European Journal o f Pharmacology, 37 (1976) 79--89

79

© North-Holland Publishing Company, Amsterdam -- Printed in The Netherlands

STUDIES ON THE MECHANISM OF THE C A R D I O V A S C U L A R EFFECTS OF METHYLDOPA MUSTAFA F. LOKHANDWALA, JOSEPH P. BUCKLEY and BHAGAVAN S. J A N D H Y A L A

Division o f Cardiovascular Research, Department o f Pharmacology, College o f Pharmacy, University o f Houston, Houston, Texas 77004, U.S.A. Received 19 September 1975, revised MS received 15 December 1975, accepted 12 January 1976

M.F. LOKHANDWALA, J.P. BUCKLEY and B.S. JANDHYALA, Studies on the mechanism o f the cardiovascular

effects o f methyldopa, European J. Pharmacol. 37 (1976) 79--89. Oral administration of methyldopa (100 mg/kg, twice daily for 3 days} to mongrel dogs produced a significant decrease in blood pressure and heart rate. The drug treatment affected neither the resting venous tone nor the cardiac output. Thus, the hypotensive effect of the drug was predominantly due to a reduction in total peripheral resistance. Vasoconstrictor responses of the renal vasculature to sympathetic nerve stimulation were significantly impaired after methyldopa at all the frequencies, while mesenteric vasoconstrictor responses to sympathetic nerve stimulation were impaired only at the lower stimulation frequencies. In addition, methylnorepinephrine was a significantly less potent vasoconstrictor than norepinephrine in the renal vasculature, but was equipotent to norepinephrine in the mesentery. The finding of a reduction in the renal vascular resistance of methyldopatreated dogs, with no such alteration in the mesenteric vascular resistance, is consistent with the nerve stimulation studies. Therefore, the results of the present investigation indicate that in addition to the existing evidence favoring a central site of action for methyldopa, the impairment of peripheral sympathetic neuronal function is also of importance in accounting for the hemodynamic alterations observed following treatment with methyldopa. Sympathetic nerve stimulation Major vessel occlusion

Systemic function curves Methylnorepinephrine

1. Introduction Studies on the mechanism of action of methyldopa in laboratory animals have so far yielded conflicting results. Although there is much evidence favoring a central site of action for methyldopa (Henning and Van Zwieten, 1968; Ingenito et al., 1970; Finch and Haeuslet, 1973; Heise and Kroneberg, 1973; Cohen et al., 1974; Nijkamp et al., 1975), controversy still exists regarding the effect of methyldopa on peripheral sympathetic nerve function and the role of such an effect in the methyldopamediated hypotension (Day and Rand, 1964; Haefely et al., 1966; Sugarman et al., 1968; Salmon and Ireson, 1970).

Cardiac output

Methyldopa

It has been recently reported from our laboratories that methyldopa treatment of mongrel dogs for a period of 3 days produces significant impairment of sympathetic nerve function to the heart and hindleg vasculature (Lokhandwala et al., 1975). The present study was undertaken to further investigate the effect of methyldopa treatment on sympathetic nerve function to the renal and the mesenteric vasculature and the role of methylnorepinephrine as the less potent false neurotransmitter in these two organs. Furthermore, regional as well as systemic hemodynamic studies were carried out to further the understanding of the actions of methyldopa on the cardiovascular system of mongrel dogs.

80 2. Materials and methods

2.1. General Mongrel dogs of either sex weighing between 15--20 kg were treated orally with 1-methyldopa, 100 mg/kg, twice daily for 3 consecutive days. Another group of dogs received a placebo (lactose) and served as controls. At the end of the treatment period in all the experiments, the animals were anesthetized with sodium pentobarbital (35 mg/kg i.v.) and a catheter placed in the abdominal aorta via one of the femoral arteries for recording of blood pressure with a Statham P23AC pressure transducer. One of the femoral veins was cannulated for injection of drugs. Heart rate was obtained from the pulse by a biotachometer BT-1200 (Narco-Bio Systems, Inc.). All of the parameters were recorded on a Narco-Bio Systems physiograph recorder. In all of these studies the animals were artificially respired and bilateral vagotomy performed.

2.2. Renal and mesenteric hemodynamics and sympathetic nerve function In order to carry out these studies, either the superior mesenteric or the renal artery was exposed retroperitonially by an incision in the left flank. Renal or mesenteric blood flow was measured with Statham electromagnetic flowmeter employing flowprobes of suitable diameter. Electronic zero in each experiment was verified by occluding either the renal or the mesenteric artery distal to the probe. To evaluate the effect of methyldopa treatment on sympathetic nerve function to the kidney, the renal nerves were carefully exposed and the central end denervated. The entire renal nerve plexus was stimulated at supramaximal voltage using a Grass stimulator at frequencies ranging from 0.5 to 8.0 Hz and impulse duration of 0.5 msec. Similarly, for studying the sympathetic nerve function to the mesentery, the thoracic splanchnic nerves to the mesenteric artery were isolated and centrally denervated. Stimulation was carried out at supramaximal voltage using a

M.F. LOKHANDWALAET AL. Grass stimulator at frequencies ranging from 0.5 to 16 Hz and an impulse duration of 0.5 msec.

2.3. Perfusion of the dog renal or mesenteric artery The left renal or superior mesenteric artery was exposed by making a retroperitonial incision in the left flank and was dissected free from the surrounding tissue. Heparin sodium (600 Units/kg) was administered and either one of the arteries was cannulated and perfused with blood obtained from the femoral artery using a Sigmamotor pump. The perfusion pressure was maintained at a level approximately equal to the mean systemic blood pressure of the dog. The relative vasoconstrictor potencies of the two amines, l-norepinephrine and 1-methylnorepinephrine, were determined in both the organs by intra-arterial administration of the drug into the perfusion circuit. The doses refer to the base.

2.4. Systemic h e m o d y n a m i c studies In order to study the effect of methyldopa treatment on cardiac output, the chest was opened at the fourth intercostal space by performing a left t h o r a c o t o m y and the heart was exposed. After opening the pericardium, the ascending aorta was freed from the surrounding tissue and a Statham electromagnetic flowprobe (12 mm internal diameter) was placed around the vessel and connected to a Statham electromagnetic flowmeter to monitor cardiac output. The electronic zero was utilized to obtain zero flow and was verified occasionally by considering the diastolic flow as zero flow.

2. 5. System ic function curves A t h o r a c o t o m y was performed on the right side of the chest wall at the fourth intercostal space and the heart was exposed. The ascending aorta was dissected free from the surrounding tissues and isolated. A Statham electromagnetic flowprobe (12 mm internal diameter) was

METHYLDOPA AND C A R D I O V A S C U L A R SYSTEM

81

placed around the ascending aorta and the cardiac o u t p u t was m o n i t o r e d as described in section 2. 4. A polyvinyl catheter and a small rubber balloon were introduced into the right atrium via the apex. T he balloon was utilized to alter the right atrial pressure to desired levels, while the catheter was used to measure right atrial pressure. An o t her polyvinyl catheter was introduced into the abdominal vein via the femoral vein to m o n i t o r abdominal venous pressure. In order to study the venous return pattern, the right atrial pressure was gradually elevated using the balloon catheter and corresponding changes in the cardiac o u t p u t were determined. Statham t y p e P23V pressure transducers were utilized to m o n i t o r the right atrial and the abdominal venous pressures.

immediately above the diaphragm. A catheter was advanced into the abdominal vein via the femoral vein to measure abdominal venous pressure utilizing a Statham P23V pressure transducer. After ligating the internal mammary vessels and the azygous vein, MVO was accomplished by simultaneously occluding the inferior vena cava and the thoracic aorta with stainless steel clamps. T he peak rise in the abdominal venous pressure was recorded during the 60 sec of MVO. During certain readings, 30 sec of control MVO was carried o u t and in the remaining 30 sec of MVO, b o t h the carotid arteries were occluded and further alterations in the abdominal vena caval pressure were recorded.

2. 6. Major vessel occlusion studies

2. 7. Computation o f various parameters and statistical analysis

The technique of major vessel occlusion (MVO) has been described in detail by Bartelstone (1960). A slightly modified version of this t ech n iq u e was e m p l o y e d for the present experiments (Cavero et al., 1973). In pentobarbital-anesthetized dogs, t h o r a c o t o m y was p e r f o r m e d on the right side of the chest wall

In the experiments described above, certain parameters were directly recorded on a NarcoBio Systems physiograph while certain o t h e r parameters were calculated. Table 1 describes in detail various measured and calculated parameters as well as the units o f measurements.

TABLE 1 Parameters measured directly or calculated and used to evaluate regional and systemic hemodynamics. Parameters

Calculations

Units

Systolic blood pressure (S.B.P.) Diastolic blood pressure (D.B.P.) Mean blood pressure (M.B.P.) Heart rate (H.R.) Right atrial pressure (R.A.P.) Left atrial pressure (L.A.P.) Renal blood flow (R.B.F.) Renal vascular resistance (R.V.R.) Mesenteric blood flow (M.B.F.) Mesenteric vascular resistance (M.V.R.) Cardiac output (C.O.) Cardiac index (C.I.) Stroke volume (S.V.) Stroke volume index (S.V.I.) Total peripheral resistance (T.P.R.) Total peripheral resistance index (T.P.R.I.)

Measured Measured 0.33 (S.B.P.--D.B.P.) + D.B.P. Measured Measured Measured Measured M.B.P./R.B.F. Measured M.B.F./M.B.F. Measured C.O./m 2 body surface C.O./H.R. C.I./H.R M.B.P./C.O. M.B.P./C.I.

mm Hg mm Hg mm Hg beats/min mm Hg mm Hg ml/min mm Hg/ml/min ml/min mm Hg/ml/min 1/min 1/min/m 2 ml/beat ml/beat/m 2 mm Hg/i/min mm Hg/1/min/m 2

82

M.F. LOKHANDWALA ET AL.

All the data are reported as mean (,~) -+ standard error of the mean (S.E.M.). Student's paired t-test was utilized to estimate significant changes within the same group, whereas differences between the means of two groups were evaluated by using the t-test for unpaired values. The difference between the means was considered statistically significant when p < 0.05.

0 Cenlrel •

Treoted

300,

._ 2O0 E

/

/

3. Results I00

3.1. Effect of methyldopa treatment on renal hemodynamics and sympathetic nerve function to the kidney Fig. 1 shows the effect of methyldopa treatment on renal blood flow and renal vascular resistance. As illustrated in the figure, there was a significant decrease in the renal vascular resistance of the treated dogs when compared to the control group. However, there was only a slight but insignificant increase in the renal blood flow following methyldopa treatment. Studies on the renal vasoconstrictor responses elicited by the stimulation of the renal nerves indicated that in the treated dogs there was significant attenuation of the vasoconstriction at all the frequencies tested when compared to the control group (fig. 2). The percent decrease in the renal blood flow reflects the relative vasoconstriction since no alterations in mean blood pressure occurred during sympathetic nerve stimulation. These results indicated that methyldopa treatment produced impairment of sympathetic neuronal function to the kidney. Relative vasoconstrictor potencies of the two amines as reflected by changes in the perfusion pressure indicated that methylnorepinephrine was about 4--5 times less potent than norepinephrine in producing vasoconstriction in the renal vasculature (fig. 3). 3.2. Effect of methyldopa treatment on mesenteric hemodynamics and sympathetic nerve function It is apparent from fig. I that treatment with methyldopa did not produce any significant

o[ 1.0 0.8

c E

-I-

E E

OlSi O.4

R.B.E

%

M.B.F.

O.2

RV.R.

M.V.R.

Fig. 1. Effect of methyldopa treatment (100 mg/kg p.o. twice daily for 3 days) on renal blood flow (R.B.F.), renal vascular resistance (R.V.R.), mesenteric blood flow (M.B.F.) and mesenteric vascular resistance (M.V.R.} in pentobarbital-anesthetized dogs (n = 6, *p < 0,05).

alterations in the mesenteric blood flow or mesenteric vascular resistance. Although there were changes in both the parameters in the treated dogs compared to the control group, these changes were not statistically significant. Studies on the sympathetic neuronal function to the mesenteric vasculature revealed that the drug treatment produced significant attenuation of the vasoconstrictor responses as reflected in percent changes in blood flow only at the lower frequencies of stimulation (fig. 4). At higher frequencies, although the responses obtained in the treated dogs were always less

METHYLDOPA AND C A R D I O V A S C U L A R SYSTEM

83

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m e t h y l d o p a t r e a t m e n t (100 mg/kg for 3 days) on vasoconstrictor renerve stimulation in pentobarbital(n = 6, *p < 0.05).

Fig. 4. Effect of m e t h y l d o p a t r e a t m e n t (100 mg/kg p.o. twice daily for 3 days) on mesenteric vasoconstriction elicited by stimulation of s y m p a t h e t i c nerves in pentobarbital-anesthetized dogs (n = 6, *p < 0.05).

20G c NE :~ - - - - - - ~ MNE

I ~ >

NE

8

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dl

0'.2

0'.4

0'.8

1~6

Fig. 3. Vasoconstrictor responses to intra-arterial administration of norepinephrine (NE) and methylnorepinephrine (MNE) in the renal vasculature o f pentobarbital-anesthetized dogs (n = 5, *p < 0.001).

~.,

~

w

~.

~,

Fig. 5. Vasoconstrictor responses to intra-arterial administration of n o r e p i n e p h r i n e (NE) and m e t h y l n o r epinephrine (MNE) in the mesenteric vasculature of pe~tobarbital-anesthetized dogs (n = 5).

84

M.F. LOKHANDWALA ET AL.

t h a n in the c o n t r o l g r o u p , t h e r e was a wide degree o f v a r i a t i o n and t h u s no statistically significant d i f f e r e n c e b e t w e e n the groups. T h e relative v a s o c o n s t r i c t o r p o t e n c i e s o f nore p i n e p h r i n e and m e t h y l n o r e p i n e p h r i n e in the m e s e n t e r i c v a s c u l a t u r e w e r e n o t significantly d i f f e r e n t (fig. 5), indicating the equip(>tency o f the t w o amines in this organ.

3.3. Effect o f methyldopa treatment on the cardiovascular system of the dog Studies in this series w e r e designed to evaluate t h e e f f e c t o f m e t h y l d o p a t r e a t m e n t o n t h e arterial as well as t h e v e n o u s side o f t h e syst e m i c circulation.

3.3.1. Studies on systemic hemodynamics T h e e f f e c t o f m e t h y l d o p a t r e a t m e n t o n systemic hemodynamics of pentobarbital-anesthetized dogs is p r e s e n t e d in t a b l e 2. T h e s e results indicate t h a t 3 d a y oral t r e a t m e n t of m o n g r e l dogs with m e t h y l d o p a p r o d u c e d significant a l t e r a t i o n s in s y s t e m i c h e m o d y n a m i c s . In addition to t h e significant decrease in b l o o d pressure and h e a r t rate p r e v i o u s l y r e p o r t e d b y us ( L o k h a n d w a l a et al., 1 9 7 5 ) , t h e r e was a signific a n t decrease in t o t a l p e r i p h e r a l resistance, while t h e r e was a significant increase in s t r o k e v o l u m e or s t r o k e v o l u m e i n d e x in t h e t r e a t e d dogs. A l t e r a t i o n s in cardiac o u t p u t or cardiac index were n o t statistically significant.

3.3. 2. Venous compliance studies T w o s e p a r a t e p r o c e d u r e s w e r e utilized to study the effect of methyldopa treatment on the v e n o u s vascular d y n a m i c s .

3.3.2. 1. Effect o f methyldopa treatment on systemic function curves. T h e p r o c e d u r e f o r s t u d y i n g the e f f e c t o f a drug on the c o m p l i a n t region o f t h e s y s t e m i c c i r c u l a t i o n using t h e s y s t e m i c f u n c t i o n curves has b e e n used extensively b y various investigators ( F e r m o s o et al., 1964; J a n d h y a l a et al., 1971). This p r o c e d u r e was used in t h e p r e s e n t s t u d y to evaluate t h e e f f e c t o f m e t h y l d o p a t r e a t m e n t on the v e n o u s s y s t e m and t h e s u b s e q u e n t i n f l u e n c e o n the v e n o u s r e t u r n to t h e heart. The effect of methyldopa treatment on the s y s t e m i c f u n c t i o n c u r v e c o m p a r e d to the control g r o u p is s h o w n in fig. 6. Analysis o f these t w o curves revealed t h a t t h e d r u g t r e a t m e n t did n o t p r o d u c e a n y essential a l t e r a t i o n s in t h e ability o f t h e v e n o u s v a s c u l a t u r e to r e t u r n b l o o d to t h e heart, suggesting t h a t m e t h y l d o p a t r e a t m e n t d o e s n o t p r o d u c e a n y changes in venous compliance.

3.3.2.2. Major vessel occlusion studies. T h e p r o c e d u r e k n o w n as ' m a j o r vessel o c c l u s i o n ' (MVO) and d e v e l o p e d b y B a r t e l s t o n e ( 1 9 6 0 ) provides an a l t e r n a t e m e a n s o f evaluating t h e p r e s s u r e - - v o l u m e r e l a t i o n s h i p s in t h e v e n o u s s y s t e m . This series o f e x p e r i m e n t s was carried

TABLE 2 Effect of methyldopa treatment on systemic hemodynamics of pentobarbital-anesthetized dogs (~ -+ S.E.M.) (n = 8). Parameters

Control

Methyldopa

% Change

Significance

Mean blood pressure (mm Hg) Heart rate (beats/min) Cardiac output (1/min) Cardiac index (1/min/m 2 ) Stroke volume (ml/beat) Stroke volume index (ml/beat/m 2 ) Total peripheral resistance (mm Hg/i/min) Total peripheral resistance index (mm Hg/1/min/m 2)

105t 3.0 160 -+ 7.0 1.41 -* 0.12 2.20 ± 0.35 8.81 ± 0.80 13.75 -+ 0.90 74.46 -* 5.20

85± 3.0 130 ± 4.0 1.50 ± 0.18 2.62 ± 0.25 11.53 -+ 1.20 20.15 ± 1.80 56.66 ± 6.0

--19.0 --18.8 +6.4 +19.1 +30.9 +46.5 --23.9

p < 0.01 p < 0.01 N.S. N.S. p < 0.05 p < 0.01 p < 0.05

47.70 ± 3.0

32.44 +- 3.0

--32.0

p < 0.01

METHYLDOPA AND CARDIOVASCULAR SYSTEM

MVO, indicating that the pressure gradient for venous return was not altered. These results confirmed the conclusions from the earlier experiments with systemic function curves. However, as shown in table 3, in the methyldopa-treated dogs the venoconstrictor response to bilateral carotid occlusion was significantly reduced.

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4 6 8 iO |2 RiQh! Atriol I~-esSure (ram HQ)

t4

1'6

Fig. 6. Comparison of the effects of elevation o f right atrial pressure on cardiac index o f m e t h y l d o p a treated (100 mg/kg p.o. twice daily for 3 days) and control animals under pentobarbital anesthesia.

out to further determine the effect of methyldopa treatment on the venous vascular tone. This procedure involves measurement of changes in the pressure gradient between the reactive venous reservoir and the central venous conduit which represents the i.v. pressure gradient between the capillaries and contiguous large veins including the vena cava. Any effect that the drug may have on the venous tone would be reflected on this pressure gradient. Table 3 summarizes the effect of methyldopa treatment on the venous tone of pentobarbital-anesthetized dogs. The drug treatment did not affect the changes in the abdominal venous pressure elicited by the application of

3.4. Studies on the effects of ganglionic blockade on the heart rate and the blood pressure of methyldopa-treated dogs At the end of some of the series of experiments, both treated and control dogs were given chlorisondamine 3 mg/kg, i.v. and atropine 1 mg/kg, i.v., in order to evaluate any direct effect that the drug treatment may have had on the heart or the vasculature in pentobarbital-anesthetized dogs (Lokhandwala et al., 1973). Table 4 summarizes the effects of ganglioplegics on the mean blood pressure and heart rate of control as well as of treated dogs. Although there were significant differences in the changes produced in these parameters by chlorisondamine plus atropine in control and in treated dogs, the basal values for these parameters following autonomic blockade were the same in both groups indicating that the drug treatment had no direct effect on either the heart or the vasculature. The failure to observe any further changes in these parameters cannot

TABLE 3 Effect of m e t h y l d o p a t r e a t m e n t on venous t o n e and reflex v e n o c o n s t r i c t i o n as d e t e r m i n e d by major vessel occlusion (MVO) in pentobarbital-anesthetized dogs (~ + S.E.M.) (n = 6). Group

A b d o m i n a l venous pressure ( m m

Hg)

Control

Control

Treated

Bilateral Carotid Occlusion ( B C O )

Pre-MVO

MVO

Change

Pre-BCO

BCO

Change

4.62 ± 0.30 4.56 +- 0.42

8.72 + 1.03 8.52 + 1.60

4.10 +- 0.40 3.96 +- 0.81

7.60 _+ 1.10 8.07 _+ 1.05

11.97 -+ 1.20 10.33 + 1.15

4.37 +- 0.61 2.25 +- 0.17 *

• Significant response (p < 0.05).

86

M.F. LOKHANDWALA

ET AL.

TABLE 4 Effect of autonomic blockade on the mean blood pressure and heart rate of control and treated dogs under p e n t o b a r b i t a l a n e s t h e s i a ( 5 + S . E . M . ) (n = 8). Treatment

Control Treated

Mean blood pressure (mm Hg)

Heart rate (beats/min)

Before blockade

After blockade

Change

Before blockade

After blockade

Change

1 1 8 +- 5 95 ~ 4 "

54 ± 6 50 +~ 7

64 ~ 5 45 + 8"

1 6 0 ~ 10 130 ~ 12"

115 ~ 6 114± 8

4 5 ± 10 16+ 7"

* S i g n i f i c a n t l y d i f f e r e n t f r o m c o n t r o l (p < 0 . 0 1 ) .

be due to low resting levels as it is possible to reduce these parameters further after the administration of agents acting directly on the myocardium and the vasculature {unpublished observations).

4. Discussion The present study has demonstrated that methyldopa treatment to mongrel dogs for a period of three days produced significant changes in certain cardiovascular parameters. Concomitant with the decrease in blood pressure, heart rate and peripheral resistance, there was a significant attenuation of peripheral sympathetic transmission to the kidney and the mesenteric vasculature. The impairment of sympathetic nerve function to these and other areas as observed in a previous study (Lokhandwala et al., 1975) could account for the hemodynamic alterations noted in the treated dog s . Hypotensive and bradycardic effects of methyldopa had been demonstrated previously in conscious as well as in anesthetized dogs (Goldberg et al., 1960; Antonaccio et al., 1974; Lokhandwala et al., 1975). The present study, while confirming these findings, further demonstrated that the antihypertensive action of methyldopa was mediated by a decrease in the peripheral resistance rather than by alterations in the cardiac output. Studies on renal hemodynamics indicated that methyldopa treatment produced a signifi-

cant decrease in renal vascular resistance and a slight but insignificant increase in renal blood flow. In addition, vasoconstrictor responses to stimulation of renal nerves were significantly attenuated in the treated dogs and methylnorepinephrine was only about 1/4 to 1/5 as potent as norepinephrine in producing vasoconstriction in the renal vasculature. These results confirm the previous findings that methylnorepinephrine may play the role of a less potent false neurotransmitter in the kidney responsible for the release of renin (Privitera and Mohammed, 1972) and for inhibition of vasoconstriction following stimulation of the renal nerves (Finch and Haeusler, 1973). From the present study and other reports it can be concluded that methyldopa treatment produces changes in renal hemodynamics possibly by an impairment of peripheral sympathetic nerve function to the kidney; the observed changes in the renal hemodynamics could largely be accounted for by the weaker potency of methylnorepinephrine. The studies on mesenteric hemodynamics revealed that unlike the changes observed in the kindey, methyldopa treatment did not produce any significant alterations in the mesenteric hemodynamics, despite the fact that sympathetic neuronal function to the mesenteric vasculature was significantly impaired in the methyldopa-treated dogs at lower stimulation frequencies. Methylnorepinephrine was equipotent to norepinephrine in producing vasoconstriction in the mesenteric vasculature.

METHYLDOPA AND CARDIOVASCULAR SYSTEM Malik and Muscholl (1969) using the isolated rat mesenteric artery preparation have observed similar impairment of sympathetic nerve function after m e t h y l d o p a treatment. The absence of any significant alterations in the mesenteric hemodynamics in spite of impaired sympathetic neuronal function to the mesenteric vasculature could be due to the extensive network of collateral vessels, in the mesenteric arterial tree. Resistance as measured in this study, utilizing blood flow in superior mesenteric artery, may not reflect changes in the total mesenteric vasculature. However, by comparing the data obtained under identical experimental conditions in control and treated animals one can conclude that m e t h y l d o p a treatment resulted in inhibition of sympathetic transmission in the mesenteric vasculature. The theory of a less potent false transmitter cannot account for this effect of m e t h y l d o p a treatment, but as reported earlier for the myocardium (Sugarman et al., 1968; Lokhandwala et al., 1975), a reduced release of either the natural and/or false transmitter could account for the impairment of nerve function at lower stimulation frequencies. The h e m o d y n a m i c mechanism(s) by which methyldopa produces a decrease in blood pressure in hypertensive patients and experimental animals are still debated (Wilson et al., 1962; Brest, 1965; Sannerstedt and Conway, 1970; Osborne et al., 1974). The current study has demonstrated that methyldopa-mediated hypotension in mongrel dogs is brought about by a decrease in total peripheral resistance with no significant changes in cardiac output. This conclusion is further substantiated by the results obtained from studies on the venous compliance. These results indicated that a 3 day oral treatment with m e t h y l d o p a does not produce any changes in the venous return of the pentobarbital-anesthetized dogs. These results are not in agreement with those of Osborne et al. (1974), who observed an increased cardiac o u t p u t due to an increase in venous return and no changes in the arterial blood pressure in methyldopa-treated dogs. However, the dose and route of administration employed by

87 Osborne et al. (1974) differed from those used in the present studies. The results of the present investigation, however, agree with clinical evidence indicating that a reduction in peripheral resistance following treatment with methyldopa is responsible for the antihypertensive action of the drug (Onesti et al., 1964; Sannerstedt and Conway, 1970). In the major vessel occlusion experiments, the reflex venoconstriction observed during bilateral carotid occlusion (BCO) was significantly attenuated in the treated dogs. Under normal physiological conditions, reflex decrease in the venous compliance caused by activation of the baroreceptor reflex plays a major role in providing an increased venous return to the heart and a subsequent increase in the systemic blood pressure {Bartelstone, 1960; Folkow and Mellander, 1964; Shepherd, 1966; Shoukas and Sagawa, 1973). The impairment of this response observed in the present study may have contributed to the reduced pressor response to BCO reported previously (Osborne et al., 1974; Lokhandwala et al., 1975) following treatment with methyldopa. Autonomic ganglionic blockade with chlorisondamine plus atropine reduced the blood pressure and heart rate of the control and treated dogs to the same resting level. These results indicate that methyldopa treatment for a period of 3 days does not have any direct effect either on the vasculature or on the heart. In addition, changes from the resting values for blood pressure and heart rate following ganglionic blockade were significantly less in the treated dogs than in the controls, indicating a decreased level of resting sympathetic activity to the heart as well as to the vasculature in the methyldopa-treated dogs. These results agree with those of Gomez et al. (1972) who used rats treated with methyldopa. Therefore, it can be concluded that all the cardiovascular effects observed after treatment with m e t h y l d o p a could be explained by the action of the drug on the sympathetic nervous system. In summary, the present findings indicate that treatment of mongrel dogs with methyldopa for a period of 3 days produces significant

88

alterations in renal and systemic hemodynamics. The hypotensive effect of the drug is essentially due to a reduction in the total peripheral resistance. The significant impairment of peripheral sympathetic neuronal function to the various organs is evidently responsible for the hemodynamic alterations produced by the drug treatment. In contrast to the pronounced effect that the drug has on the arterial side of the systemic circulation, it does not alter the resting venous vascular tone with this treatment schedule. Finally, it can be concluded that the hemodynamic alterations observed following methyldopa administration are mediated by the action of the drug on the sympathetic nervous system and while the present study does not rule out the involvement of the central sympathetic mechanisms in methyldopa-induced hypotension, it establishes that peripheral autonomic mechanisms may also play a major role in accounting for the hemodynamic effects of the compound.

Acknowledgements The authors wish to express their thanks to Dr. C.A. Stone, Merck Institute for Therapeutic Research for a generous supply of methyldopa and Sterling-Winthrop Research Institute for methylnorepinephrine. We also acknowledge the skillful technical assistance of Drs. Regis R. Vollmer and Magdi M. Asaad and the expert secretarial assistance of Mrs. Yzelda V. Hinojosa. This work was supported by PHS Grant HL 17401.

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Studies on the mechanism of the cardiovascualr effects of methyldopa.

European Journal o f Pharmacology, 37 (1976) 79--89 79 © North-Holland Publishing Company, Amsterdam -- Printed in The Netherlands STUDIES ON THE M...
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