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α-Methylnoradrenaline induced hypotension and bradycardia after administration into the area of the nucleus tractus solitarii
European Journal o f Pharmacology, 32 ( 1 9 7 5 ) 3 6 1 - - 3 6 4 © N o r t h - H o l l a n d Publishing C o m p a n y , A m s t e r d a m -- P r i n t e d in T h e N e t h e r l a n d s
Short communication ~-METHYLNORADRENALINE INDUCED HYPOTENSION AND BRADYCARDIA AFTER ADMINISTRATION INTO THE AREA OF THE NUCLEUS TRACTUS SOLITARII F R A N S P. N I J K A M P a n d W Y B R E N DE J O N G
R u d o l f Magnus Institute for Pharmacology, Medical Faculty, University o f Utrecht, Vondellaan 6, Utrecht, The Netherlands Received 1 April 1975, a c c e p t e d 14 April 1 9 7 5
F.P. N I J K A M P a n d W. DE J O N G , a-Methylnoradrenaline induced hypotension and bradycardia after administration into the area o f the nucleus tractus solitarii, E u r o p e a n J. Pharmacol. 32 ( 1 9 7 5 ) 3 6 1 - - 3 6 4 . Bilateral i n j e e t i o n s of a - m e t h y l n o r a d r e n a l i n e i n t o t h e area of the n u c l e u s t r a c t u s solitarii of the b r a i n s t e m caused a d o s e - d e p e n d e n t decrease of s y s t e m i c arterial b l o o d pressure a n d h e a r t rate of a n e s t h e t i z e d rats. T h e effects were p r e v e n t e d and even reversed b y a p r e c e d i n g i n j e c t i o n of t h e a - a d r e n o c e p t o r b l o c k i n g a g e n t p h e n t o l a m i n e . Pressor doses o f a n g i o t e n s i n II a n d of arginine-vasopressin at the same site failed to decrease b l o o d pressure a n d h e a r t rate. Nucleus t r a c t u s solitarii Arginine-vasopressin
-Methylnoradrenaline Blood pressure
A n g i o t e n s i n II
H e a r t rate
1. Introduction
2. Materials and methods
The central blood pressure lowering effect of a-methyldopa may be mediated by its metabolite a-methylnoradrenaline (Henning and Rubenson, 1971). The brain structures at which this action is exerted so far remain to be identified. Experiments in cats in which am e t h y l d o p a was infused into the vertebral artery indicate that the main site may be located in the medulla oblongata (Henning and Van Zwieten, 1968). Administration of noradrenaline into the medulla oblongata in the area of the nucleus tractus solitarii in the rat decreased blood pressure and heart rate (De Jong, 1974}. The present report describes the effect o f local administration of a-methylnoradrenaline at the same medullary site on blood pressure and heart rate of anesthetized rats.
The experiments were performed on male Wistar rats (outbred stock, Wi/Cpb, TNO, Zeist), weighing 200--250 g. The rats were anesthetized with urethane (1.25 g/kg i.p.). After fixation of the rat in a stereotaxic apparatus (David Kopf) with the head flexed to 45 °, the dorsal surface of the lower brainstem was exposed by a limited occipital craniotomy. Bilateral injections into the nucleus tractus solitarii (0.5 mm lateral of the obex and at a depth of 1.0 ram) were carried out with a stainless steel needle (outer diameter 0.2 mm) connected via a polyethylene cannula with an Agla micrometer syringe and a Shardlow micrometer. Various doses of a-methylnoradrenaline dissolved in 0.9% NaC1 solution were administered bilaterally in a volume of 0.6 pl over
362
F.P. NYKAMP, W. DE J O N G
a period of 30 sec. Control rats received 0.6 pl of vehicle only. Blood pressure was continuously recorded from a permanent indwelling iliac cannula (Nijkamp et al., 1975) with a Statham transducer (Model P23-AC) connected to a Grass polygraph. The iliac cannula had been implanted under ether anesthesia at least 24 hr prior to the experiment. Heart rate was c o m p u t e d from the blood pressure pulse wave by a cardiotachometer (Narco Bio-systems, Model BT 1200). Drugs given i.v. were dissolved in 0.2 ml 0.9% NaC1 solution and administered into the right jugular vein. Control rats received 0.2 ml vehicle only. After termination of th~ experiments, the brains were fixed in 5% formalin. Frozen sections of 100 p were cut and stained with 0.1% thionine. The injection sites were controlled microscopically. The following drugs were used: (+)-erythroa-methylnoradrenaline " HC1 (Cobrefin ®, Winthrop Laboratories); phentolamine methane sulphonate (Regitine ®, Ciba); 1-asp-5-val-angiotensin II am±de (Hypertensin ®, Ciba); argininevasopressin (232 U/mg, Organon).
3. Results and discussion Local injection of a-methylnoradrenaline (5.8, 23.0, 92.0 nmol) into the area of the nucleus tractus solitarii at the level of the obex caused a dose-dependent decrease in mean blood pressure and heart rate (table 1). Blood pressure started to fall immediately after completion of the bilateral injections. The lowest dose (5.8 nmol) caused a short lasting decrease in blood pressure which reached a maximum after 5 min. The higher doses (23.0 and 92.0 nmol) decreased blood pressure and heart rate for over 30 min. Maximal effects were observed after 5--10 min. Compared to the effect of noradrenaline injected at the same site in doses causing a similar degree of hypotensin (De Jong, 1974), the effect of a-methylnoradrenaline appears to be of longer duration. Heise and Kroneberg (1972), showed that the central inhibitory action on blood pressure and heart rate of intraventricularly administered a-methylnoradrenaline was blocked by phentolamine given by the same route. In this study, phentolamine in a dose of 23 nmol ad-
TABLE 1 E f f e c t o f graded doses o f ( ~ - m e t h y l n o r a d r e n a l i n e o n m e a n b l o o d pressure a n d h e a r t rate a f t e r local a d m i n i s t r a t i o n i n t o t h e area o f t h e n u c l e u s t r a c t u s solitarii o f t h e b r a i n s t e m o f a n e s t h e t i z e d rats. B l o o d pressure a n d h e a r t rate, a n d t h e changes in t h e s e p a r a m e t e r s are given in m m o f m e r c u r y ( m m Hg) a n d in beats per m i n ( b p m ) . T h e m e a n s -+ S.E.M. are listed. T h e n u m b e r s in p a r e n t h e s e s i n d i c a t e t h e n u m b e r o f animals used in e a c h group. Microinjection of
n
B l o o d pressure in m m Hg ( h e a r t rate in b p m ) Basal values
Vehicle ~-Methylnoradrenaline
9 5.8 n m o l
7
~ - M e t h y l n o r a d r e n a l i n e 23.0 n m o l
6
~ - M e t h y l n o r a d r e n a l i n e 92.0 n m o l
6
123+- 5 3 9 1 + 18 122 -+ 5 386 + 18 117 +- 4 423+ 5 126 ± 6 4 2 1 2 12
Changes a f t e r 5 rain
10 rain
20 m i n
30 rain
+6! 6 -4-+ 9 - 1 6 ± 2* - 2 7 +- 11 - 3 3 -+ 6* -56±14" - 3 9 ± 5* -38± 7
+5+ 6 +12+- 4 - 1 3 +- 2 - 1 2 -+ 14 - 3 3 + 6* -66+ 7* - 4 3 ± 4* -75±11"
+1+ 4 + 3 9 + 15 -3 + 2 +13 + 21 - 2 3 + 5* 7* -43+ - 3 2 +- 3* -86± 11"
-2 +39 +1 +42 -15 -28 -20 -74
+ 3 + 17 + 26 -+ 26 + 4* ± 9* -+ 3* + 14"
* c ~ - M e t h y l n o r a d r e n a l i n e - i n d u c e d c h a n g e s t h a t are significantly d i f f e r e n t (p < 0.01) f r o m t h e respective c o n t r o l values a f t e r t h e a d m i n i s t r a t i o n o f vehicle (1 n m o l o f ~ - m e t h y l n o r a d r e n a l i n e c o r r e s p o n d s t o 2 1 9 . 5 ng).
CENTRAL HYPOTENSIVE ACTION OF ~-METHYLNORADRENALINE m i n i s t e r e d i n t o t h e area o f t h e n u c l e u s t r a c t u s solitarii f o l l o w e d a f t e r 10 m i n b y a d o s e o f 23 n m o l o f ~ - m e t h y l n o r a d r e n a l i n e applied at t h e s a m e site c o m p l e t e l y p r e v e n t e d a n d even reversed t h e fall in b l o o d pressure a n d h e a r t rate, with a maximum reached within approximately 2 min. T h e m e a n changes (± S.E.M.) observed in 6 rats at this t i m e were +31 ± 6 m m Hg a n d +27 ± 9 b p m in t h e p h e n t o l a m i n e - t r e a t e d rats c o m p a r e d t o - - 1 4 ± 5 m m Hg a n d - - 2 8 +- 15 b p m f o r t h e c o n t r o l s w h i c h h a d received 0.9% NaC1 10 m i n p r i o r to a - m e t h y l n o r a d r e n a l i n e . In t h e c o n t r o l g r o u p a m a x i m a l decrease in b l o o d pressure a n d h e a r t r a t e o f t h e s a m e m a g n i t u d e as t h a t s h o w n in t a b l e 1 was r e a c h e d w i t h i n 10 min. In t h e p h e n t o l a m i n e - t r e a t e d a n i m a l s n o s e c o n d a r y d e c r e a s e in b l o o d pressure a n d h e a r t r a t e o c c u r r e d . P h e n t o l a m i n e b y itself c a u s e d no significant c h a n g e in b l o o d p r e s s u r e b u t r e d u c e d h e a r t r a t e b y 20% w i t h i n 2 - - 3 min. In o r d e r to establish t h e specificity o f t h e s t r u c t u r e s in t h e m e d u l l a o b l o n g a t a f o r a - m e t h y l n o r a d r e n a l i n e , t h e e f f e c t of t h e 2 pressor p e p t i d e s a n g i o t e n s i n II a n d arginine-vasopressin was studied in e q u i p r e s s o r a m o u n t s . I.v. a d m i n i s t r a t i o n o f t h e 3 d i f f e r e n t doses o f a - m e t h y l n o r a d r e n a l i n e u s e d caused a dosed e p e n d e n t increase in m e a n b l o o d pressure o f u r e t h a n i z e d rats o f 2 0 - - 6 0 m m Hg ( u n p u b lished data). Basal values o f b l o o d pressure and h e a r t r a t e in t h e s e g r o u p s o f rats (table 2) did n o t d i f f e r f r o m t h o s e r e p o r t e d in t a b l e 1. I.v. a d m i n i s t r a t i o n o f t h e 2 p e p t i d e s caused a dosed e p e n d e n t increase in b l o o d pressure. O n l y t h e highest d o s e o f vasopressin ( 4 8 0 p m o l ) decreased h e a r t rate. H o w e v e r , bilateral applicat i o n o f t h e s e doses into t h e area o f t h e n u c l e u s t r a c t u s solitarii in a n o t h e r g r o u p of rats failed t o cause c a r d i o v a s c u l a r changes ( t a b l e 2 ) . T h e s e o b s e r v a t i o n s suggest t h e r e f o r e t h a t t h e observed inhibitory cardiovascular action of a - m e t h y l n o r a d r e n a l i n e c a n n o t be e x p l a i n e d b y local v a s o c o n s t r i c t i o n . a - M e t h y l n o r a d r e n a l i n e p r e s u m a b l y is t h e m e t a b o l i t e w h i c h m e d i a t e s t h e b l o o d pressure lowering effect of a-methyldopa (Henning and R u b e n s o n , 1971). Central a - a d r e n o c e p t o r
363
TABLE 2 Effect of graded doses of angiotensin II and of arginine-vasopressin on mean blood pressure and heart rate after local administration into the area of the nucleus tractus solitarii and after i.v. administration. Maximal changes in blood pressure and heart rate which were observed within 5 rain are given in mm of mercury (mm Hg) and in beats per min (bpm). The means +- S.E.M. of 4--5 anesthetized rats are listed. Treatment
Blood pressure in mm Hg Heart rate in bpm Nucleus tractus solitarii
Intravenous
Vehicle
-5 + 4 -16-+ 6
+2 + 2 -31-+11
Angiotensin II 39 pmol
-6 +- 8 -13 + 13
+18 + 3** -33 + 10
Angiotensin II 117pmol
+5 +- 5 -28+- 7
+41 + 7** -31± 8
Angiotensin II 351pmol
+17 +- 7 -31+ 6
+68 + 5** -38 +13
Arg-vasopressin 41 pmol
0 + 12 -30 +- 14
+22 -+ 5** -38 +- 4
Arg-vasopressin 164 pmol
-1 + 5 -21 + 11
+47 + 11"* -52 + 18
Arg-vasopressin 656pmol
0 ± 11 -20+ 8
+54 + 3** -128+32"
* p ( 0.05 * * p ~ 0.01 ~ compared to the control values after the administration of vehicle (1 pmol of angiotensin II corresponds to 1030 pg and 1 pmol of arg-vasopressin corresponds to 1051 pg = 0.244 mU). blockade by intraventricularly administered p h e n t o l a m i n e p r e v e n t e d t h e fall in b l o o d pressure c a u s e d b y a - m e t h y l d o p a ( F i n c h a n d Hauesler, 1 9 7 3 ) . In t h e p r e s e n t s t u d y i n j e c t i o n o f p h e n t o l a m i n e i n t o t h e area o f t h e n u c l e u s t r a c t u s solitarii i n h i b i t e d b o t h t h e h y p o t e n s i o n and b r a d y c a r d i a i n d u c e d b y ~ - m e t h y l n o r a d r e n aline. T h e p r e s e n t findings t h u s s u p p o r t t h e h y p o t h e s i s t h a t a - m e t h y l d o p a acts t h r o u g h
364
a-adrenoceptor stimulation in this particular region of the medulla oblongata (Van Zwieten, 1973).
Acknowledgement We thank Dr. F.C. Nachod of Sterling--Winthrop Research Institute for providing us with a-methylnoradrenaline • HC1 (Cobrefin ®).
References De Jong, W., 1974, Noradrenaline: Central inhibitory control of blood pressure and heart rate, European J. Pharmacol. 29, 179.
F.P. NYKAMP, W. DE JONG Finch, L. and G. Hauesler, 1973, Further evidence for a central hypotensive action of a-methyldopa in both rat and cat, Brit. J. Pharmacol. 47,217. Heise, A. and G. Kroneberg, 1972, a-Sympathetic receptor stimulation in the brain and hypotensive activity of a-methyldopa, European J. Pharmacol. 17,315. Henning, M. and A. Rubenson, 1971, Evidence that the hypotensive action of methyldopa is mediated by central actions of methylnoradrenaline, J. Pharm. Pharmacol. 23, 407. Henning, M. and P.A. Van Zwieten, 1968, Central hypotensive effect of a-methyldopa, J. Pharm. Pharmacol. 20, 409. Nijkamp, F.P., J. Ezer and W. De Jong, 1975, Central inhibitory effect of a-methyldopa on blood pressure, heart rate and body temperature of renal hypertensive rats, European J. Pharmacol. 31,243. Van Zwieten, P.A., 1973, The central action of antihypertensive drugs, mediated via central a-receptor, J. Pharm. Pharmacol. 25, 89.
Short communication ~-METHYLNORADRENALINE INDUCED HYPOTENSION AND BRADYCARDIA AFTER ADMINISTRATION INTO THE AREA OF THE NUCLEUS TRACTUS SOLITARII F R A N S P. N I J K A M P a n d W Y B R E N DE J O N G
R u d o l f Magnus Institute for Pharmacology, Medical Faculty, University o f Utrecht, Vondellaan 6, Utrecht, The Netherlands Received 1 April 1975, a c c e p t e d 14 April 1 9 7 5
F.P. N I J K A M P a n d W. DE J O N G , a-Methylnoradrenaline induced hypotension and bradycardia after administration into the area o f the nucleus tractus solitarii, E u r o p e a n J. Pharmacol. 32 ( 1 9 7 5 ) 3 6 1 - - 3 6 4 . Bilateral i n j e e t i o n s of a - m e t h y l n o r a d r e n a l i n e i n t o t h e area of the n u c l e u s t r a c t u s solitarii of the b r a i n s t e m caused a d o s e - d e p e n d e n t decrease of s y s t e m i c arterial b l o o d pressure a n d h e a r t rate of a n e s t h e t i z e d rats. T h e effects were p r e v e n t e d and even reversed b y a p r e c e d i n g i n j e c t i o n of t h e a - a d r e n o c e p t o r b l o c k i n g a g e n t p h e n t o l a m i n e . Pressor doses o f a n g i o t e n s i n II a n d of arginine-vasopressin at the same site failed to decrease b l o o d pressure a n d h e a r t rate. Nucleus t r a c t u s solitarii Arginine-vasopressin
-Methylnoradrenaline Blood pressure
A n g i o t e n s i n II
H e a r t rate
1. Introduction
2. Materials and methods
The central blood pressure lowering effect of a-methyldopa may be mediated by its metabolite a-methylnoradrenaline (Henning and Rubenson, 1971). The brain structures at which this action is exerted so far remain to be identified. Experiments in cats in which am e t h y l d o p a was infused into the vertebral artery indicate that the main site may be located in the medulla oblongata (Henning and Van Zwieten, 1968). Administration of noradrenaline into the medulla oblongata in the area of the nucleus tractus solitarii in the rat decreased blood pressure and heart rate (De Jong, 1974}. The present report describes the effect o f local administration of a-methylnoradrenaline at the same medullary site on blood pressure and heart rate of anesthetized rats.
The experiments were performed on male Wistar rats (outbred stock, Wi/Cpb, TNO, Zeist), weighing 200--250 g. The rats were anesthetized with urethane (1.25 g/kg i.p.). After fixation of the rat in a stereotaxic apparatus (David Kopf) with the head flexed to 45 °, the dorsal surface of the lower brainstem was exposed by a limited occipital craniotomy. Bilateral injections into the nucleus tractus solitarii (0.5 mm lateral of the obex and at a depth of 1.0 ram) were carried out with a stainless steel needle (outer diameter 0.2 mm) connected via a polyethylene cannula with an Agla micrometer syringe and a Shardlow micrometer. Various doses of a-methylnoradrenaline dissolved in 0.9% NaC1 solution were administered bilaterally in a volume of 0.6 pl over
362
F.P. NYKAMP, W. DE J O N G
a period of 30 sec. Control rats received 0.6 pl of vehicle only. Blood pressure was continuously recorded from a permanent indwelling iliac cannula (Nijkamp et al., 1975) with a Statham transducer (Model P23-AC) connected to a Grass polygraph. The iliac cannula had been implanted under ether anesthesia at least 24 hr prior to the experiment. Heart rate was c o m p u t e d from the blood pressure pulse wave by a cardiotachometer (Narco Bio-systems, Model BT 1200). Drugs given i.v. were dissolved in 0.2 ml 0.9% NaC1 solution and administered into the right jugular vein. Control rats received 0.2 ml vehicle only. After termination of th~ experiments, the brains were fixed in 5% formalin. Frozen sections of 100 p were cut and stained with 0.1% thionine. The injection sites were controlled microscopically. The following drugs were used: (+)-erythroa-methylnoradrenaline " HC1 (Cobrefin ®, Winthrop Laboratories); phentolamine methane sulphonate (Regitine ®, Ciba); 1-asp-5-val-angiotensin II am±de (Hypertensin ®, Ciba); argininevasopressin (232 U/mg, Organon).
3. Results and discussion Local injection of a-methylnoradrenaline (5.8, 23.0, 92.0 nmol) into the area of the nucleus tractus solitarii at the level of the obex caused a dose-dependent decrease in mean blood pressure and heart rate (table 1). Blood pressure started to fall immediately after completion of the bilateral injections. The lowest dose (5.8 nmol) caused a short lasting decrease in blood pressure which reached a maximum after 5 min. The higher doses (23.0 and 92.0 nmol) decreased blood pressure and heart rate for over 30 min. Maximal effects were observed after 5--10 min. Compared to the effect of noradrenaline injected at the same site in doses causing a similar degree of hypotensin (De Jong, 1974), the effect of a-methylnoradrenaline appears to be of longer duration. Heise and Kroneberg (1972), showed that the central inhibitory action on blood pressure and heart rate of intraventricularly administered a-methylnoradrenaline was blocked by phentolamine given by the same route. In this study, phentolamine in a dose of 23 nmol ad-
TABLE 1 E f f e c t o f graded doses o f ( ~ - m e t h y l n o r a d r e n a l i n e o n m e a n b l o o d pressure a n d h e a r t rate a f t e r local a d m i n i s t r a t i o n i n t o t h e area o f t h e n u c l e u s t r a c t u s solitarii o f t h e b r a i n s t e m o f a n e s t h e t i z e d rats. B l o o d pressure a n d h e a r t rate, a n d t h e changes in t h e s e p a r a m e t e r s are given in m m o f m e r c u r y ( m m Hg) a n d in beats per m i n ( b p m ) . T h e m e a n s -+ S.E.M. are listed. T h e n u m b e r s in p a r e n t h e s e s i n d i c a t e t h e n u m b e r o f animals used in e a c h group. Microinjection of
n
B l o o d pressure in m m Hg ( h e a r t rate in b p m ) Basal values
Vehicle ~-Methylnoradrenaline
9 5.8 n m o l
7
~ - M e t h y l n o r a d r e n a l i n e 23.0 n m o l
6
~ - M e t h y l n o r a d r e n a l i n e 92.0 n m o l
6
123+- 5 3 9 1 + 18 122 -+ 5 386 + 18 117 +- 4 423+ 5 126 ± 6 4 2 1 2 12
Changes a f t e r 5 rain
10 rain
20 m i n
30 rain
+6! 6 -4-+ 9 - 1 6 ± 2* - 2 7 +- 11 - 3 3 -+ 6* -56±14" - 3 9 ± 5* -38± 7
+5+ 6 +12+- 4 - 1 3 +- 2 - 1 2 -+ 14 - 3 3 + 6* -66+ 7* - 4 3 ± 4* -75±11"
+1+ 4 + 3 9 + 15 -3 + 2 +13 + 21 - 2 3 + 5* 7* -43+ - 3 2 +- 3* -86± 11"
-2 +39 +1 +42 -15 -28 -20 -74
+ 3 + 17 + 26 -+ 26 + 4* ± 9* -+ 3* + 14"
* c ~ - M e t h y l n o r a d r e n a l i n e - i n d u c e d c h a n g e s t h a t are significantly d i f f e r e n t (p < 0.01) f r o m t h e respective c o n t r o l values a f t e r t h e a d m i n i s t r a t i o n o f vehicle (1 n m o l o f ~ - m e t h y l n o r a d r e n a l i n e c o r r e s p o n d s t o 2 1 9 . 5 ng).
CENTRAL HYPOTENSIVE ACTION OF ~-METHYLNORADRENALINE m i n i s t e r e d i n t o t h e area o f t h e n u c l e u s t r a c t u s solitarii f o l l o w e d a f t e r 10 m i n b y a d o s e o f 23 n m o l o f ~ - m e t h y l n o r a d r e n a l i n e applied at t h e s a m e site c o m p l e t e l y p r e v e n t e d a n d even reversed t h e fall in b l o o d pressure a n d h e a r t rate, with a maximum reached within approximately 2 min. T h e m e a n changes (± S.E.M.) observed in 6 rats at this t i m e were +31 ± 6 m m Hg a n d +27 ± 9 b p m in t h e p h e n t o l a m i n e - t r e a t e d rats c o m p a r e d t o - - 1 4 ± 5 m m Hg a n d - - 2 8 +- 15 b p m f o r t h e c o n t r o l s w h i c h h a d received 0.9% NaC1 10 m i n p r i o r to a - m e t h y l n o r a d r e n a l i n e . In t h e c o n t r o l g r o u p a m a x i m a l decrease in b l o o d pressure a n d h e a r t r a t e o f t h e s a m e m a g n i t u d e as t h a t s h o w n in t a b l e 1 was r e a c h e d w i t h i n 10 min. In t h e p h e n t o l a m i n e - t r e a t e d a n i m a l s n o s e c o n d a r y d e c r e a s e in b l o o d pressure a n d h e a r t r a t e o c c u r r e d . P h e n t o l a m i n e b y itself c a u s e d no significant c h a n g e in b l o o d p r e s s u r e b u t r e d u c e d h e a r t r a t e b y 20% w i t h i n 2 - - 3 min. In o r d e r to establish t h e specificity o f t h e s t r u c t u r e s in t h e m e d u l l a o b l o n g a t a f o r a - m e t h y l n o r a d r e n a l i n e , t h e e f f e c t of t h e 2 pressor p e p t i d e s a n g i o t e n s i n II a n d arginine-vasopressin was studied in e q u i p r e s s o r a m o u n t s . I.v. a d m i n i s t r a t i o n o f t h e 3 d i f f e r e n t doses o f a - m e t h y l n o r a d r e n a l i n e u s e d caused a dosed e p e n d e n t increase in m e a n b l o o d pressure o f u r e t h a n i z e d rats o f 2 0 - - 6 0 m m Hg ( u n p u b lished data). Basal values o f b l o o d pressure and h e a r t r a t e in t h e s e g r o u p s o f rats (table 2) did n o t d i f f e r f r o m t h o s e r e p o r t e d in t a b l e 1. I.v. a d m i n i s t r a t i o n o f t h e 2 p e p t i d e s caused a dosed e p e n d e n t increase in b l o o d pressure. O n l y t h e highest d o s e o f vasopressin ( 4 8 0 p m o l ) decreased h e a r t rate. H o w e v e r , bilateral applicat i o n o f t h e s e doses into t h e area o f t h e n u c l e u s t r a c t u s solitarii in a n o t h e r g r o u p of rats failed t o cause c a r d i o v a s c u l a r changes ( t a b l e 2 ) . T h e s e o b s e r v a t i o n s suggest t h e r e f o r e t h a t t h e observed inhibitory cardiovascular action of a - m e t h y l n o r a d r e n a l i n e c a n n o t be e x p l a i n e d b y local v a s o c o n s t r i c t i o n . a - M e t h y l n o r a d r e n a l i n e p r e s u m a b l y is t h e m e t a b o l i t e w h i c h m e d i a t e s t h e b l o o d pressure lowering effect of a-methyldopa (Henning and R u b e n s o n , 1971). Central a - a d r e n o c e p t o r
363
TABLE 2 Effect of graded doses of angiotensin II and of arginine-vasopressin on mean blood pressure and heart rate after local administration into the area of the nucleus tractus solitarii and after i.v. administration. Maximal changes in blood pressure and heart rate which were observed within 5 rain are given in mm of mercury (mm Hg) and in beats per min (bpm). The means +- S.E.M. of 4--5 anesthetized rats are listed. Treatment
Blood pressure in mm Hg Heart rate in bpm Nucleus tractus solitarii
Intravenous
Vehicle
-5 + 4 -16-+ 6
+2 + 2 -31-+11
Angiotensin II 39 pmol
-6 +- 8 -13 + 13
+18 + 3** -33 + 10
Angiotensin II 117pmol
+5 +- 5 -28+- 7
+41 + 7** -31± 8
Angiotensin II 351pmol
+17 +- 7 -31+ 6
+68 + 5** -38 +13
Arg-vasopressin 41 pmol
0 + 12 -30 +- 14
+22 -+ 5** -38 +- 4
Arg-vasopressin 164 pmol
-1 + 5 -21 + 11
+47 + 11"* -52 + 18
Arg-vasopressin 656pmol
0 ± 11 -20+ 8
+54 + 3** -128+32"
* p ( 0.05 * * p ~ 0.01 ~ compared to the control values after the administration of vehicle (1 pmol of angiotensin II corresponds to 1030 pg and 1 pmol of arg-vasopressin corresponds to 1051 pg = 0.244 mU). blockade by intraventricularly administered p h e n t o l a m i n e p r e v e n t e d t h e fall in b l o o d pressure c a u s e d b y a - m e t h y l d o p a ( F i n c h a n d Hauesler, 1 9 7 3 ) . In t h e p r e s e n t s t u d y i n j e c t i o n o f p h e n t o l a m i n e i n t o t h e area o f t h e n u c l e u s t r a c t u s solitarii i n h i b i t e d b o t h t h e h y p o t e n s i o n and b r a d y c a r d i a i n d u c e d b y ~ - m e t h y l n o r a d r e n aline. T h e p r e s e n t findings t h u s s u p p o r t t h e h y p o t h e s i s t h a t a - m e t h y l d o p a acts t h r o u g h
364
a-adrenoceptor stimulation in this particular region of the medulla oblongata (Van Zwieten, 1973).
Acknowledgement We thank Dr. F.C. Nachod of Sterling--Winthrop Research Institute for providing us with a-methylnoradrenaline • HC1 (Cobrefin ®).
References De Jong, W., 1974, Noradrenaline: Central inhibitory control of blood pressure and heart rate, European J. Pharmacol. 29, 179.
F.P. NYKAMP, W. DE JONG Finch, L. and G. Hauesler, 1973, Further evidence for a central hypotensive action of a-methyldopa in both rat and cat, Brit. J. Pharmacol. 47,217. Heise, A. and G. Kroneberg, 1972, a-Sympathetic receptor stimulation in the brain and hypotensive activity of a-methyldopa, European J. Pharmacol. 17,315. Henning, M. and A. Rubenson, 1971, Evidence that the hypotensive action of methyldopa is mediated by central actions of methylnoradrenaline, J. Pharm. Pharmacol. 23, 407. Henning, M. and P.A. Van Zwieten, 1968, Central hypotensive effect of a-methyldopa, J. Pharm. Pharmacol. 20, 409. Nijkamp, F.P., J. Ezer and W. De Jong, 1975, Central inhibitory effect of a-methyldopa on blood pressure, heart rate and body temperature of renal hypertensive rats, European J. Pharmacol. 31,243. Van Zwieten, P.A., 1973, The central action of antihypertensive drugs, mediated via central a-receptor, J. Pharm. Pharmacol. 25, 89.