arter-ies (Stiefel and Kreye, 1984; MacKenzie ana Parratt, 1977). Although results from in vivo plethysmography studies indicate that nitroglycerin is more effective in di lating the veins than arteries of isolatea vascular beds (Miller et a l . , 1976; Col l ier et a l . , 1978; lmhof et a l . , 1980), in vivo studies using MCFP as an index of venous tone in anaesthetized, open-chest dogs suggest that nitroglycerin is a more effective arteriolar than venous D'uYltY, H. M. 46 dilator (Ito ana Hirakawa, 1984). It was proposed by Ito and hirakawa (1984) that the venodilator effect of nitroglycerin may have been nul l i f ied by reflex constriction init iated by the decrease in MAP. In the present stuay it was founa that endogenous sympathetic tone hao to be attenuated in order to disclose both the arteriolar dilator and venodilator effects of nitroglycerin. hydralazine caused a aose-aepenaent aecrease in MAP that was parallelea by a dose-aependent increase in MCFP when administered to intact rats. Increased venous tone may therefore have been part ia l ly responsible for the hydralazine-inducea increase in cardiac output previously observed in hyper-tensive and healthy human subjects (Freis et a l . , 1953; Ablad, 1963). The observed increase in MCFP was probably a result of reflex sympathetic nerve act iv i ty as it was prevented upon the impairment of ganglionic transmission. During the administration of hexamethonium, i t was found that hydralazine decreased MCFP below control levels. Although the hydralazine-inaucea decrease in MCFP was lower than the vehicle at every dose, the difference was not founa to be signif icant. Therefore, our results suggest that hydralazine has insignificant dilatory effects on the veins in both intact animals, as reflected by the reflexly meaiatea increase in MCFP, as well as in animals with impairea sympathetic reflexes. The results from _in_ vitro stuaies on isolatea vessels (Moulds et a l . , 1981) and j_n vivo studies on single vascular beds (Ablad and Mellander, 1963; Col l ier et a l . , 1978) have shown that the direct effect of hydralazine on various venous preparations is minimal. In intact rats i t was found that sodium nitroprusside causea a dose-de-penaent decrease in MAP that was accompanied by a concomitant increase in HK. The nitroprusside-inaucea f a l l in arteriolar resistance was partial ly D'OYLEY, h. h. 47 opposed by reflex arteriolar constriction in intact rats; the magnitude of the f a l l in MAP was increased in the presence of hexamethonium. The nitro-prusside-inducec tachycardia appeared to have been due to reflex sympathetic nerve activity as i t was abolished by hexamethonium. Unlike hydralazine, however, the increase in sympathetic nerve act iv ity did not induce an appre-ciable increase in MCFP suggesting that nitroprusside has a direct venodila-tor effect that prevents the reflex-mediated increase in MCFP. Indeed, when ganglionic transmission was impaired it was found that nitroprussiae induceo considerable direct venoailation that was not yet maximal at the conclusion of the dosing-interval. Altnough i t has previously been demonstrated in  vitro that nitroprusside effectively relaxes both preconstricteo arteries and veins (Vernaeghe ana Shepherd, 1976; Col l ier et a l . , 197b; Moulas et a l . , 1981; St iefel and Kreye, 1984), the relative magnitude of its arter io-lar ano venous effects vn vivo are somewhat uncertain. Results using the long c i rcu i t technique suggest that nitroprusside is a more effective arteriolar di lator (Oerson et a l . , 1982) while measurements of forearm tone by plethysmography (Miller et a l . , 1976) ano central venous pressure in anaesthetized dogs (Ross ano Cole, 1973) indicate that the compound is effective in di lat ing both the resistance anc capacitance vessels. Uur results suggest that nitroprusside is an effective arteriolar and venous di lator. Under normal conditions, however, the compound's venous effect may be concealed ano its arteriolar effect attenuated, by a reflex increase in sympathetic nerve act iv i ty. 4.2 a-adrenoceptor Antagonists ano hexamethonium ln the intact rat i t was found that both nonselective ano selective antagonism of a-aarenoceptors was effective in s ignif icantly reducing MAP. Although the high doses of phentolamine, prazosin ana rauwolscine causea D'OYLEY, h. M. 48 similar decreases in MAP, the pattern of the vasodilation induced by the three antagonists differea. Prazosin reauceo MAP signif icantly at the f i r s t dose while the i n i t i a l doses of phentolamine and rauwolscine had no effect. Significant reductions in MAP with phentolamine and rauwolscine were observed at the second ana the fourth dose, respectively. Administra- tion of hexamethonium to intact rats was also found to aecrease MAP. The hypotensive effect of the ganglionic blocker appeared to be maximal by the f inal dose and was similar in magnitude to that inducea by the a-adrenocep-tor antagonists. These results indicate that basal tone in the resistance vessels is maintained in part by the stimulation of both a^- ana a^-adrenoceptors. The calibre of the arterioles can he effect ively altered by specif ical ly antagonising the postjunctional a-aarenoceptors or by interfer-ing with sympathetic nerve transmission at the level of the ganglion. The infusion of hyaralazine causea a significant aecrease in MAP to values ranging between 70 ana 80 mmHg. The hypotensive actions of prazosin ana rauwolscine were greatly reauceo when the antagonists were aOministerea to rats treated with hyaralazine. Although both antagonists causea a s igni -ficant decrease in MAP in comparison to the time-control, only rauwolscine could s ignif icantly reduce MAP below the post-hyaralazine control. None of the a-aarenoceptor antagonists s ignif icantly affectea H k . This was unexpected since one would have expected the arterial hypotension to ref lexly increase sympathetic nerve activity ano thereby Hk by e-aarenocep-tor activation. It has previously been demonstrated in our laboratory that the admini-stration of phentolamine, prazosin ana rauwolscine decreases MAP in halo-thane-anaesthetizea surgically-stressea rats (Tabrizchi ana Pang, 1987). The mechanism by which these compounas exert their hypotensive effects 1 D'OYLEY, h. M. 49 appears to d i f fer , however. Both phentolamine ana rauwolscine were founa to decrease caraiac output without a reauction in total peripheral resistance, while prazosin decreased total peripheral resistance without affecting caroiac output (Tabrizchi and Pang, 1987). Thus, in anaesthetized, surgi-cally-stressed rats, the blockade of o^- but not o^-aarenoceptors reduces cardiac output. The mechanisms by which cardiac output is aecreasea is not clear. It has also been shown that MCFP is increasea in the presence of nonspecific a- and specific o^-adrenoceptor agonists but not by agonism of the a^-aorenoceptor (Pang ana Tabrizchi, l&bb). Therefore, we specu-latea that perhaps the blockaae of a.,-aarenoceptors by phentolamine ana rauwolscine reduced venous tone. In the present stuay we examinea the effects of the a-aarenoceptor antagonists on MCFP, an important determinant of cardiac output (buyton, I9bb; buyton et a l . , 1973). In intact rats, rauwolscine signif icantly reduced MCFP at the f inal administered aose. Yet, this oose of the antagon-ist was not founa to be selective for the a^-receptor. Neither phentol-amine nor prazosin decreased MCFP, however, making i t l ikely that a^-adrenoceptor blockaae was primarily responsible for rauwolseine1s ven-ous actions. It was somewhat unexpected to find that phentolamine, a non-selective a-antagonist, haa no effect on MCFP when administered to intact rats. The compound has been shown to decrease caraiac output ana MCFP when administerea to intact anaesthetized rats and aogs, respectively (Tabrizchi ana Pang, 1987; Ito et a l . , 1984). A large increase in plasma catecholamine levels, more than three-fold for adrenaline ana two-fold for noraarenaline, has been founa to occur after the aaministration of phentol-amine to halothane-anaesthetized, surgically-stressea rats. An equihypoten-sive dose of rauwolscine, however, has no effect on noraarenaline levels and D'OYLEY, H. M. 50 a smaller effect on the adrenaline levels (Tabrizchi ana Pang, 1987). Phen-tolamine has been found to cause similar elevations of plasma noraarenaline and adrenaline levels in conscious rats (Tabrizchi et a l . , in press). Therefore, in the present stuay i t is l ikely that the degree of o^-block-ade induced by phentolamine, due to displacement from the a^-acirenoceptor by circulating catecholamines, was less than that induced by rauwolscine. The absence of extensive venooilation in the intact rat after the infusion of the a-adrenoceptor antagonists was not due to a lack of basal venous tone; hexamethonium markedly decreased MCFP unaer identical experi-mental conditions. Previous studies using MCFP as an index of venous tone have shown that while a single dose of hexamethonium wil l decrease MCFP (Drees and kothe, 1974; Yamamoto et a l . , 1980), the aaministration of prazosin or phenoxybenzamine — at ooses that abolish phenylephrine-inaucea increases in MCFP — have no effect on basal venous tone (Ito et a l . , 1984; hirakawa et a l . , 1984). The reason for the Oiscrepancy between the effec-tiveness of ganglionic blocking agents ana postjunctionally acting antagon-ists is unclear. It has been shown in vitro (Docherty and Starke, 1982) ana in vivo (Docherty ana Mcbrath, 1980; Wilffert ...et a l . , 1982) that a^-adrenoceptors are locatea primarily extrajunctionally ana are stimul-ated by exogenously administered or circulating catecholamines, while a^-receptors are found postsynaptically and are stimulated by nerve stimu-lation, however, previous work has demonstrated that i t is d i f f i cu l t to induce venoconstriction in rats using a specific a^-adrenoceptor agonist as opposed to an a2~agonist (Pang and Tabrizchi, 1986). ln this study i t was found that prazosin hao no effect while a high oose of rauwolscine caused a significant decrease in the venous capacitance implying that enao-genously releasee catecholamines have greater influence on a^-aarenocep-D'OYLEY, H. h. 51 tors. There is evidence suggesting that basal venous tone is part ia l ly maintained by sympathetic nerve act iv i ty; venoailation has been observed in the l iver after sectioning of the sympathetic nerves (Donegan, 1921; Rothe, 1983). In the present study we founo that hexamethonium had marked dose-re-lated venodilatory e f f e c t s . One may speculate that hexamethoniurn's actions in the capacitance vessels were related to the innibition of ganglionic sympathetic nerve act iv ity since discrete regulation of venous tone would be d i f f i cu l t to achieve i f MCFP were maintained solely by circulating rather than neurogenically released catecholamines. The discrepancy between the venous response to ganglionic blockade as opposed to antagonism ot the a^-aorenoceptor may simply reflect the ease with which these substances gain access to their respective sites of action. The infusion of hydralazine to intact rats was found to increase both HR ano MCFP. In the f i r s t set of experiments i t was found that the hydrala-zine-induced increase in venous tone was abolished after the administration of a ganglionic blocker implying that the vasodilator increased MCFP by reflex rather than direct venoconstriction. Under these conditions the venous actions of prazosin ana rauwolscine were accentuated; both antagon-ists dose-oependently decreased MCFP suggesting that stimulation of both a-adrenoceptors participate in the maintenance of reflexly induced venous tone. Rauwolscine's venodilatory actions were signif icantly greater than those of prazosin, however. Previous work both _i_n vitro (De May and Vanhoutte, 1980; Steen et a l . , 1984; Alabaster et a l . , 1985) and in vivo (Hirakawa et a l . , 1984; Kalkman et a l . , 1984); Appleton et a l . , 1986) has demonstrated that a^- as well as a^-receptors are present in the capaci-tance vessels of various species. There seems to be variation, however, between species as to the distribution of these receptors within the veins. D'OYLEY, H. M. 52 Although i t has been demonstrated in pithed cats (Kalkman et a l . , 1984) ana in seaatea aogs (Appleton et a l . , 1986) that stimulation of the o^-recep-tor is the dominant factor in aetermining caraiac output and MCFP, respec-t ive ly , our results ana those of others (Kalkman et a l . , 1984; Pang ana Tabrizchi, 1986) demonstrate that although both o-adrenoceptors are present in the veins of rats, stimulation of the o^-adrenoceptor contributes more than its counterpart in the control of basal and reflexly-induced tone. This, however, may simply be a function of the accessiblity of o^- ano c^-adrenoceptor agonists ana antagonists to their relative receptor sites. 4.3 Summary we have founa that in intact rats the aaministration of hydralazine ana nitroprusside, but not nitroglycerin, decreasea MAP. however, MCFP was not aecreasea by these airectly acting smooth muscle relaxants; inaeea, hyaral-azine causea a aose-dependent increase in MCFP. After the impairment of ganglionic transmission by hexamethonium, both nitroglycerin and soaium nitroprusside decreased while hydralazine had no significant effect on MCFP. we concluae tnat soaium nitroprussiae ana hydralazine are effective arterio-lar ailators while nitroglycerin induces only weak di lation of the res is-tance vessels. In the intact animal, the airect venoailator actions of nitroprusside ana nitroglycerin are maskea by endogenous sympathetic tone. When sympathetic nerve activity is attenuatea, the venodilator actions of nitroprussiae ana nitroglycerin are revealed. Hydralazine, on the other nana, has insignificant venoailator effects both in the presence and absence of sympathetic reflexes. Administration of phentolamine to intact rats or the aaministration of prazosin and rauwolscine to intact and hypotensive rats inducea a reduction in MP implying that arteriolar tone is maintained by the stimulation of D'UYLtY, H. to. 53 both 0 ^ - and a^-adrenoceptors. MAP was also effectively reauced by the administration of hexamethonium Demons t ra t i ng that the state of the res is -tance vessels may be effect ively altered by interfering with sympathetic transmission at the ganglionic or the postjunctional level . ln intact rats neither prazosin nor phentolamine affected the basal value of MCFP while rauwolscine s l ightly decreasea venous tone implying that o^-adrenoceptors are important in the maintenance of basal venous tone. Hexamethonium, on the other hand, markedly ana dose-depenaently reducea MCFP. Therefore unlike the arterioles, in intact rats the capacitance vessels are somewhat resistant to effects of postjunctional^ acting antagonists. After MCFP was raiseo via reflex venoconstriction, i t was founa that antagonism of both the c^- ana the a^-receptor reaucea venous tone. Venoai 1 ation was greater in the presence of rauwolscine than prazosin, however, suggesting that although both a-aarenoceptors are present in the veins o^-receptor activation may be more important for sympathetically-meaiated control o f basal as well as reflexly-inaucea venous tone. D'OYLtY, h. to. 54 5. REFERENCES Ablao b. A stuay of the mechanism of the hemodynamic effects of hydralazine in man. Acta Pharmacol et Toxicol, 20(Suppl 1):1246, 1963. Ablao B, Mel lander S. 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