European Journal of Pharmacology, 44 (1977) 57--63 © Elsevier/North-Holland Biomedical Press
A POSSIBLE MECHANISM
57
OF THE ANTIARRHYTHMIC
ACTION OF GLUCAGON
*
R. DOUGLAS WILKERSON **, DAVID B. PARTLOW ***, JACK K. PRUETT and CHARLES W. PATTERSON
Department of Pharmacology, University of South Alabama, College of Medicine, Mobile, Alabama 36688, and Department of Pharmacology, Medical University of South Carolina, Charleston, South Carolina 29401, U.S.A. Received 30 November 1976, revised MS received 15 February 1977, accepted 17 March 1977
R.D. WILKERSON, D.B. PARTLOW, J.K. PRUETT and C.W. PATTERSON, A possible mechanism of the antiarrhythmic action ofglucagon, European J. Pharmacol. 44 (1977) 57---63. The effects of glucagon on ouabain-induced ventricular arrhythmias were studied in dogs with normal cardiac conduction systems and in dogs with surgically induced complete heart block. Glucagon, 20 pg/kg, effected conversion to a sinus rhythm in eight of nine animals with normal conduction systems, and in each instance the conversion occurred with a sinus tachycardia whose rate exceeded the rate of pre-existing ventricular arrhythmia. The one animal which failed to convert to a sinus rythm did not develop a sinus nodal rate faster than the rate of the ventricular arrhythmia. In 10 animals with ouabain-induced ventricular arrhythmias atrial pacing was employed to overdrive the arrhythmia, and in 6 cases this was successful. In all 6 experiments subsequent administration of glucagon after pacing resulted in conversion to a sinus rhythm at a rate greater than the minimum atrial pacing rate required to effect capture of the ventricules. In 4 of the 10 animals neither atrial overdrive nor glucagon were successful in effecting sinus conversion, but ventricular rate was slowed by lidocaine. In addition, in 5 animals with complete heart block and ouabain-induced ventricular tachycardia which were given glucagon, 20 pg/kg, there was no change in the tachycardia and no change in the ECG pattern associated with the arrhythmia. It was concluded that the antiarrhytmic action of glucagon in this arrhythmia is mediated through a supraventricular action to elevate sinus rate above that of the dominant ventricular focus and thus allow a return to dominance of the sinus node. Heart block, complete
Arrhythmia
Ouabain
1. I n t r o d u c t i o n Glucagon has received considerable attent i o n as a c a r d i a c d r u g w h e n a d m i n i s t e r e d i n pharmacologic doses (Parmley and Sonnenblick, 1971; Kones and Phillips, 1972), and o n e o f i t s i n v e s t i g a t e d u s e s h a s b e e n as a n a n t i a r r h y t h m i c d r u g in d i g i t a l i s - i n d u c e d v e n tricular arrhythmias ( C o h n e t al., 1 9 7 0 ; * Supported by a grant from the Alabama Heart Association. * * A d d r e s s reprint requests to: R.D. Wilkerson, Ph.D., Department of Pharmacology, College of Medicine, University of South Alabama, Mobile, Alabama 36688, U.S.A. *** Supported by a summer fellowship from the University of South Alabama, College of Medicine.
Glucagon
Lidocaine
Atrial pacing
M a d a n e t al., 1 9 7 1 ; P r a s a d a n d D e S o u s a , 1 9 7 2 ) as w e l l as a r r h y t h m i a s p r o d u c e d b y coronary artery occlusion (Madan, 1971). The present study was designed to provide a better understanding of the mechanism of this antiarrhythmic activity. The positive inotropic and chronotropic effects of glucagon were first documented by Farah and Tuttle (1960). Lucchesi (1968) subsequently demonstrated that these actions p e r s i s t e d in t h e p r e s e n c e o f b e t a a d r e n e r g i c blockade by propranolol, thus distinguishing the cardiac actions of glucagon from those of the catecholamines. In addition, electrophysiologic studies demonstrated that glucagon increases atrioventricular conduction velocity ( S t e i n e r e t al., 1 9 6 9 ; L i p s k i e t al., 1 9 7 2 ) a n d AV nodal automaticity ( L u c c h e s i e t al.,
58 1969). A direct action to enhance SA nodal automaticity is u n d o u b t e d l y responsible for its positive chronotropic effect (Whitehouse et al., 1966). Conflicting results have been obtained in studies evaluating the action of glucagon on ventricular automaticity. While some investigators have described no change in ventricular automaticity (Steiner et al., 1969; Lucchesi et al., 1969), others have reported an enhancement of automaticity in experimental animals (Wilkerson et al., 1971; Lipski et al., 1972) and man (Hurwitz, 1973) and isolated Purkinje fibers (Pruett et al., 1971). Daniell and coworkers (1970) also demonstrated an interesting difference in the ventricular automaticity response to glucagon between animals with acute and chronic heart block. They demonstrated that the ventricular automaticity enhancement noted in dogs with chronic heart block could not be demonstrated in animals after acute heart block. This observation was confirmed by Lavarenne, and his associates (1976) in a study which also demonstrated the effects of different experimental conditions on the cardiac response to glucagon. Wilkerson and coworkers (1971) demonstrated that the effects of glucagon on ventricular automaticity in animals with normal AV conduction are masked by increased overdrive suppression resulting from the positive chronotropic effect of this agent. Cohn and coworkers (1970) suggested that a number of factors may be involved in the antiarrhythmic activity of glucagon. These included increased SA nodal automaticity, enhanced AV conduction, alteration in serum potassium levels and possibly decreased ventricular automaticity. Prasad and DeSousa (1972} speculated that only increased SA nodal automaticity, in the absence of alterations in ventricular automaticity, was responsible for the conversion to a sinus tachycardia. The present study was designed to evaluate the antiarrhythmic activity of glucagon on digitalis-induced arrhythmias in dogs with normal cardiac conduction systems and in dogs with surgically produced complete heart
R.D. WILKERSON ET AL. block. Our basic hypothesis was that glucagon exerts its antiarrhythmic activity by increasing SA nodal rate to a point that allows it to successfully compete with the d o m i n a n t ventricular pacemaker, quickly surpassing it and returning pacemaker dominance to the SA node.
2. Materials and methods 27 dogs, not selected for sex or breed, ranging in weight from 7 to 18 kg were anesthetized with a-chloralose (100 mg/kg i.v.). In all experiments a femoral artery was cannulated for continuous monitoring of blood pressure using a Statham pressure transducer and a femoral vein was cannulated for administration of drugs. Lead II electrocardiogram (ECG) and heart rate were also continuously recorded. All parameters were recorded on a Beckman Dynograph. In addition, the ECG was displayed on a storage oscilloscope (Textronix) to allow continuous monitoring at a fast trace speed. After allowing for stabilization of the animal after surgical preparation, an initial intravenous loading dose of 40 pg/kg of ouabain (ouabain octahydrate, Sigma) was administered, and this was followed by a second dose of 20 pg/kg after 30 min. Subsequent doses of 10 pg/kg were administered at 15 min intervals until ECG evidence of a d o m i n a n t ventricular r h y t h m was obtained. In experiments where glucagon was employed, it was diluted in the commercial solvent and given in a dose of 20 pg/kg after 20--30 min of persistent ventricular arrhythmia had elapsed. In several experiments in which glucagon was ineffective, lidocaine, 4 m g / k g (Xylocaine, Astra) was administered so as to establish that the arrhythmia could, in fact, be suppressed by pharmacologic means. 11 animals with normal cardiac conduction systems were administered toxic doses of ouabain as described. 9 of these subsequently received glucagon. The other 2 animals were treated with only the glucagon diluent. In 6 animals, a complete heart block was
ANTIARRHYTHMIC ACTION OF GLUCAGON produced surgically. The preparation and experimental procedure for these dogs was identical to the above, except that a right t h o r a c o t o m y at the fourth intercostal space was performed in order to expose the right atrium. Complete AV block was produced by cauterizing a selected area on the atrial septum according to the procedure described by Pruett and Woods (1967). Successful AV block was verified by ECG tracings showing complete AV dissociation. Following a period of stabilization, ouabain was administered according to the regimen previously described. 5 arrhythmic heart-block animals subsequently received glucagon. 1 animal was treated with only the glucagon diluent. An additional 10 animals were prepared in a manner identical to that described above for animals with normal conduction systems, except that 2 electrodes were attached on the right atrium through a right t h o r a c o t o m y . Connection of these electrodes to a Grass Model S-48 stimulator provided for electrical pacing of the atria at selected rates. In these experiments a 4 V, 2 msec square wave pulse was employed after establishing a stable ventricular arrhythmia with ouabain, but prior to glucagon treatment. In each case, the minimum atrial pacing rate required to effect an atrial r h y t h m was determined. After this determination was made, glucagon 20 pg/kg was administered and the rate of the resulting sinus r h y t h m was noted and compared to the minimum atrial pacing rate required to overcome the arrhythmia. All animals utilized in these studies were normokalemic (3.8 + 0.3 mEq/liter).
3. Results
3.1. Untreated ouabain toxicity In 3 animals, 2 with intact AV conduction systems and 1 with complete AV block, the ventricular arrhythmia induced by ouabain was allowed to run its course with only the commercial diluent for glucagon being admin-
59 istered. In all 3 animals the ventricular arrhythmia was observed to persist for more than 1 h.
3.2. Glucagon treatment o f ouabain toxicity in animals with normal conduction systems 8 of 9 animals with normal AV conduction systems receiving glucagon, 20 pg/kg, following the establishment of a persistent ouabaininduced ventricular arrhythmia were converted to a sinus r h y t h m by glucagon within 1 min after its administration. 3 of the 8 sinus conversions lasted less than 5 min while 5 persisted for greater than 30 min. In all 8 animals which were converted by glucagon, the rate of the sinus r h y t h m after conversion was faster than the pre-glucagon ventricular arrhythmia rate. Ventricular rates d u r i n g ouabain toxicity averaged 211 + 6.7 beats/min compared to rates of 262 + 7.1 beats/min following glucagon conversion to a sinus r h y t h m (p < 0.001) (fig. 1). The 1 animal which was not converted to a sinus r h y t h m by glucagon did not attain a sinus nodal rate (as determined by p--p intervals) more rapid than the ventricular arrhythmia rate after glucagon administration.
3.3. Glucagon treatment o f ouabain toxicity in animals with complete heart block In all 5 animals with interrupted AV conduction systems, conversion to a sinus r h y t h m was not observed following glucagon administration after the establishment of a ventricular tachycardia by ouabain, nor was there a significant alteration in either ventricular rate or the QRS pattern of the ECG. Using paired t-analysis, comparison of heart rates (fig. 2) for all 5 heart block animals before and after glucagon administration (170 + 7.7 and 136 -+ 27 beats/min, respectively) did not demonstrate a significant effect of glucagon on ventricular rate in blocked animals (p > 0.25). These values include the experiment in which ventricular rate dropped from 170 to 40 beats/min, a drop that we
60
R.D. W I L K E R S O N ET AL. 280
180
T
260
240
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=o 200
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(5)
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CONTROL
CONTROL
OUABAIN OUABAIN +GLUCAGON HO* 5L ~g/kg I10~=51.,ug/kg 2:O.yg/kg
Fig. 1. S u m m a r y of heart rate changes produced by ouabain toxicity and subsequent administration of glucagon in animals with normal c o n d u c t i o n systems. Bars represent mean heart rates ± standard deviation necessary to produce arrhythmia is noted at b o t t o m of the fig. p values were determined by paired t-analysis.
feel is d i f f i c u l t t o a t t r i b u t e to glucagon a c t i o n because o f its long d u r a t i o n (greater t h a n 90 min). 3 h e a r t b l o c k animals were t r e a t e d with lidocaine, 4 mg/kg, 30 min a f t e r it was n o t e d t h a t glucagon a d m i n i s t r a t i o n was w i t h o u t effect. In all 3 cases, t h e r e was a significant r e d u c t i o n in ventricular rate t o 123 +- 9.3 beats/min. 3.4. Comparison of atrial pacing and gIucagon
treatment In 1 0 a n i m a l s w i t h n o r m a l c o n d u c t i o n s y s t e m s p r e p a r e d f o r e l e c t r i c a l atrial p a c i n g , o u a -
(5)
(5)
OUABAIN OUABAIN+ GLUCAGON 82 ±12 ~q/kg B2* 12 ug/kg 20yg/kg
Fig. 2. S u m m a r y of heart rate changes in heart blocked animals (left bar); following production of a ventricular tachycardia by toxic doses of ouabain (middle bar); and 1 m i n after administration of glucagon (right bar). Bars represent mean heart rate ± standard error. Mean ouabain dose ± standard deviation necessary to produce ventricular tachycardia is noted at the b o t t o m of the fig. p values were determined by paired t-analysis.
bain t o x i c i t y was p r o d u c e d in a m a n n e r identical t o t h a t o f the above g r o u p with n o r m a l c o n d u c t i o n systems. The rate o f the ventricular a r r h y t h m i a in this group was 197 ± 8.4 b e a t s / m i n . T h e m i n i m u m atrial pacing rate required t o e f f e c t c a p t u r e o f the ventricles was 215 +_ 7.2 b e a t s / m i n in the 6 animals in which atrial pacing was successful. A f t e r glucagon a d m i n i s t r a t i o n to these 6 animals, t h e i r sinus rate were elevated to 237 + 12.8 beats/ min with a shift of p a c e m a k e r site to the SA node. In 4 animals successful c a p t u r e b y atrial pacing was n o t possible after the d e v e l o p m e n t
ANTIARRHYTHMIC ACTION OF GLUCAGON of the ouabain arrhythmia. These 4 animals also did not revert to a sinus r h y t h m after the administration of glucagon even though the sinus nodal rate (as determined by p - - p intervals) was slightly greater than the minimum atrial pacing rate required to overcome the arrhythmia in the 8 animals where atrial pacing was successful (219 + 8.1 beats/min). This finding suggests that in these 4 animals the degree of AV block associated with ouabain toxicity was such conduction of the atrial impulse to the ventricle was not possible. Even though glucagon is known to enhance AV conduction, the conduction deficit in these 4 animals was apparently so severe that this dose of glucagon was not capable of overcoming it.
4. Discussion Although a number of investigators have demonstrated that glucagon exerts an antiarrhythmic effect on arrhythmias produced by digitalis (Cohn et al., 1970; Madan et al., 1971 and Prasad and DeSousa, 1972) and coronary artery occlusion (Madan, 1971), there is evidence to suggest that this compound is unique among antiarrhythmic agents. It is n o t a cardiac depressant (Farah and Tuttle, 1960) and unlike all other antiarrhythmic drugs there have been a number of reports demonstrating that glucagon enhances ventricular automaticity in experimental animals (Daniell et al., 1970; Wilkerson et al., 1971; Lipski et al., 1972) and man (Hurwitz, 1973) as well as in isolated false tendons (Pruett et al., 1971). There are, however conflicting reports concerning this latter action (Steiner et al., 1969; Lucchesi et al., 1969). Since increased ventricular automaticity is likely to be a major contributing factor in the ventricular arrhythmias associated with digitalis toxicity, it would seem contradictory that glucagon should be effective in the management of this arrhythmia unless other actions of this drug such as enhanced sinus node automaticity and increased atrioven-
61 tricular conduction play a major role in the shift of pacemaker site from a ventricular focus back to the sinus node. Previous work by Cohn and his coworkers (1970), Prasad and DeSousa (1972) and Einzig and his associates (1971) suggested the possibility that the action of glucagon to increase the sinus node rate may be instrumental in this antiarrhythmic action. These groups observed that in the majority of cases when conversion to a sinus r h y t h m occurred the resulting sinus rate was higher than the rate of the arrhythmia prior to glucagon administration. Results of the present study are in agreement with these findings in that in all instances in which conversion of a digitalis-induced ventricular arrhythmia to a sinus r h y t h m occurred it did so at a sinus rate which was faster than the rate of the arrhythmia prior to glucagon administration. In addition, electrical pacing of the atria in 6 animals resulted in successful capture of the ventricles at a pacing rate which was lower than the sinus rate attained after subsequent administration of glucagon. Thus it would appear that the enhancement of sinus node rate alone would be sufficient to explain the antiarrhythmic action of glucagon in these animals, although the enhanced atrioventricular conduction after glucagon administration (Steiner et al., 1969 and Lipski et al., 1972) surely played a facilitatory role. Animals that could not be converted to sinus r h y t h m by atrial pacing, presumably due to a high degree of AV block resulting from digitalis action, were also not converted by glucagon administration. It was also noted that glucagon had no effect on the rate or ECG pattern of the arrhythmia in these animals but administration of lidocaine (4 mg/kg) significantly reduced the rate of the ventricular arrhythmia, but did not restore a sinus r h y t h m . Thus our data on dogs with normal conduction systems support the contention that the antiarrhythmic action of glucagon in digitalis toxicity is the result of an action of this drug at a supraventricular site. Additional data supporting this hypothesis were obtained from dogs with acute heart
62 block, a preparation in which glucagon has been demonstrated to have no effect on ventricular automaticity (Daniell et al., 1970; Lavarenne et al., 1976). It was noted that glucagon administration had no effect on the rate or ECG pattern of the digitalis-induced arrhythmia in this preparation. This finding further suggests a supraventricular site of action, since other suggested mechanisms such as alterations in serum potassium resulting from the hyperglycemic action of this hormone (Cohn et al., 1970) or an action to decrease ventricular automaticity would be expected to be effective even in the absence of atrioventricular conduction. To demonstrate the effects of decreasing ventricular automaticity in this preparation, lidocaine (4 mg/kg) was administered to 3 animals which failed to respond to glucagon, and in each instance the ventricular rate was slowed significantly. Although other investigators have shown that glucagon may have antiarrhythmic actions, presumably at a ventricular site of action, in arrhythmias induced by coronary artery ligation (Madan, 1971; Lavarenne et al., 1975), work done with arrhythmias resulting from administration of digitalis glycosides almost universally points to a supraventricular site of action. The present study presents direct evidence to support this supraventricular site of action. More specifically, our data demonstrate that although the action of glucagon to improve AV conduction may play a permissive role in its antiarrhythmic action the positive chronotropic effect of this agent appears indispensible, since in all instances where glucagon was effective rapid atrial pacing was equally as effective. It is difficult to understand how glucagon might exert an effect via a ventricular action in arrhythmias associated with myocardial ischemia, while apparently not producing a similar effect on arrhythmias associated with digitalis toxicity, but one possible explanation is that this action on arrhythmias due to cardiac ischemia may be the result of an action of glucagon to improve intraventricular conduction and thus
R.D. WILKERSON ET AL. abolish re-entry circuits that are undoubtedly present in this form of arrhythmia. The finding that glucagon enhances membrane responsiveness is suggestive of such an action in arrhythmias with a large re-entry component (Prasad and Bharadwaj, 1975), but an action to increase intraventricular conduction could not be demonstrated in normal dogs by Lipski and coworkers (1972). The mechanism of action of glucagon in digitalis arrhythmias is further complicated by the findings of Einzig and coworkers (1971). They showed that although glucagon converted ventricular tachycardias caused by digitalis to a sinus tachycardia with a rate greater than the preceeding ventricular tachycardia rate in all animals so treated, glucagon failed to prevent digitalis-induced ventricular tachycardia in normokalemic animals despite the development of a sinus tachycardia. This same dose, however, prevented these arrhythmias in hypokalemic animals even though they did not develop a sinus rate that exceeded the rate of the ventricular tachycardia. Thus not only are experimental conditions such as recent surgical trauma and anesthesia important in determining the cardiac actions of this drug, but electrolyte balance has also been shown to influence its antiarrhythmic action. In conclusion, although the exact mechanism of glucagon in the treatment of digitalisinduced arrhythmias is unclear, we have presented the first direct evidence to suggest that this action is supraventricular, with no ventricular component that could be identified. References Cohn, K.E,, J. Agmon and O.W. Gamble, 1970, The effect of glucagon on arrythmias due to digitalis, Amer. J. Cardiol. 25,683. Daniell, H.B., J.E. Holl, J.K. Pruett, E.E. Bagwell and E.F. Woods, 1970, Cardiovascular effects of glucagon in dogs with non-nodal pacemakers, J. Electrocardiol. 3, 117. Einzig, S., E.P. Todd, D.M. Nicoloff and R.V. Lucas, 1971, Glucagon in prevention and abolition of ouabain-induced ventricular tachycardia in normokalemic and hypokalemic dogs, Circulation Res. 24, 88.
ANTIARRHYTHMIC ACTION OF GLUCAGON Farah, A. and R. Tuttle, 1960, Studies on the pharmacology of glucagon, J. Pharmacol. Exptl. Therap. 129, 49. Hurwitz, R.A., 1973, Effect of glucagon on infants and children with atrioventricular heart block, Brit. Heart J. 35, 1260. Kones, R.J. and J.H. Phillips, 1972, Glucagon: present status in cardiovascular disease, Clin. Pharmacol. Therap. 3,427. Lavarenne, J., N. Moins, M. Boucher and G. Barthelemy, 1976, The chronotropic action of glucagon in the dog, under different experimental conditions, J. Pharmacol. (Paris), 7,5. Lavarenne, J., Y. Mongheal, G. Barthelemy and P. Duchene Marullaz, 1975, Effects of glucagon on arrhythmia induced by ligation of the interventricular artery in unanesthetized dogs, J. Pharmacol. (Paris) 6, 73. Lipski, J.I., D.M. Kaminsky, E. Donoso and C.K. Friedberg, 1972, The electrophysiological properties of glucagon on the normal canine heart, Amer. J. Physiol. 5, 1107. Lucchesi, B.R., 1968, Cardiac actions of glucagon, Circulation Res. 22,777. Lucchesi, B.R., D.F. Stutz and R.A. Winfield, 1969, Glucagon: its enhancement of atrioventricular nodal pacemaker activity and failure to increase ventricular automaticity in dogs, Circulation Res. 25, 183. Madan, B.R., 1971, Effect of glucagon on ventricular arrhythmias after coronary artery occlusion and on ventricular automaticity in the dog, Brit. Heart J. 43, 279.
63 Madan, B.R., B.K. Jain and R.S. Gupta, 1971, Actions and interactions of glucagon and propranolol in ouabain-induced arrhythmias in the rabbit, Arch. Intern. Pharmacodyn. 194, 78. Parmley, W.W. and E.H. Sonnenblick, 1971, Glucagon a new agent in cardiac therapy, Amer. J. Cardiol. 27, 298. Prasad, K. and B. Bharadwaj, 1975, Electrophysiologic basis of use of glucagon in ouabain-induced cardiac arrhythmias in man, (6th International Congress of Pharmacology, Helsinki, Finland) p. 507. Prasad, K. and H.H. DeSousa, 1972, Glucagon in the treatment of ouabain-induced cardiac arrhythmias in dogs, Cardiovasc. Res. 4,333. Pruett, J.K. and E.F. Woods, 1967, Technique for experimental complete heart block, J. Appl. Physiol. 22,830. Pruett, J.K., E.F. Woods and H.B. Daniell, 1971, Glucagon enhanced automaticity in spontaneously beating purkinje fibres of canine false tendons, Cardiovasc. Res. 5,436. Steiner, C., Al.L. Wit and A.N. Damato, 1969, Effects of glucagon on atrioventricular conduction and ventricular automaticity in dogs, Circularion Res. 24, 167. Whitehouse, A., F.W. James and T.N. James, 1966, Chronotropic actions of glucagon on the sinus node, Proc. Soc. Exptl. Biol. Med. 122, 823. Wilkerson, R.D., J.K. Pruett and E.F. Woods, 1971, Glucagon enhanced ventricular automaticity in dogs: its concealment by positive chronotropism, Circulation Res. 29,616.
A POSSIBLE MECHANISM
57
OF THE ANTIARRHYTHMIC
ACTION OF GLUCAGON
*
R. DOUGLAS WILKERSON **, DAVID B. PARTLOW ***, JACK K. PRUETT and CHARLES W. PATTERSON
Department of Pharmacology, University of South Alabama, College of Medicine, Mobile, Alabama 36688, and Department of Pharmacology, Medical University of South Carolina, Charleston, South Carolina 29401, U.S.A. Received 30 November 1976, revised MS received 15 February 1977, accepted 17 March 1977
R.D. WILKERSON, D.B. PARTLOW, J.K. PRUETT and C.W. PATTERSON, A possible mechanism of the antiarrhythmic action ofglucagon, European J. Pharmacol. 44 (1977) 57---63. The effects of glucagon on ouabain-induced ventricular arrhythmias were studied in dogs with normal cardiac conduction systems and in dogs with surgically induced complete heart block. Glucagon, 20 pg/kg, effected conversion to a sinus rhythm in eight of nine animals with normal conduction systems, and in each instance the conversion occurred with a sinus tachycardia whose rate exceeded the rate of pre-existing ventricular arrhythmia. The one animal which failed to convert to a sinus rythm did not develop a sinus nodal rate faster than the rate of the ventricular arrhythmia. In 10 animals with ouabain-induced ventricular arrhythmias atrial pacing was employed to overdrive the arrhythmia, and in 6 cases this was successful. In all 6 experiments subsequent administration of glucagon after pacing resulted in conversion to a sinus rhythm at a rate greater than the minimum atrial pacing rate required to effect capture of the ventricules. In 4 of the 10 animals neither atrial overdrive nor glucagon were successful in effecting sinus conversion, but ventricular rate was slowed by lidocaine. In addition, in 5 animals with complete heart block and ouabain-induced ventricular tachycardia which were given glucagon, 20 pg/kg, there was no change in the tachycardia and no change in the ECG pattern associated with the arrhythmia. It was concluded that the antiarrhytmic action of glucagon in this arrhythmia is mediated through a supraventricular action to elevate sinus rate above that of the dominant ventricular focus and thus allow a return to dominance of the sinus node. Heart block, complete
Arrhythmia
Ouabain
1. I n t r o d u c t i o n Glucagon has received considerable attent i o n as a c a r d i a c d r u g w h e n a d m i n i s t e r e d i n pharmacologic doses (Parmley and Sonnenblick, 1971; Kones and Phillips, 1972), and o n e o f i t s i n v e s t i g a t e d u s e s h a s b e e n as a n a n t i a r r h y t h m i c d r u g in d i g i t a l i s - i n d u c e d v e n tricular arrhythmias ( C o h n e t al., 1 9 7 0 ; * Supported by a grant from the Alabama Heart Association. * * A d d r e s s reprint requests to: R.D. Wilkerson, Ph.D., Department of Pharmacology, College of Medicine, University of South Alabama, Mobile, Alabama 36688, U.S.A. *** Supported by a summer fellowship from the University of South Alabama, College of Medicine.
Glucagon
Lidocaine
Atrial pacing
M a d a n e t al., 1 9 7 1 ; P r a s a d a n d D e S o u s a , 1 9 7 2 ) as w e l l as a r r h y t h m i a s p r o d u c e d b y coronary artery occlusion (Madan, 1971). The present study was designed to provide a better understanding of the mechanism of this antiarrhythmic activity. The positive inotropic and chronotropic effects of glucagon were first documented by Farah and Tuttle (1960). Lucchesi (1968) subsequently demonstrated that these actions p e r s i s t e d in t h e p r e s e n c e o f b e t a a d r e n e r g i c blockade by propranolol, thus distinguishing the cardiac actions of glucagon from those of the catecholamines. In addition, electrophysiologic studies demonstrated that glucagon increases atrioventricular conduction velocity ( S t e i n e r e t al., 1 9 6 9 ; L i p s k i e t al., 1 9 7 2 ) a n d AV nodal automaticity ( L u c c h e s i e t al.,
58 1969). A direct action to enhance SA nodal automaticity is u n d o u b t e d l y responsible for its positive chronotropic effect (Whitehouse et al., 1966). Conflicting results have been obtained in studies evaluating the action of glucagon on ventricular automaticity. While some investigators have described no change in ventricular automaticity (Steiner et al., 1969; Lucchesi et al., 1969), others have reported an enhancement of automaticity in experimental animals (Wilkerson et al., 1971; Lipski et al., 1972) and man (Hurwitz, 1973) and isolated Purkinje fibers (Pruett et al., 1971). Daniell and coworkers (1970) also demonstrated an interesting difference in the ventricular automaticity response to glucagon between animals with acute and chronic heart block. They demonstrated that the ventricular automaticity enhancement noted in dogs with chronic heart block could not be demonstrated in animals after acute heart block. This observation was confirmed by Lavarenne, and his associates (1976) in a study which also demonstrated the effects of different experimental conditions on the cardiac response to glucagon. Wilkerson and coworkers (1971) demonstrated that the effects of glucagon on ventricular automaticity in animals with normal AV conduction are masked by increased overdrive suppression resulting from the positive chronotropic effect of this agent. Cohn and coworkers (1970) suggested that a number of factors may be involved in the antiarrhythmic activity of glucagon. These included increased SA nodal automaticity, enhanced AV conduction, alteration in serum potassium levels and possibly decreased ventricular automaticity. Prasad and DeSousa (1972} speculated that only increased SA nodal automaticity, in the absence of alterations in ventricular automaticity, was responsible for the conversion to a sinus tachycardia. The present study was designed to evaluate the antiarrhythmic activity of glucagon on digitalis-induced arrhythmias in dogs with normal cardiac conduction systems and in dogs with surgically produced complete heart
R.D. WILKERSON ET AL. block. Our basic hypothesis was that glucagon exerts its antiarrhythmic activity by increasing SA nodal rate to a point that allows it to successfully compete with the d o m i n a n t ventricular pacemaker, quickly surpassing it and returning pacemaker dominance to the SA node.
2. Materials and methods 27 dogs, not selected for sex or breed, ranging in weight from 7 to 18 kg were anesthetized with a-chloralose (100 mg/kg i.v.). In all experiments a femoral artery was cannulated for continuous monitoring of blood pressure using a Statham pressure transducer and a femoral vein was cannulated for administration of drugs. Lead II electrocardiogram (ECG) and heart rate were also continuously recorded. All parameters were recorded on a Beckman Dynograph. In addition, the ECG was displayed on a storage oscilloscope (Textronix) to allow continuous monitoring at a fast trace speed. After allowing for stabilization of the animal after surgical preparation, an initial intravenous loading dose of 40 pg/kg of ouabain (ouabain octahydrate, Sigma) was administered, and this was followed by a second dose of 20 pg/kg after 30 min. Subsequent doses of 10 pg/kg were administered at 15 min intervals until ECG evidence of a d o m i n a n t ventricular r h y t h m was obtained. In experiments where glucagon was employed, it was diluted in the commercial solvent and given in a dose of 20 pg/kg after 20--30 min of persistent ventricular arrhythmia had elapsed. In several experiments in which glucagon was ineffective, lidocaine, 4 m g / k g (Xylocaine, Astra) was administered so as to establish that the arrhythmia could, in fact, be suppressed by pharmacologic means. 11 animals with normal cardiac conduction systems were administered toxic doses of ouabain as described. 9 of these subsequently received glucagon. The other 2 animals were treated with only the glucagon diluent. In 6 animals, a complete heart block was
ANTIARRHYTHMIC ACTION OF GLUCAGON produced surgically. The preparation and experimental procedure for these dogs was identical to the above, except that a right t h o r a c o t o m y at the fourth intercostal space was performed in order to expose the right atrium. Complete AV block was produced by cauterizing a selected area on the atrial septum according to the procedure described by Pruett and Woods (1967). Successful AV block was verified by ECG tracings showing complete AV dissociation. Following a period of stabilization, ouabain was administered according to the regimen previously described. 5 arrhythmic heart-block animals subsequently received glucagon. 1 animal was treated with only the glucagon diluent. An additional 10 animals were prepared in a manner identical to that described above for animals with normal conduction systems, except that 2 electrodes were attached on the right atrium through a right t h o r a c o t o m y . Connection of these electrodes to a Grass Model S-48 stimulator provided for electrical pacing of the atria at selected rates. In these experiments a 4 V, 2 msec square wave pulse was employed after establishing a stable ventricular arrhythmia with ouabain, but prior to glucagon treatment. In each case, the minimum atrial pacing rate required to effect an atrial r h y t h m was determined. After this determination was made, glucagon 20 pg/kg was administered and the rate of the resulting sinus r h y t h m was noted and compared to the minimum atrial pacing rate required to overcome the arrhythmia. All animals utilized in these studies were normokalemic (3.8 + 0.3 mEq/liter).
3. Results
3.1. Untreated ouabain toxicity In 3 animals, 2 with intact AV conduction systems and 1 with complete AV block, the ventricular arrhythmia induced by ouabain was allowed to run its course with only the commercial diluent for glucagon being admin-
59 istered. In all 3 animals the ventricular arrhythmia was observed to persist for more than 1 h.
3.2. Glucagon treatment o f ouabain toxicity in animals with normal conduction systems 8 of 9 animals with normal AV conduction systems receiving glucagon, 20 pg/kg, following the establishment of a persistent ouabaininduced ventricular arrhythmia were converted to a sinus r h y t h m by glucagon within 1 min after its administration. 3 of the 8 sinus conversions lasted less than 5 min while 5 persisted for greater than 30 min. In all 8 animals which were converted by glucagon, the rate of the sinus r h y t h m after conversion was faster than the pre-glucagon ventricular arrhythmia rate. Ventricular rates d u r i n g ouabain toxicity averaged 211 + 6.7 beats/min compared to rates of 262 + 7.1 beats/min following glucagon conversion to a sinus r h y t h m (p < 0.001) (fig. 1). The 1 animal which was not converted to a sinus r h y t h m by glucagon did not attain a sinus nodal rate (as determined by p--p intervals) more rapid than the ventricular arrhythmia rate after glucagon administration.
3.3. Glucagon treatment o f ouabain toxicity in animals with complete heart block In all 5 animals with interrupted AV conduction systems, conversion to a sinus r h y t h m was not observed following glucagon administration after the establishment of a ventricular tachycardia by ouabain, nor was there a significant alteration in either ventricular rate or the QRS pattern of the ECG. Using paired t-analysis, comparison of heart rates (fig. 2) for all 5 heart block animals before and after glucagon administration (170 + 7.7 and 136 -+ 27 beats/min, respectively) did not demonstrate a significant effect of glucagon on ventricular rate in blocked animals (p > 0.25). These values include the experiment in which ventricular rate dropped from 170 to 40 beats/min, a drop that we
60
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OUABAIN OUABAIN +GLUCAGON HO* 5L ~g/kg I10~=51.,ug/kg 2:O.yg/kg
Fig. 1. S u m m a r y of heart rate changes produced by ouabain toxicity and subsequent administration of glucagon in animals with normal c o n d u c t i o n systems. Bars represent mean heart rates ± standard deviation necessary to produce arrhythmia is noted at b o t t o m of the fig. p values were determined by paired t-analysis.
feel is d i f f i c u l t t o a t t r i b u t e to glucagon a c t i o n because o f its long d u r a t i o n (greater t h a n 90 min). 3 h e a r t b l o c k animals were t r e a t e d with lidocaine, 4 mg/kg, 30 min a f t e r it was n o t e d t h a t glucagon a d m i n i s t r a t i o n was w i t h o u t effect. In all 3 cases, t h e r e was a significant r e d u c t i o n in ventricular rate t o 123 +- 9.3 beats/min. 3.4. Comparison of atrial pacing and gIucagon
treatment In 1 0 a n i m a l s w i t h n o r m a l c o n d u c t i o n s y s t e m s p r e p a r e d f o r e l e c t r i c a l atrial p a c i n g , o u a -
(5)
(5)
OUABAIN OUABAIN+ GLUCAGON 82 ±12 ~q/kg B2* 12 ug/kg 20yg/kg
Fig. 2. S u m m a r y of heart rate changes in heart blocked animals (left bar); following production of a ventricular tachycardia by toxic doses of ouabain (middle bar); and 1 m i n after administration of glucagon (right bar). Bars represent mean heart rate ± standard error. Mean ouabain dose ± standard deviation necessary to produce ventricular tachycardia is noted at the b o t t o m of the fig. p values were determined by paired t-analysis.
bain t o x i c i t y was p r o d u c e d in a m a n n e r identical t o t h a t o f the above g r o u p with n o r m a l c o n d u c t i o n systems. The rate o f the ventricular a r r h y t h m i a in this group was 197 ± 8.4 b e a t s / m i n . T h e m i n i m u m atrial pacing rate required t o e f f e c t c a p t u r e o f the ventricles was 215 +_ 7.2 b e a t s / m i n in the 6 animals in which atrial pacing was successful. A f t e r glucagon a d m i n i s t r a t i o n to these 6 animals, t h e i r sinus rate were elevated to 237 + 12.8 beats/ min with a shift of p a c e m a k e r site to the SA node. In 4 animals successful c a p t u r e b y atrial pacing was n o t possible after the d e v e l o p m e n t
ANTIARRHYTHMIC ACTION OF GLUCAGON of the ouabain arrhythmia. These 4 animals also did not revert to a sinus r h y t h m after the administration of glucagon even though the sinus nodal rate (as determined by p - - p intervals) was slightly greater than the minimum atrial pacing rate required to overcome the arrhythmia in the 8 animals where atrial pacing was successful (219 + 8.1 beats/min). This finding suggests that in these 4 animals the degree of AV block associated with ouabain toxicity was such conduction of the atrial impulse to the ventricle was not possible. Even though glucagon is known to enhance AV conduction, the conduction deficit in these 4 animals was apparently so severe that this dose of glucagon was not capable of overcoming it.
4. Discussion Although a number of investigators have demonstrated that glucagon exerts an antiarrhythmic effect on arrhythmias produced by digitalis (Cohn et al., 1970; Madan et al., 1971 and Prasad and DeSousa, 1972) and coronary artery occlusion (Madan, 1971), there is evidence to suggest that this compound is unique among antiarrhythmic agents. It is n o t a cardiac depressant (Farah and Tuttle, 1960) and unlike all other antiarrhythmic drugs there have been a number of reports demonstrating that glucagon enhances ventricular automaticity in experimental animals (Daniell et al., 1970; Wilkerson et al., 1971; Lipski et al., 1972) and man (Hurwitz, 1973) as well as in isolated false tendons (Pruett et al., 1971). There are, however conflicting reports concerning this latter action (Steiner et al., 1969; Lucchesi et al., 1969). Since increased ventricular automaticity is likely to be a major contributing factor in the ventricular arrhythmias associated with digitalis toxicity, it would seem contradictory that glucagon should be effective in the management of this arrhythmia unless other actions of this drug such as enhanced sinus node automaticity and increased atrioven-
61 tricular conduction play a major role in the shift of pacemaker site from a ventricular focus back to the sinus node. Previous work by Cohn and his coworkers (1970), Prasad and DeSousa (1972) and Einzig and his associates (1971) suggested the possibility that the action of glucagon to increase the sinus node rate may be instrumental in this antiarrhythmic action. These groups observed that in the majority of cases when conversion to a sinus r h y t h m occurred the resulting sinus rate was higher than the rate of the arrhythmia prior to glucagon administration. Results of the present study are in agreement with these findings in that in all instances in which conversion of a digitalis-induced ventricular arrhythmia to a sinus r h y t h m occurred it did so at a sinus rate which was faster than the rate of the arrhythmia prior to glucagon administration. In addition, electrical pacing of the atria in 6 animals resulted in successful capture of the ventricles at a pacing rate which was lower than the sinus rate attained after subsequent administration of glucagon. Thus it would appear that the enhancement of sinus node rate alone would be sufficient to explain the antiarrhythmic action of glucagon in these animals, although the enhanced atrioventricular conduction after glucagon administration (Steiner et al., 1969 and Lipski et al., 1972) surely played a facilitatory role. Animals that could not be converted to sinus r h y t h m by atrial pacing, presumably due to a high degree of AV block resulting from digitalis action, were also not converted by glucagon administration. It was also noted that glucagon had no effect on the rate or ECG pattern of the arrhythmia in these animals but administration of lidocaine (4 mg/kg) significantly reduced the rate of the ventricular arrhythmia, but did not restore a sinus r h y t h m . Thus our data on dogs with normal conduction systems support the contention that the antiarrhythmic action of glucagon in digitalis toxicity is the result of an action of this drug at a supraventricular site. Additional data supporting this hypothesis were obtained from dogs with acute heart
62 block, a preparation in which glucagon has been demonstrated to have no effect on ventricular automaticity (Daniell et al., 1970; Lavarenne et al., 1976). It was noted that glucagon administration had no effect on the rate or ECG pattern of the digitalis-induced arrhythmia in this preparation. This finding further suggests a supraventricular site of action, since other suggested mechanisms such as alterations in serum potassium resulting from the hyperglycemic action of this hormone (Cohn et al., 1970) or an action to decrease ventricular automaticity would be expected to be effective even in the absence of atrioventricular conduction. To demonstrate the effects of decreasing ventricular automaticity in this preparation, lidocaine (4 mg/kg) was administered to 3 animals which failed to respond to glucagon, and in each instance the ventricular rate was slowed significantly. Although other investigators have shown that glucagon may have antiarrhythmic actions, presumably at a ventricular site of action, in arrhythmias induced by coronary artery ligation (Madan, 1971; Lavarenne et al., 1975), work done with arrhythmias resulting from administration of digitalis glycosides almost universally points to a supraventricular site of action. The present study presents direct evidence to support this supraventricular site of action. More specifically, our data demonstrate that although the action of glucagon to improve AV conduction may play a permissive role in its antiarrhythmic action the positive chronotropic effect of this agent appears indispensible, since in all instances where glucagon was effective rapid atrial pacing was equally as effective. It is difficult to understand how glucagon might exert an effect via a ventricular action in arrhythmias associated with myocardial ischemia, while apparently not producing a similar effect on arrhythmias associated with digitalis toxicity, but one possible explanation is that this action on arrhythmias due to cardiac ischemia may be the result of an action of glucagon to improve intraventricular conduction and thus
R.D. WILKERSON ET AL. abolish re-entry circuits that are undoubtedly present in this form of arrhythmia. The finding that glucagon enhances membrane responsiveness is suggestive of such an action in arrhythmias with a large re-entry component (Prasad and Bharadwaj, 1975), but an action to increase intraventricular conduction could not be demonstrated in normal dogs by Lipski and coworkers (1972). The mechanism of action of glucagon in digitalis arrhythmias is further complicated by the findings of Einzig and coworkers (1971). They showed that although glucagon converted ventricular tachycardias caused by digitalis to a sinus tachycardia with a rate greater than the preceeding ventricular tachycardia rate in all animals so treated, glucagon failed to prevent digitalis-induced ventricular tachycardia in normokalemic animals despite the development of a sinus tachycardia. This same dose, however, prevented these arrhythmias in hypokalemic animals even though they did not develop a sinus rate that exceeded the rate of the ventricular tachycardia. Thus not only are experimental conditions such as recent surgical trauma and anesthesia important in determining the cardiac actions of this drug, but electrolyte balance has also been shown to influence its antiarrhythmic action. In conclusion, although the exact mechanism of glucagon in the treatment of digitalisinduced arrhythmias is unclear, we have presented the first direct evidence to suggest that this action is supraventricular, with no ventricular component that could be identified. References Cohn, K.E,, J. Agmon and O.W. Gamble, 1970, The effect of glucagon on arrythmias due to digitalis, Amer. J. Cardiol. 25,683. Daniell, H.B., J.E. Holl, J.K. Pruett, E.E. Bagwell and E.F. Woods, 1970, Cardiovascular effects of glucagon in dogs with non-nodal pacemakers, J. Electrocardiol. 3, 117. Einzig, S., E.P. Todd, D.M. Nicoloff and R.V. Lucas, 1971, Glucagon in prevention and abolition of ouabain-induced ventricular tachycardia in normokalemic and hypokalemic dogs, Circulation Res. 24, 88.
ANTIARRHYTHMIC ACTION OF GLUCAGON Farah, A. and R. Tuttle, 1960, Studies on the pharmacology of glucagon, J. Pharmacol. Exptl. Therap. 129, 49. Hurwitz, R.A., 1973, Effect of glucagon on infants and children with atrioventricular heart block, Brit. Heart J. 35, 1260. Kones, R.J. and J.H. Phillips, 1972, Glucagon: present status in cardiovascular disease, Clin. Pharmacol. Therap. 3,427. Lavarenne, J., N. Moins, M. Boucher and G. Barthelemy, 1976, The chronotropic action of glucagon in the dog, under different experimental conditions, J. Pharmacol. (Paris), 7,5. Lavarenne, J., Y. Mongheal, G. Barthelemy and P. Duchene Marullaz, 1975, Effects of glucagon on arrhythmia induced by ligation of the interventricular artery in unanesthetized dogs, J. Pharmacol. (Paris) 6, 73. Lipski, J.I., D.M. Kaminsky, E. Donoso and C.K. Friedberg, 1972, The electrophysiological properties of glucagon on the normal canine heart, Amer. J. Physiol. 5, 1107. Lucchesi, B.R., 1968, Cardiac actions of glucagon, Circulation Res. 22,777. Lucchesi, B.R., D.F. Stutz and R.A. Winfield, 1969, Glucagon: its enhancement of atrioventricular nodal pacemaker activity and failure to increase ventricular automaticity in dogs, Circulation Res. 25, 183. Madan, B.R., 1971, Effect of glucagon on ventricular arrhythmias after coronary artery occlusion and on ventricular automaticity in the dog, Brit. Heart J. 43, 279.
63 Madan, B.R., B.K. Jain and R.S. Gupta, 1971, Actions and interactions of glucagon and propranolol in ouabain-induced arrhythmias in the rabbit, Arch. Intern. Pharmacodyn. 194, 78. Parmley, W.W. and E.H. Sonnenblick, 1971, Glucagon a new agent in cardiac therapy, Amer. J. Cardiol. 27, 298. Prasad, K. and B. Bharadwaj, 1975, Electrophysiologic basis of use of glucagon in ouabain-induced cardiac arrhythmias in man, (6th International Congress of Pharmacology, Helsinki, Finland) p. 507. Prasad, K. and H.H. DeSousa, 1972, Glucagon in the treatment of ouabain-induced cardiac arrhythmias in dogs, Cardiovasc. Res. 4,333. Pruett, J.K. and E.F. Woods, 1967, Technique for experimental complete heart block, J. Appl. Physiol. 22,830. Pruett, J.K., E.F. Woods and H.B. Daniell, 1971, Glucagon enhanced automaticity in spontaneously beating purkinje fibres of canine false tendons, Cardiovasc. Res. 5,436. Steiner, C., Al.L. Wit and A.N. Damato, 1969, Effects of glucagon on atrioventricular conduction and ventricular automaticity in dogs, Circularion Res. 24, 167. Whitehouse, A., F.W. James and T.N. James, 1966, Chronotropic actions of glucagon on the sinus node, Proc. Soc. Exptl. Biol. Med. 122, 823. Wilkerson, R.D., J.K. Pruett and E.F. Woods, 1971, Glucagon enhanced ventricular automaticity in dogs: its concealment by positive chronotropism, Circulation Res. 29,616.
