European Journal of Pharmacology, 47 (1978) 443--449
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© Elsevier/North-Holland Biomedical Press
EFFECTS OF SEROTONIN ANTAGONISTS ON DIGITALIS-INDUCED VENTRICULAR ARRHYTHMIAS
*
CINDA J. HELKE, JOHN A. QUEST ** and RICHARD A. GILLIS ***
Department of Pharmacology, Georgetown University, School of Medicine and Dentistry, Washington, D.C. 20007, U.S.A. Received 31 August 1977, revised MS received 4 November 1977, accepted 10 November 1977
C.J. HELKE, J.A. QUEST and R.A. GILLIS, Effects of serotonin antagonists on digitalis-induced ventricular arrhythmias, European J. Pharmacol. 47 (1978)443--449. The present study was performed to determine whether pharmacological blockade of serotonin receptors would counteract digitalis-induced ventricular arrhythmias. The effect of the serotonin receptor blocking drugs, methysergide, cinansersin, and cyproheptadine on ventricular arrhythmias produced by ouabain was studied in anesthetized dogs. Each of the three serotonin receptor blocking drugs given as a bolus i.v. injection of 1.5--3.0 mg/kg produced an antiarrhythmic effect. In addition, methysergide administered in the above doses to cats intoxicated with deslanoside, restored an abnormal ventricular arrhythmia to either sinus or junctional rhythm. Methysergide, administered to cats intoxicated with deslanoside but pretreated with p-chlorophenylalanine, exerted an antiarrhythmic effect in less than half of the animals tested. These data indicate that serotonin antagonists are effective in counteracting digitalis-induced ventricular arrhythmias and support the notion that a serotonergic mechanism may be mediating the arrhythmogenic effect of digitalis. Serotonin antagonists
Cardiac glycosides
Ventricular arrhythmias
1. I n t r o d u c t i o n
There is evidence that serotonin might be involved in the toxic effects of digitalis drugs. Buterbaugh and Spratt (1970) reported that rats pretreated with p~hlorophenylalanine (PCPA), a drug that interferes with serotonin synthesis, required a significantly larger i.v. dose of digitoxigenin to produce a lethal response (respiratory arrest). Gaitonde and Joglekar (1977) reported that cats pretreated with either P-CPA or 2-bromolysergic acid diethylamide, a drug that blocks serotonin receptors, were protected from the neuro* Supported by a grant from the U.S.P.H.S. (HE13675). ** Present address: Bureau of Drugs, Division of Cardio-Renal Drug Products, Food and Drug Administration, Rockville, Maryland 20852. *** Recipient of Research Career Development Award HL-70778 from the National Heart and Lung Institute.
toxic effects of intracerebroventricular injections of peruvoside. In a preliminary report (Helke et al., 1976) we demonstrated that cats pretreated with either P-CPA or methysergide require significantly greater doses of deslanoside to produce ventricular arrhythmias. The purpose of the present study was to determine whether or not drugs that antagonize serotonin on postsynaptic receptors would counteract digitalis-induced ventricular arrhythmias. The antagonists studied were methysergide, cinanserin, and cyproheptadine.
2. Materials and m e t h o d s
2.1. Studies in anesthetized dogs
Mongrel dogs weighing between 14.5 and 19.0 kg and unselected as to sex were anesthetized with pentobarbital sodium, 30 mg/kg i.v. Tracheal cannulation was performed and
444 all animals were artificially respired with room air. Blood pressure was measured from the left femoral artery with a pressure transducer and standard ECG limb leads were used to measure heart rate and monitor cardiac r h y t h m . Drugs were administered via the left external jugular vein. The right cervical vagus was sectioned in each animal and in selected experiments, electrically stimulated as described below. Rectal temperature was monitored and maintained between 36 and 38°C by radiant heat. To evaluate the antiarrhythmic effect of serotonin antagonists, the methods of Lucchesi and Hardman (1961) were employed. In detail, an initial dose of 40 pg/kg ouabain was administered i.v., and this dose was supplemented 30 min later by a dose of 2 0 p g / k g , followed thereafter by 1 0 p g / k g repeated every 15 min. Once a ventricular arrhythmia was established, i.e. 10 min of uninterrupted run of ventricular ectopic beats, ouabain administration was discontinued and the serotonin antagonist was administered. Criteria used to determine antiarrhythmic activity were: (1) reversion to normal sinus r h y t h m within a few min following drug administration and maintenance of a normal sinus r h y t h m for at least 30 min, and (2) the failure of electrical stimulation of the right vagus to expose automatic ectopic ventricular activity. The distal end of the cut vagus was stimulated with bipolar electrodes using a Grass stimulator, 1--7 V, 20 Hz, and 1 msec duration pulses. Stimulus parameters were ascertained for each animal in the control condition before drug administration. The serotonin antagonists were administered as i.v. bolus injections ranging between 1.5 and 3.0 mg/kg increments, separated by 1--2 min intervals. In an a t t e m p t to determine grossly the site of antiarrhythmic action of the serotonin antagonists, intra-arterial injections of one of these drugs (methysergide) were made. Our intent was to determine whether the drug was acting primarily at a central nervous system site or a cardiac site. To do this we adminis-
c.J. HELKE ET AL tered the agent by one of the following three routes: vertebral artery (3 expts.), internal carotid artery (3 expts.) and left atrium (5 expts.). At the termination of the experiments dealing with intra-arterial injections directed to the central nervous system, Evans Blue dye was injected into either the vertebral or internal carotid arteries and the brains examined to confirm the success of the injection techniques. 2.2. S t u d i e s in a n e s t h e t i z e d cats
Methysergide was also evaluated in cats. Cats of either sex with weights ranging from 1.7--3.2 kg were anesthetized with 35 mg/kg pentobarbital administerecl i.p. Tracheal cannulation was performed and all animals were artificially respired with room air. Catheters were inserted into the right femoral artery and vein for the purpose of measuring arterial blood pressure and administering drugs, respectively. The right cervical vagus was also sectioned. Blood pressure, ECG, and temperature monitoring (and maintenance) were performed as described for the dog studies. To study the antiarrhythmic activity of methysergide, the m e t h o d described by Gillis et al., 1973, was used. This consisted of administering deslanoside at a constant continuous rate of 0.75 ttg/kg/min until a ventricular arrhythmia of 3 min duration occurred. Deslanoside infusion was terminated and methysergide was administered 7 rain later in the same doses and manner as described for the dog experiments. Similar experiments were performed in animals pretreated with p-chlorophenylalanine. This agent was administered as a dose of 300 mg/kg i.p. once a day for 3 days prior to intoxicating the animals with deslanoside. This dose regimen has been d e m o n s t r a t e d in our laboratory to significantly decrease brain and duodenal s e r o t o n i n c o n t e n t to approximately 16 and 44% respectively of the control values w i t h o u t significantly affecting brain or heart norepinephrine c o n t e n t (Helke and colleagues, unpublished observations).
SEROTONIN ANTAGONISTSAND DIGITALIS-ARRHYTHMIAS 2.3. Statistical analysis and materials The data were analyzed by paired comparisons and grouped Student's t-tests. The criterion for significance was p < 0.05. Drugs used included pentobarbital sodium (Abbott Laboratories, North Chicago, Illinois), ouabain octahydrate (Sigma Chemical Co., St. Louis, Missouri), deslanoside (Sandoz Pharmaceuticals, Hanover, New Jersey, Courtesy of Kathleen D. Roskoz), cinanserin hydrochloride (Gift from E.R. Squibb and Sons, Inc., Princeton, N.Y., courtesy of S.J. Lucania), cyproheptadine hydrochloride (Gift from Merck Institute for Therapeutic Research, West Point, Pa., courtesy of Dr. A. Scriabine), and D,L-p-chlorophenylalanine methyl ester (Regis Chemical Co., Morton Grove, Ill.). All drugs (except cyproheptadine) were dissolved in normal saline and doses calculated and administered as the salts or the octahydrate. Cyproheptadine was dissolved in polyethylene glycol 400. p-Chlorophenylalanine was dissolved in bacteriostatic NaC1 (Abbott Laboratories, North Chicago, Ill. ) for injection.
3. Results
3.1. Results in anesthetized dogs 3 serotonin antagonists were tested for their ability to influence ventricular arrhythmias in the dog. These were methysergide, cinanserin, and cyproheptadine and the results are summarized in table 1. Methysergide was tested in 7 dogs and in each case the drug was found effective in reversing ventricular arrhythmias resulting from toxic concent-rations of ouabain. The antiarrhythmic dose of methysergide ranged from 6 to 30 mg/kg and averaged 13.9 + 3.1 mg/kg. The duration of reversal was greater than 60 rain in 6 of the 7 experiments. In one of the 7 experiments, the arrhythmia reappeared after 34 min. In 5 of the 7 experiments, right vagal stimulation was employed before ouabain
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intoxication, during the ouabain-induced ventricular arrhythmia, and after methysergide had counteracted the ventricular arrhythmia. Vagal stimulation resulted in sino-atrial arrest and exposure of ventricular pacemaker activity with an idioventricular rate of 45 + 2.2 beats/min before ouabain administration. N o effect of vagal stimulation was observed during the ouabain-induced ventricular arrhythmia, and asystole was observed in each instance when vagal~ stimulation was performed after methysergide conversion. Finally, associated with the antiarrhythmic effect of methysergide were significant decreases in heart rate and arterial pressure (table 1). Three experiments were performed with cinanserin and in each case there was restoration of the ouabain-induced ventricular arrhythmia to sinus rhythm. The antiarrhythmic dose of cinanserin ranged from 9 to 12 mg/kg and averaged 10.0 + 0.8 mg/kg. The duration of reversal was greater than 60 min in all 3 experiments. Vagal stimulation was performed in 2 experiments during the period of conversion and exposed an idioventricular rate of 35 and 44 beats/min. As in the case of methysergide administration, the antiarrhythmic effect of cinanserin was associated with decreases in blood pressure and heart rate (table 1). Four experiments were performed with cyproheptadine and an antiarrhythmic effect was observed in 2 of the 4 animals. This effect was also associated with decreases in blood pressure and heart rate. Antiarrhythmic activity was seen with 15 mg/kg (in both animals) and conversion lasted for more than 60 min. When dogs are intoxicated with ouabain and left untreated, the ventricular tachycardia lasts 87 + 7 min; range: 60--110 min (Somani and Lum, 1965). In one untreated dog in our study, the ventricular tachycardia lasted 204 min. We observed that conversion of the abnormal rhythm to sinus rhythm with the serotonin antagonists always occurred within 20 min after initiation of treatment. In the experiments where methysergide was
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C.J. HELKE ET AL.
TABLE 1 Effects of serotonin antagonists on toxic cardiovascular changes produced by ouabain in the dog.
Number of animals in each group Initial Heart rate(beats/min) Mean blood pressure (mm Hg)
Methysergide-treated
Cinanserin-treated
Cyproheptadine-treated
7
3
4
155± 8.8 121 ± 10.0
153± 13.3 137 ± 14.2
154_+ 11.0 132 ± 13.5
146 _+ 11.0 212 ± 17.5 1
174 ± 8.2 1 171 +_ 15.1 1
1 min before development of ventricular arrhytbmia Heart rate (beats/min) 158 ± 11.8 Mean blood pressure 163 ± 15.7 1 (mm Hg)
After 10 min of arrhythmia and just prior to administration of serotonin antagonist Heart rate (beats/min) 209 ± 15.2 1 204 ± 22.7 1 Mean blood pressure 155 ± 24.6 213 ___23.4 1 (mm Hg) After administration of antiarrhythmic doses of serotonin antagonist Heart rate (beats/min) 118 ± 10.00 2 153 ± 11.5 2 Mean blood pressure 67 ± 6.9 2 85 ± 4.0 2 (mm Hg)
221 ± 8.9 1 167 + 18.2 1
88 + 42.0 50 ± 1.5
Outcome: n of N animals exhibited conversion of the abnormal ventricular rhythm to sinus rhythm n=N=7 n=N=3 n=2, N = 4 1 p < 0.05 with paired comparisons in reference to control values. p < 0.05 with paired comparisons in reference to values obtained during ventricular arrhythmia.
2
given l o c a l l y b y i n t r a - a r t e r i a l b o l u s i n j e c t i o n s ( d o s e s r a n g i n g b e t w e e n 0 . 6 - - 3 m g / k g ) , n o sign i f i c a n t a l t e r a t i o n f r o m t h e i.v. a n t i a r r h y t h m i c d o s e was o b s e r v e d . In t h e case o f v e r t e bral artery injection, conversion of the a b n o r m a l v e n t r i c u l a r r h y t h m t o sinus r h y t h m o c c u r r e d w i t h 10.6 m g / k g in o n e a n i m a l ; n o c o n v e r s i o n was o b s e r v e d w i t h 15 m g / k g a n d 1 3 . 2 m g / k g in t h e o t h e r t w o a n i m a l s , r e s p e c t i v e l y . In o n e o f t h e s e l a t t e r t w o a n i m a l s , an a d d i t i o n a l 7.8 m g / k g b y t h e i.v. r o u t e d i d r e s t o r e t h e a b n o r m a l r h y t h m t o sinus r h y t h m . In t h e 3 a n i m a l s w h e r e m e t h y s e r g i d e was administered by the internal carotid artery r o u t e , d o s e s o f 3.0, 13.2 a n d 1 5 . 0 m g / k g w e r e r e q u i r e d f o r e x h i b i t i n g an a n t i a r r y t h m i c e f f e c t (i.e. a sinus r h y t h m o f a t l e a s t 30 m i n duration). Doses effective by the left atrial
r o u t e w e r e 5.4, 9.2, 13.2 a n d 1 8 . 2 m g / k g . In a f i f t h a n i m a l , 25 m g / k g was i n e f f e c t i v e b u t an a d d i t i o n a l 12.5 m g / k g b y t h e i.v. r o u t e was effective.
3.2. R e s u l t s in a n e s t h e t i z e d cats T h e e f f e c t s o f m e t h y s e r g i d e on t o x i c cardiovascular changes produced by a continuous i n f u s i o n o f d e s l a n o s i d e in c a t s are s u m m a r i z e d in t a b l e 2. M e t h y s e r g i d e was also a d m i n i s t e r e d t o c a t s i n t o x i c a t e d w i t h d e s l a n o s i d e b u t pret r e a t e d w i t h p - c h l o r o p h e n y l a l a n i n e (P-CPA), an a g e n t t h a t i n h i b i t s t r y p t o p h a n h y d r o x y l a s e a n d r e d u c e s t h e b r a i n c o n c e n t r a t i o n o f serot o n i n . T h e s e d a t a are also s u m m a r i z e d in t a b l e 2. In c o n t r o l a n i m a l s , v e n t r i c u l a r t a c h y c a r d i a d e v e l o p e d a f t e r an average d o s e o f 167 -+ 9.2
SEROTONIN ANTAGONISTS AND DIGITALIS-ARRHYTHMIAS TABLE 2 Effect of methysergide administration on t o x i c cardiovascular changes p r o d u c e d by c o n t i n u o u s infusions of deslanoside in control and P-CPA-treated cats.
N u m b e r o f animals in each group Initial Heart rate (beats/rain) Mean b l o o d pressure (ram Hg)
Control
p-CPA pretreated
6
9
184 _+ 4.6
174 -+ 10.5
106 _+ 8.7
91 -+ 8.6
I rain before d e v e l o p m e n t of ventricular arrhythmia Heart rate 205 _+ 11.2 186 _+ 8.3 (beats/rain) Mean b l o o d pressure 113 +_ 4.3 92 -+ 3.9 (ram Hg) A f t e r 10 rain o f arrhythmia and just prior to m e t h y sergide administration Heart rate 228_+ 6.0 1 212-+ 7.2 1 (beats/rain) Mean blood pressure 113 + 5.0 85 _+ 5.8 (ram Hg) A f t e r either conversion with methysergide or after 30 mg/kg methysergide Heart rate 135 _+ 15.2 2 124 + 8.1 2 (beats/rain) Mean blood pressure 60 + 5.9 2 53 + 4.4 2 (ram Hg) O u t c o m e : n o f N animals exhibited conversion o f the abnormal ventricular r h y t h m to sinus r h y t h m n=N=6 nf4, N=9 l p < 0.05 with paired comparisons in reference to control values. 2 p < 0.05 with paired comparisons in reference to values o b t a i n e d during ventricular arrhythmias.
#g/kg deslanoside. Methysergide administration resulted in an antiarrhythmic effect in each instance, with restoration of the abnormal rhythm to either sinus (4 of 6) or junctional rhythm (remaining 2 cats). This antiarrhythmic effect was associated with significant reductions in arterial pressure and heart rate.
The doses of methysergide required for
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producing an antiarrhythmic effect ranged from 15 to 30 mg/kg, and averaged 21.5 + 2.5 mg/kg. Conversion of the ventricular rhythm to a supraventricular rhythm seemed to be long lasting as 3 of the 6 animals were followed for 90 min after an antiarrhythmic effect was noticed with methysergide and the supraventricular rhythm was still present in each. The other 3 animals were followed for 20, 37 and 70 min and all were in a supraventricular rhythm at those times. When cats are intoxicated with deslanoside and left untreated approximately one half die from either ventricular fibrillation or hypotension, and the other half remain in a ventricular rhythm for 65 -+ 8.0 min (Gillis et al., 1973). Two animals were left untreated in the present study; one remained in a ventricular rhythm for 5 7 m i n before sinus rhythm appeared and the other remained in a ventricular arrhythmia for 246 min, and then died from hypotension. We observed that the conversion of the abnormal rhythm to sinus rhythm with methysergide always occurred within 25 min after treatment was started. These same doses of methysergide (up to 30 mg/kg) were tested in 9 animals pretreated with P-CPA. Ventricular tachycardia developed after an average dose of 205 + 13.5 #g/ kg deslanoside. In contrast to the observations made in the control animals, the effects of methysergide in these animals were distinctly different from the control group. In 5 of the 9 animals methysergide failed to restore the abnormal ventricular rhythm to either a sinus or junctional rhythm. In the remaining 4 animals, there was conversion and the dose ranged from 3 to 30 mg/kg and averaged 20.0 + 6.0 mg/kg. In these 4 animals, methysergide conversion seemed long lasting as supraventricular rhythm was still present at the times that the experiments were terminated (i.e. 79, 81, 86 and 96 min after methysergide administration). It was also observed that in the 5 animals that did n o t convert, a significantly greater dose of deslanoside was required to produce ventricular tachycardia as compared to the control group ( 2 2 0 + 2 1 . 0 p g / k g vs.
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167 + 9.2 #g/kg). Finally, although P-CPA pretreatment decreased the percentage of animals exhibiting an antiarrhythmic effect with methysergide (44 vs. 100%), pretreatment did n o t appear to m o d i f y the heart rate and blood pressure effects of methysergide (table 2). 4. Discussion The purpose of our study was to examine whether drugs that block postsynaptic serotonin receptors would counteract digitalisinduced ventricular arrhythmias. Three agents were tested and each effectively converted the abnormal ventricular rhythm to a normal sinus rhythm. Although these drugs all reduced heart rate and arterial blood pressure, they appeared to be relatively non-toxic to the cardiovascular system. Usually, the peak depressor response would occur before a converting dose was given and the level of arterial pressure would remain unchanged thereafter even though repeated injections of the serotonin antagonists were being administered. Of the agents tested, cinanserin appeared to be the least depressant to the cardiovascular system (table 1). Presumably the mechanism of the antiarrhythmic effect of methysergide, cinanserin, and cyproheptadine involved antagonism of serotonin receptors. Evidence for this is that two effects that all these drugs have in common are antagonism of serotonin receptors and reversal of digitalis-induced arrhythmias. In addition, the ability of a representative agent, methysergide, to counteract digitalisinduced arrhythmias is reduced by prior depletion of serotonin with p-chlorophenylalanine. The few P-CPA-pretreated animals that did exhibit an antiarrhythmic effect with methysergide administration may have done so because of either incomplete depletion of serotonin or subnormal amounts of serotonin being released and acting on receptors that are hypersensitive to the effects of transmitters. Hypersensitivity of serotonin receptors has been demonstrated after transmitter depletion
C.J. H E L K E ET AL.
(Trulson et al., 1976; Stewart et al., 1976). In an a t t e m p t to delineate the site of antiarrhythmic action of the serotonin antagonists, we utilized the technique employed by Wang and colleagues (Lisander et al., 1975; Chai et al., 1976). That is, we administered one of these agents (methysergide) by various routes to determine the relative effectiveness of the drug in counteracting digitalisinduced arrhythmias. We were not able to detect any important differences in doses necessary to block digitalis-induced ventricular arrhythmias with methysergide. These results suggest that serotonin receptor antagonists, as exemplified by methysergide, may be exerting their antiarrhythmic effect at several sites. The heart might be the most important site, as a brief period of cardiac arrest was sometimes observed when vagal stimulation was performed after administration of an antiarrhythmic dose of the serotonin receptor blocking agents. Some of the findings of the present study add to our previous evidence (Helke et al., 1976) demonstrating that serotonin is involved in the arrhythmogenic effects of digitalis. This is indicated in the cat experiments with deslanoside wherein animals depleted of serotonin after P-CPA administration required significantly larger doses of deslanoside to produce ventricular arrhythmias as compared to control animals. This was especially true of the P-CPA-pretreated animals that failed to respond to methysergide. It was true of all the P-CPA animals treated as a group. Application of the " l - s i d e d " t-test to these data and the data of the control group revealed that the P-CPA-pretreated group required a significantly greater dose of deslanoside to produce ventricular arrhythmias. We are aware of only one other study that has been published regarding possible antiarrhythmic effects of serotonin antagonists. This study (Singh et al., 1974) dealt with cyproheptadine and demonstrated that this agent was effective in countering ventricular arrhythmias produced by epinephrine in dogs, chloroform in mice, and a combination
SEROTONIN ANTAGONISTS AND DIGITALIS-ARRHYTHMIAS
of pentothal sodium and barium chloride in dogs. In addition, they demonstrated that cyproheptadine exerted no beta adrenergic blocking activity in dogs but did produce local anesthesia in rabbits and guinea pigs. They concluded that cyproheptadine's antiarrhythmic effect was related to its local anesthetic activity. They did not test cyproheptadine against ventricular arrhythmias produced by digitalis. Their data do suggest that cyproheptadine has an antiarrhythmic effect that may be unrelated to an effect on serotonin. This may also be true of the 5-HT antagonists tested in the present study. This is suggested by the findings that methysergide did exert an antiarrhythmic effect in a few animals that were pretreated with P-CPA. It is also suggested by the finding that vagal stimulation produced cardiac arrest in dogs treated with methysergide. As indicated above, our data do suggest that the antiarrhythmic effect of serotonin antagonists may be in part mediated by blockade of serotonin receptors. This does not seem to be the case with the hypotensive and bradycardic effects of these drugs. We found that methysergide produces the same decreases in blood pressure and heart rate in P-CPA-treated animals as in controls, indicating that these effects may be independent of blockade of 5-HT receptors. Antonaccio and colleagues (1975) reached a similar conclusion in their study of the cardiovascular effects of methysergide and cyproheptadine in dogs. They demonstrated that methysergide, but not cyproheptadine caused reduction in arterial pressure and heart rate, and counteracted the pressor response to bilateral carotid occlusion. Since cyproheptadine did not produce these changes they reasoned that the cardiovascular effects of methysergide were not the results of 5-HT receptor blockade. We did, however, observe cardiovascular effects (i.e. falls in blood pressure and heart rate) with cyproheptadine that were similar to those obtained with methysergide. In summary, we found drugs which are effective as serotonin antagonists effective in counteracting digitalis-induced ventricular
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arrhythmias. In addition, in view of the efficacy and safety of these agents, they may be worth investigating further in patients with digitalis-induced ventricular arrhythmias.
References Antonaccio, M.J., E. Kelly and J. Haley, 1975, Centrally mediated hypotension and bradycardia by methysergide in anesthetized dogs, European J. Pharmacol. 33, 107. Buterbaugh, G.G. and J.L. Spratt, 1970, The possible role of brain monoamines in the acute toxicity of digitoxigenin, J. Pharmacol. Exptl. Therap. 175, 121. Chai, C.Y., T.M. Lee and S.C. Wang, 1976, Effects of diphenylhydantoin on cardiac arrhythmias induced by carotid occlusion in the cat, Arch. Intern. Pharmacodyn. 219, 180. Gaitonde, B.B. and S.N. Joglekar, 1977, Mechanism of neurotoxicity of cardiotonic glycosides, Brit. J. Pharmacol. 59, 223. Gillis, R.A., M.M. Clancy and R.J. Anderson, 1973, Deleterious effects of bretylium in cats with digitalis-induced ventricular tachycardia, Circulation 47,974. Helke, C.J., J. Dias Souza, B.L. Hamilton, V.H. Morgenroth, III and R.A. Gillis, 1976, Evidence for a role of central serotonergic neurons in digitalis-induced cardiac arrhythmias, Nature 263, 246. Lisander, B.J., J. Bhruvaneshwar and S.C. Wang, 1975, CNS site of antiarrhythmic action of diphenylhydantoin (DPH) in the cat, European J. Pharmacol. 31, 53. Lucchesi, B.R. and H.F. Hardman, 1961, The influence of dichloroisoproterenol (DCI) and related compounds upon ouabain and acetylstrophanthidin-induced cardiac arrhythmias, J. Pharmacol. Exptl. Therap. 132, 372. Singh, K.P., V.K. Pendse and D.S. Bhandari, 1974, Cyproheptadine in ventricular arrhythmias, Indian Heart J. 27,120. Somani, P. and B.K.B. Lum, 1965, The antiarrhythmic actions of ~-adrenergic blocking agents, J. Pharmacol. Exptl. Therap. 147, 194. Stewart, R.M., J.H. Growdon, D. Cancian and R.J. Baldessarini, 1976, 5-hydroxytryptophan-induced myoclonus: increased sensitivity to serotonin after intracranial 5,7-dihydroxytryptamine in the adult rat, Neuropharmacol. 15, 449. Trulson, M.E., E.E. Eubanks and B.L. Jacobs, 1976, Behavioral evidence for supersensitivity following destruction of central serotonergic nerve terminals by 5,7-dihydroxytryptamine, J. Pharmacol. Exptl. Therap. 198, 23.
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© Elsevier/North-Holland Biomedical Press
EFFECTS OF SEROTONIN ANTAGONISTS ON DIGITALIS-INDUCED VENTRICULAR ARRHYTHMIAS
*
CINDA J. HELKE, JOHN A. QUEST ** and RICHARD A. GILLIS ***
Department of Pharmacology, Georgetown University, School of Medicine and Dentistry, Washington, D.C. 20007, U.S.A. Received 31 August 1977, revised MS received 4 November 1977, accepted 10 November 1977
C.J. HELKE, J.A. QUEST and R.A. GILLIS, Effects of serotonin antagonists on digitalis-induced ventricular arrhythmias, European J. Pharmacol. 47 (1978)443--449. The present study was performed to determine whether pharmacological blockade of serotonin receptors would counteract digitalis-induced ventricular arrhythmias. The effect of the serotonin receptor blocking drugs, methysergide, cinansersin, and cyproheptadine on ventricular arrhythmias produced by ouabain was studied in anesthetized dogs. Each of the three serotonin receptor blocking drugs given as a bolus i.v. injection of 1.5--3.0 mg/kg produced an antiarrhythmic effect. In addition, methysergide administered in the above doses to cats intoxicated with deslanoside, restored an abnormal ventricular arrhythmia to either sinus or junctional rhythm. Methysergide, administered to cats intoxicated with deslanoside but pretreated with p-chlorophenylalanine, exerted an antiarrhythmic effect in less than half of the animals tested. These data indicate that serotonin antagonists are effective in counteracting digitalis-induced ventricular arrhythmias and support the notion that a serotonergic mechanism may be mediating the arrhythmogenic effect of digitalis. Serotonin antagonists
Cardiac glycosides
Ventricular arrhythmias
1. I n t r o d u c t i o n
There is evidence that serotonin might be involved in the toxic effects of digitalis drugs. Buterbaugh and Spratt (1970) reported that rats pretreated with p~hlorophenylalanine (PCPA), a drug that interferes with serotonin synthesis, required a significantly larger i.v. dose of digitoxigenin to produce a lethal response (respiratory arrest). Gaitonde and Joglekar (1977) reported that cats pretreated with either P-CPA or 2-bromolysergic acid diethylamide, a drug that blocks serotonin receptors, were protected from the neuro* Supported by a grant from the U.S.P.H.S. (HE13675). ** Present address: Bureau of Drugs, Division of Cardio-Renal Drug Products, Food and Drug Administration, Rockville, Maryland 20852. *** Recipient of Research Career Development Award HL-70778 from the National Heart and Lung Institute.
toxic effects of intracerebroventricular injections of peruvoside. In a preliminary report (Helke et al., 1976) we demonstrated that cats pretreated with either P-CPA or methysergide require significantly greater doses of deslanoside to produce ventricular arrhythmias. The purpose of the present study was to determine whether or not drugs that antagonize serotonin on postsynaptic receptors would counteract digitalis-induced ventricular arrhythmias. The antagonists studied were methysergide, cinanserin, and cyproheptadine.
2. Materials and m e t h o d s
2.1. Studies in anesthetized dogs
Mongrel dogs weighing between 14.5 and 19.0 kg and unselected as to sex were anesthetized with pentobarbital sodium, 30 mg/kg i.v. Tracheal cannulation was performed and
444 all animals were artificially respired with room air. Blood pressure was measured from the left femoral artery with a pressure transducer and standard ECG limb leads were used to measure heart rate and monitor cardiac r h y t h m . Drugs were administered via the left external jugular vein. The right cervical vagus was sectioned in each animal and in selected experiments, electrically stimulated as described below. Rectal temperature was monitored and maintained between 36 and 38°C by radiant heat. To evaluate the antiarrhythmic effect of serotonin antagonists, the methods of Lucchesi and Hardman (1961) were employed. In detail, an initial dose of 40 pg/kg ouabain was administered i.v., and this dose was supplemented 30 min later by a dose of 2 0 p g / k g , followed thereafter by 1 0 p g / k g repeated every 15 min. Once a ventricular arrhythmia was established, i.e. 10 min of uninterrupted run of ventricular ectopic beats, ouabain administration was discontinued and the serotonin antagonist was administered. Criteria used to determine antiarrhythmic activity were: (1) reversion to normal sinus r h y t h m within a few min following drug administration and maintenance of a normal sinus r h y t h m for at least 30 min, and (2) the failure of electrical stimulation of the right vagus to expose automatic ectopic ventricular activity. The distal end of the cut vagus was stimulated with bipolar electrodes using a Grass stimulator, 1--7 V, 20 Hz, and 1 msec duration pulses. Stimulus parameters were ascertained for each animal in the control condition before drug administration. The serotonin antagonists were administered as i.v. bolus injections ranging between 1.5 and 3.0 mg/kg increments, separated by 1--2 min intervals. In an a t t e m p t to determine grossly the site of antiarrhythmic action of the serotonin antagonists, intra-arterial injections of one of these drugs (methysergide) were made. Our intent was to determine whether the drug was acting primarily at a central nervous system site or a cardiac site. To do this we adminis-
c.J. HELKE ET AL tered the agent by one of the following three routes: vertebral artery (3 expts.), internal carotid artery (3 expts.) and left atrium (5 expts.). At the termination of the experiments dealing with intra-arterial injections directed to the central nervous system, Evans Blue dye was injected into either the vertebral or internal carotid arteries and the brains examined to confirm the success of the injection techniques. 2.2. S t u d i e s in a n e s t h e t i z e d cats
Methysergide was also evaluated in cats. Cats of either sex with weights ranging from 1.7--3.2 kg were anesthetized with 35 mg/kg pentobarbital administerecl i.p. Tracheal cannulation was performed and all animals were artificially respired with room air. Catheters were inserted into the right femoral artery and vein for the purpose of measuring arterial blood pressure and administering drugs, respectively. The right cervical vagus was also sectioned. Blood pressure, ECG, and temperature monitoring (and maintenance) were performed as described for the dog studies. To study the antiarrhythmic activity of methysergide, the m e t h o d described by Gillis et al., 1973, was used. This consisted of administering deslanoside at a constant continuous rate of 0.75 ttg/kg/min until a ventricular arrhythmia of 3 min duration occurred. Deslanoside infusion was terminated and methysergide was administered 7 rain later in the same doses and manner as described for the dog experiments. Similar experiments were performed in animals pretreated with p-chlorophenylalanine. This agent was administered as a dose of 300 mg/kg i.p. once a day for 3 days prior to intoxicating the animals with deslanoside. This dose regimen has been d e m o n s t r a t e d in our laboratory to significantly decrease brain and duodenal s e r o t o n i n c o n t e n t to approximately 16 and 44% respectively of the control values w i t h o u t significantly affecting brain or heart norepinephrine c o n t e n t (Helke and colleagues, unpublished observations).
SEROTONIN ANTAGONISTSAND DIGITALIS-ARRHYTHMIAS 2.3. Statistical analysis and materials The data were analyzed by paired comparisons and grouped Student's t-tests. The criterion for significance was p < 0.05. Drugs used included pentobarbital sodium (Abbott Laboratories, North Chicago, Illinois), ouabain octahydrate (Sigma Chemical Co., St. Louis, Missouri), deslanoside (Sandoz Pharmaceuticals, Hanover, New Jersey, Courtesy of Kathleen D. Roskoz), cinanserin hydrochloride (Gift from E.R. Squibb and Sons, Inc., Princeton, N.Y., courtesy of S.J. Lucania), cyproheptadine hydrochloride (Gift from Merck Institute for Therapeutic Research, West Point, Pa., courtesy of Dr. A. Scriabine), and D,L-p-chlorophenylalanine methyl ester (Regis Chemical Co., Morton Grove, Ill.). All drugs (except cyproheptadine) were dissolved in normal saline and doses calculated and administered as the salts or the octahydrate. Cyproheptadine was dissolved in polyethylene glycol 400. p-Chlorophenylalanine was dissolved in bacteriostatic NaC1 (Abbott Laboratories, North Chicago, Ill. ) for injection.
3. Results
3.1. Results in anesthetized dogs 3 serotonin antagonists were tested for their ability to influence ventricular arrhythmias in the dog. These were methysergide, cinanserin, and cyproheptadine and the results are summarized in table 1. Methysergide was tested in 7 dogs and in each case the drug was found effective in reversing ventricular arrhythmias resulting from toxic concent-rations of ouabain. The antiarrhythmic dose of methysergide ranged from 6 to 30 mg/kg and averaged 13.9 + 3.1 mg/kg. The duration of reversal was greater than 60 rain in 6 of the 7 experiments. In one of the 7 experiments, the arrhythmia reappeared after 34 min. In 5 of the 7 experiments, right vagal stimulation was employed before ouabain
445
intoxication, during the ouabain-induced ventricular arrhythmia, and after methysergide had counteracted the ventricular arrhythmia. Vagal stimulation resulted in sino-atrial arrest and exposure of ventricular pacemaker activity with an idioventricular rate of 45 + 2.2 beats/min before ouabain administration. N o effect of vagal stimulation was observed during the ouabain-induced ventricular arrhythmia, and asystole was observed in each instance when vagal~ stimulation was performed after methysergide conversion. Finally, associated with the antiarrhythmic effect of methysergide were significant decreases in heart rate and arterial pressure (table 1). Three experiments were performed with cinanserin and in each case there was restoration of the ouabain-induced ventricular arrhythmia to sinus rhythm. The antiarrhythmic dose of cinanserin ranged from 9 to 12 mg/kg and averaged 10.0 + 0.8 mg/kg. The duration of reversal was greater than 60 min in all 3 experiments. Vagal stimulation was performed in 2 experiments during the period of conversion and exposed an idioventricular rate of 35 and 44 beats/min. As in the case of methysergide administration, the antiarrhythmic effect of cinanserin was associated with decreases in blood pressure and heart rate (table 1). Four experiments were performed with cyproheptadine and an antiarrhythmic effect was observed in 2 of the 4 animals. This effect was also associated with decreases in blood pressure and heart rate. Antiarrhythmic activity was seen with 15 mg/kg (in both animals) and conversion lasted for more than 60 min. When dogs are intoxicated with ouabain and left untreated, the ventricular tachycardia lasts 87 + 7 min; range: 60--110 min (Somani and Lum, 1965). In one untreated dog in our study, the ventricular tachycardia lasted 204 min. We observed that conversion of the abnormal rhythm to sinus rhythm with the serotonin antagonists always occurred within 20 min after initiation of treatment. In the experiments where methysergide was
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C.J. HELKE ET AL.
TABLE 1 Effects of serotonin antagonists on toxic cardiovascular changes produced by ouabain in the dog.
Number of animals in each group Initial Heart rate(beats/min) Mean blood pressure (mm Hg)
Methysergide-treated
Cinanserin-treated
Cyproheptadine-treated
7
3
4
155± 8.8 121 ± 10.0
153± 13.3 137 ± 14.2
154_+ 11.0 132 ± 13.5
146 _+ 11.0 212 ± 17.5 1
174 ± 8.2 1 171 +_ 15.1 1
1 min before development of ventricular arrhytbmia Heart rate (beats/min) 158 ± 11.8 Mean blood pressure 163 ± 15.7 1 (mm Hg)
After 10 min of arrhythmia and just prior to administration of serotonin antagonist Heart rate (beats/min) 209 ± 15.2 1 204 ± 22.7 1 Mean blood pressure 155 ± 24.6 213 ___23.4 1 (mm Hg) After administration of antiarrhythmic doses of serotonin antagonist Heart rate (beats/min) 118 ± 10.00 2 153 ± 11.5 2 Mean blood pressure 67 ± 6.9 2 85 ± 4.0 2 (mm Hg)
221 ± 8.9 1 167 + 18.2 1
88 + 42.0 50 ± 1.5
Outcome: n of N animals exhibited conversion of the abnormal ventricular rhythm to sinus rhythm n=N=7 n=N=3 n=2, N = 4 1 p < 0.05 with paired comparisons in reference to control values. p < 0.05 with paired comparisons in reference to values obtained during ventricular arrhythmia.
2
given l o c a l l y b y i n t r a - a r t e r i a l b o l u s i n j e c t i o n s ( d o s e s r a n g i n g b e t w e e n 0 . 6 - - 3 m g / k g ) , n o sign i f i c a n t a l t e r a t i o n f r o m t h e i.v. a n t i a r r h y t h m i c d o s e was o b s e r v e d . In t h e case o f v e r t e bral artery injection, conversion of the a b n o r m a l v e n t r i c u l a r r h y t h m t o sinus r h y t h m o c c u r r e d w i t h 10.6 m g / k g in o n e a n i m a l ; n o c o n v e r s i o n was o b s e r v e d w i t h 15 m g / k g a n d 1 3 . 2 m g / k g in t h e o t h e r t w o a n i m a l s , r e s p e c t i v e l y . In o n e o f t h e s e l a t t e r t w o a n i m a l s , an a d d i t i o n a l 7.8 m g / k g b y t h e i.v. r o u t e d i d r e s t o r e t h e a b n o r m a l r h y t h m t o sinus r h y t h m . In t h e 3 a n i m a l s w h e r e m e t h y s e r g i d e was administered by the internal carotid artery r o u t e , d o s e s o f 3.0, 13.2 a n d 1 5 . 0 m g / k g w e r e r e q u i r e d f o r e x h i b i t i n g an a n t i a r r y t h m i c e f f e c t (i.e. a sinus r h y t h m o f a t l e a s t 30 m i n duration). Doses effective by the left atrial
r o u t e w e r e 5.4, 9.2, 13.2 a n d 1 8 . 2 m g / k g . In a f i f t h a n i m a l , 25 m g / k g was i n e f f e c t i v e b u t an a d d i t i o n a l 12.5 m g / k g b y t h e i.v. r o u t e was effective.
3.2. R e s u l t s in a n e s t h e t i z e d cats T h e e f f e c t s o f m e t h y s e r g i d e on t o x i c cardiovascular changes produced by a continuous i n f u s i o n o f d e s l a n o s i d e in c a t s are s u m m a r i z e d in t a b l e 2. M e t h y s e r g i d e was also a d m i n i s t e r e d t o c a t s i n t o x i c a t e d w i t h d e s l a n o s i d e b u t pret r e a t e d w i t h p - c h l o r o p h e n y l a l a n i n e (P-CPA), an a g e n t t h a t i n h i b i t s t r y p t o p h a n h y d r o x y l a s e a n d r e d u c e s t h e b r a i n c o n c e n t r a t i o n o f serot o n i n . T h e s e d a t a are also s u m m a r i z e d in t a b l e 2. In c o n t r o l a n i m a l s , v e n t r i c u l a r t a c h y c a r d i a d e v e l o p e d a f t e r an average d o s e o f 167 -+ 9.2
SEROTONIN ANTAGONISTS AND DIGITALIS-ARRHYTHMIAS TABLE 2 Effect of methysergide administration on t o x i c cardiovascular changes p r o d u c e d by c o n t i n u o u s infusions of deslanoside in control and P-CPA-treated cats.
N u m b e r o f animals in each group Initial Heart rate (beats/rain) Mean b l o o d pressure (ram Hg)
Control
p-CPA pretreated
6
9
184 _+ 4.6
174 -+ 10.5
106 _+ 8.7
91 -+ 8.6
I rain before d e v e l o p m e n t of ventricular arrhythmia Heart rate 205 _+ 11.2 186 _+ 8.3 (beats/rain) Mean b l o o d pressure 113 +_ 4.3 92 -+ 3.9 (ram Hg) A f t e r 10 rain o f arrhythmia and just prior to m e t h y sergide administration Heart rate 228_+ 6.0 1 212-+ 7.2 1 (beats/rain) Mean blood pressure 113 + 5.0 85 _+ 5.8 (ram Hg) A f t e r either conversion with methysergide or after 30 mg/kg methysergide Heart rate 135 _+ 15.2 2 124 + 8.1 2 (beats/rain) Mean blood pressure 60 + 5.9 2 53 + 4.4 2 (ram Hg) O u t c o m e : n o f N animals exhibited conversion o f the abnormal ventricular r h y t h m to sinus r h y t h m n=N=6 nf4, N=9 l p < 0.05 with paired comparisons in reference to control values. 2 p < 0.05 with paired comparisons in reference to values o b t a i n e d during ventricular arrhythmias.
#g/kg deslanoside. Methysergide administration resulted in an antiarrhythmic effect in each instance, with restoration of the abnormal rhythm to either sinus (4 of 6) or junctional rhythm (remaining 2 cats). This antiarrhythmic effect was associated with significant reductions in arterial pressure and heart rate.
The doses of methysergide required for
447
producing an antiarrhythmic effect ranged from 15 to 30 mg/kg, and averaged 21.5 + 2.5 mg/kg. Conversion of the ventricular rhythm to a supraventricular rhythm seemed to be long lasting as 3 of the 6 animals were followed for 90 min after an antiarrhythmic effect was noticed with methysergide and the supraventricular rhythm was still present in each. The other 3 animals were followed for 20, 37 and 70 min and all were in a supraventricular rhythm at those times. When cats are intoxicated with deslanoside and left untreated approximately one half die from either ventricular fibrillation or hypotension, and the other half remain in a ventricular rhythm for 65 -+ 8.0 min (Gillis et al., 1973). Two animals were left untreated in the present study; one remained in a ventricular rhythm for 5 7 m i n before sinus rhythm appeared and the other remained in a ventricular arrhythmia for 246 min, and then died from hypotension. We observed that the conversion of the abnormal rhythm to sinus rhythm with methysergide always occurred within 25 min after treatment was started. These same doses of methysergide (up to 30 mg/kg) were tested in 9 animals pretreated with P-CPA. Ventricular tachycardia developed after an average dose of 205 + 13.5 #g/ kg deslanoside. In contrast to the observations made in the control animals, the effects of methysergide in these animals were distinctly different from the control group. In 5 of the 9 animals methysergide failed to restore the abnormal ventricular rhythm to either a sinus or junctional rhythm. In the remaining 4 animals, there was conversion and the dose ranged from 3 to 30 mg/kg and averaged 20.0 + 6.0 mg/kg. In these 4 animals, methysergide conversion seemed long lasting as supraventricular rhythm was still present at the times that the experiments were terminated (i.e. 79, 81, 86 and 96 min after methysergide administration). It was also observed that in the 5 animals that did n o t convert, a significantly greater dose of deslanoside was required to produce ventricular tachycardia as compared to the control group ( 2 2 0 + 2 1 . 0 p g / k g vs.
448
167 + 9.2 #g/kg). Finally, although P-CPA pretreatment decreased the percentage of animals exhibiting an antiarrhythmic effect with methysergide (44 vs. 100%), pretreatment did n o t appear to m o d i f y the heart rate and blood pressure effects of methysergide (table 2). 4. Discussion The purpose of our study was to examine whether drugs that block postsynaptic serotonin receptors would counteract digitalisinduced ventricular arrhythmias. Three agents were tested and each effectively converted the abnormal ventricular rhythm to a normal sinus rhythm. Although these drugs all reduced heart rate and arterial blood pressure, they appeared to be relatively non-toxic to the cardiovascular system. Usually, the peak depressor response would occur before a converting dose was given and the level of arterial pressure would remain unchanged thereafter even though repeated injections of the serotonin antagonists were being administered. Of the agents tested, cinanserin appeared to be the least depressant to the cardiovascular system (table 1). Presumably the mechanism of the antiarrhythmic effect of methysergide, cinanserin, and cyproheptadine involved antagonism of serotonin receptors. Evidence for this is that two effects that all these drugs have in common are antagonism of serotonin receptors and reversal of digitalis-induced arrhythmias. In addition, the ability of a representative agent, methysergide, to counteract digitalisinduced arrhythmias is reduced by prior depletion of serotonin with p-chlorophenylalanine. The few P-CPA-pretreated animals that did exhibit an antiarrhythmic effect with methysergide administration may have done so because of either incomplete depletion of serotonin or subnormal amounts of serotonin being released and acting on receptors that are hypersensitive to the effects of transmitters. Hypersensitivity of serotonin receptors has been demonstrated after transmitter depletion
C.J. H E L K E ET AL.
(Trulson et al., 1976; Stewart et al., 1976). In an a t t e m p t to delineate the site of antiarrhythmic action of the serotonin antagonists, we utilized the technique employed by Wang and colleagues (Lisander et al., 1975; Chai et al., 1976). That is, we administered one of these agents (methysergide) by various routes to determine the relative effectiveness of the drug in counteracting digitalisinduced arrhythmias. We were not able to detect any important differences in doses necessary to block digitalis-induced ventricular arrhythmias with methysergide. These results suggest that serotonin receptor antagonists, as exemplified by methysergide, may be exerting their antiarrhythmic effect at several sites. The heart might be the most important site, as a brief period of cardiac arrest was sometimes observed when vagal stimulation was performed after administration of an antiarrhythmic dose of the serotonin receptor blocking agents. Some of the findings of the present study add to our previous evidence (Helke et al., 1976) demonstrating that serotonin is involved in the arrhythmogenic effects of digitalis. This is indicated in the cat experiments with deslanoside wherein animals depleted of serotonin after P-CPA administration required significantly larger doses of deslanoside to produce ventricular arrhythmias as compared to control animals. This was especially true of the P-CPA-pretreated animals that failed to respond to methysergide. It was true of all the P-CPA animals treated as a group. Application of the " l - s i d e d " t-test to these data and the data of the control group revealed that the P-CPA-pretreated group required a significantly greater dose of deslanoside to produce ventricular arrhythmias. We are aware of only one other study that has been published regarding possible antiarrhythmic effects of serotonin antagonists. This study (Singh et al., 1974) dealt with cyproheptadine and demonstrated that this agent was effective in countering ventricular arrhythmias produced by epinephrine in dogs, chloroform in mice, and a combination
SEROTONIN ANTAGONISTS AND DIGITALIS-ARRHYTHMIAS
of pentothal sodium and barium chloride in dogs. In addition, they demonstrated that cyproheptadine exerted no beta adrenergic blocking activity in dogs but did produce local anesthesia in rabbits and guinea pigs. They concluded that cyproheptadine's antiarrhythmic effect was related to its local anesthetic activity. They did not test cyproheptadine against ventricular arrhythmias produced by digitalis. Their data do suggest that cyproheptadine has an antiarrhythmic effect that may be unrelated to an effect on serotonin. This may also be true of the 5-HT antagonists tested in the present study. This is suggested by the findings that methysergide did exert an antiarrhythmic effect in a few animals that were pretreated with P-CPA. It is also suggested by the finding that vagal stimulation produced cardiac arrest in dogs treated with methysergide. As indicated above, our data do suggest that the antiarrhythmic effect of serotonin antagonists may be in part mediated by blockade of serotonin receptors. This does not seem to be the case with the hypotensive and bradycardic effects of these drugs. We found that methysergide produces the same decreases in blood pressure and heart rate in P-CPA-treated animals as in controls, indicating that these effects may be independent of blockade of 5-HT receptors. Antonaccio and colleagues (1975) reached a similar conclusion in their study of the cardiovascular effects of methysergide and cyproheptadine in dogs. They demonstrated that methysergide, but not cyproheptadine caused reduction in arterial pressure and heart rate, and counteracted the pressor response to bilateral carotid occlusion. Since cyproheptadine did not produce these changes they reasoned that the cardiovascular effects of methysergide were not the results of 5-HT receptor blockade. We did, however, observe cardiovascular effects (i.e. falls in blood pressure and heart rate) with cyproheptadine that were similar to those obtained with methysergide. In summary, we found drugs which are effective as serotonin antagonists effective in counteracting digitalis-induced ventricular
449
arrhythmias. In addition, in view of the efficacy and safety of these agents, they may be worth investigating further in patients with digitalis-induced ventricular arrhythmias.
References Antonaccio, M.J., E. Kelly and J. Haley, 1975, Centrally mediated hypotension and bradycardia by methysergide in anesthetized dogs, European J. Pharmacol. 33, 107. Buterbaugh, G.G. and J.L. Spratt, 1970, The possible role of brain monoamines in the acute toxicity of digitoxigenin, J. Pharmacol. Exptl. Therap. 175, 121. Chai, C.Y., T.M. Lee and S.C. Wang, 1976, Effects of diphenylhydantoin on cardiac arrhythmias induced by carotid occlusion in the cat, Arch. Intern. Pharmacodyn. 219, 180. Gaitonde, B.B. and S.N. Joglekar, 1977, Mechanism of neurotoxicity of cardiotonic glycosides, Brit. J. Pharmacol. 59, 223. Gillis, R.A., M.M. Clancy and R.J. Anderson, 1973, Deleterious effects of bretylium in cats with digitalis-induced ventricular tachycardia, Circulation 47,974. Helke, C.J., J. Dias Souza, B.L. Hamilton, V.H. Morgenroth, III and R.A. Gillis, 1976, Evidence for a role of central serotonergic neurons in digitalis-induced cardiac arrhythmias, Nature 263, 246. Lisander, B.J., J. Bhruvaneshwar and S.C. Wang, 1975, CNS site of antiarrhythmic action of diphenylhydantoin (DPH) in the cat, European J. Pharmacol. 31, 53. Lucchesi, B.R. and H.F. Hardman, 1961, The influence of dichloroisoproterenol (DCI) and related compounds upon ouabain and acetylstrophanthidin-induced cardiac arrhythmias, J. Pharmacol. Exptl. Therap. 132, 372. Singh, K.P., V.K. Pendse and D.S. Bhandari, 1974, Cyproheptadine in ventricular arrhythmias, Indian Heart J. 27,120. Somani, P. and B.K.B. Lum, 1965, The antiarrhythmic actions of ~-adrenergic blocking agents, J. Pharmacol. Exptl. Therap. 147, 194. Stewart, R.M., J.H. Growdon, D. Cancian and R.J. Baldessarini, 1976, 5-hydroxytryptophan-induced myoclonus: increased sensitivity to serotonin after intracranial 5,7-dihydroxytryptamine in the adult rat, Neuropharmacol. 15, 449. Trulson, M.E., E.E. Eubanks and B.L. Jacobs, 1976, Behavioral evidence for supersensitivity following destruction of central serotonergic nerve terminals by 5,7-dihydroxytryptamine, J. Pharmacol. Exptl. Therap. 198, 23.
