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Quinidine-like activity of atropine in rabbit atria
European Journal of Pharmacology, 47 (1978) 423--430
423
© Elsevier/North-Holland Biomedical Press
QUINIDINE-LIKE ACTIVITY OF ATROPINE IN RABBIT ATRIA J.E. HOLL
Department of Pharmacology, School of Pharmacy, University of Georgia, Athens, Georgia 30602, U.S.A. Received 8 July 1977, revised MS received 18 October 1977, accepted 4 November 1977
J.E. HOLL, Quinidine-like activity of atropine in rabbit atria, European J. Pharmacol. 47 (1978) 423--430. Direct effects of atropine and quinidine on contractile force, overdrive suppression, and effective refractory period were studied in rabbit atria. Left atrial contractile force at normal CaCI2 concentration (2.2 raM) and the contractile force response to elevated CaCl2 (5 mM) were unchanged over 30 min. Atropine exposure (1.7 × 10-s M) for 30 min significantly reduced contractile force at both normal and elevated CaCl2 levels. Quinidine exposure (6.2 × 10-6 M) for 30 min produced similar but statistically insignificant changes. At 4 h, control contractile force at 2.2 mM CaCl2 decreased to the post-atropine level, but the response to 5 mM CaC12 was unchanged. The asystolic interval of right atria following overdrive (for 2 rain at 3 rates) was increased after 30 m i n by 4% for controls, 14% after atropine (1.7 × 10 -s M) and 45% after quinidine. The effective refractory period of left atria (evaluated by paired pulse stimulation) was unchanged after 30 min for controls, but increased by 20% after atropine and 45% after quinidine. Effective refractory period Overdrive suppression
Contractile force
1. Introduction The use of atropine for management of bradyarrhythmias following acute myocardial infarction has been evaluated in dogs (Goldstein et al., 1973) and man (8heinman et al., 1975), conflicting results provide no firm conclusion concerning the efficacy or safety of atropine. In these reports atropine perhaps enhanced arrhythmia generation in acutely infarcted dogs, whereas it protected from premature ventricular contractions in humans. The electrophysiologic properties of atropine have been evaluated in humans, again with inconclusive results. According to Akhtar et al. (1974) vagolytic concentration of atropine increased sino-atrial rate and AV conduction, but did not alter His--Purkinje conduction. Atrial refractoriness was increased, decreased and unaltered. Similar changes were found when human ventricular refractoriness
Quinidine
CaCI2
Isolated atria
was evaluated before and after atropine (Guss et al., 1976). Vagal innervation of atria is well documented, ant it is possible that erratic atropine effects on atrial refractoriness may derive from atropine action in relation to background vagal tone. Vagal innervation of the ventricles is less well known. It does exist (Stanton and Vick, 1968; Kent et al., 1974}, but its function is obscure. Therefore attribution of inconsistent atropine effects on ventricular refractoriness to variation in vagal tone is more hypothetical. In view of the unknown contribution of background nervous activity to atropine action, and the contradictory reports of atropine arrhythmic/antiarrhythmic properties, atropine effects on the heart were evaluated in isolated tissue preparations in which nervous activity was eliminated. The influence of atropine on the mechanical and electrophysio-
424
logic properties of the heart were specifically evaluated and compared to those of quinidine to determine whether atropine affects the heart similarly and has potential antiarrythmic properties as suggested by Scheinman et al. (1976).
JE. HOLL
2. Materials and methods
drug. 30 rain or 4 h after the first calcium response the calcium levels were again increased to 5 mM and the parameters remeasured. Time control baths into which no drugs were introduced between calcium responses were included to determine the effect of time lapse and prior exposure to calcium on subsequent responses.
2.1. Contractile force
2.3. Overdrive suppression
New Zealand rabbits of either sex, weighing approximately 2 kg were killed by a blow on the head and their hearts removed and placed in an cooled, aerated (95% 02, 5% CO2)solution containing (mM); NaC1 120, KC1 5.6, MgC12 2.1, NaHCO3 25, CaC12 2.2 and dextrose 10 (Chenoweth and Koelle, 1946). Right and left atria were dissected free, separated and m o u n t e d in 100 ml tissue baths at 30°C. Atria were connected by threads to Grass FT-03 Force Displacement Transducers for contractile force monitoring (developed peak systolic tension) on a Grass Model 7 Polygraph. A 1.0 g diastolic load was imposed and a 1 h equilibration period was allowed. Right atria were connected to a Grass Model 7P44A Tachograph to provide linear recording of frequency changes in beats/min. Left atria were driven at 120 beats/min with a Grass $48 Stimulator set to deliver suprathreshold electrical pulses of 1.0 msec duration through stainless steel electrodes which were embedded in the muscle holders. Voltages employed ranged between O.9--4.5 V.
After a 1 h equilibration and the establishm e n t of constant spontaneous rate, overdrive suppression of the right atrial pacemaker was measured to evaluate automaticity of the tissue. In a variation of the m e t h o d of Kelliher et al. (1972), the right atrium was electrically driven for 2 min. When stimulation was stopped the period of asystole between the last driven beat and the first spontaneous beat was measured with the recorder chart speed set at 50 mm/sec. This procedure was repeated so that three rates of drive were used: 90, 120 and 150 beats/min. In approximately 95% of atria studied the duration of asystole was proportional to the rate of stimulation; as the frequency of stimulation was increased, the asystolic period was lengthened. At least 2 observations were made at each drive rate and 2 min rest periods were allowed between observations. From these control data, a linear regression analysis of asystolic period vs. drive frequency was obtained. Only those tissues with an r > 0.95 were studied further. In the approximately 5% of atria where r ~ 0.95, the atria appeared to contain at least 2 primary pacemakers, one normally latent which become d o m i n a n t at some appropriate stimulation frequency. Therefore, an r > 0.95 was taken to indicate the probability that only one pacemaker was influenced by electrical stimulation, and that drug studies were conducted on a single pacemaker system. Right atria qualifying for further study were exposed either to 1.7 X 10 -6 M or 1.7 X 10 -s M atropine or 6.2 X 10 -6 M quinidine for
2.2. Elevated calcium concentration After equilibration, the calcium concentration in the tissue baths containing left atria was raised to 5 mM and contractile force increases were measured. The excess calcium was then washed o u t by at least four bath fluid exchanges and contractile force at 2.2 mM CaC12 re-established. Atropine (1.7X 10 -s M) or quinidine (6.2 X 10 -6 M) was then added to some baths while others received no
CARDIODEPRESSANT ACTIVITY OF ATROPINE 30 min, and overdrive studies were repeated. Time control atria were retested for overdrive suppression after 30 min with no drug intervention.
425
P,
B T C.0.5 hr.
4'
2.4. Effective refractory period The functional refractory period of left atria was determined by the m e t h o d of Govier (1965). The basic stimulation rate was 60 beats/min and the refractory period was determined by delivering a second stimulus identical to the first. Initially, the second stimulus was delivered within the refractory period of the muscle; then the interval to the second stimulus was increased until a mechanical response to both stimuli and was recorded. At endpoint, the interval between stimuli was measured with Tektronix R5103/ D13 Storage Oscilloscope. After control readings were taken, atria were exposed to 1.7 × 10 -s M atropine or 6.2 × 10 -4 M quinidine for 30 min and refractory period redetermined. In a time control group with no drug intervention, the elapsed time between refractory period determinations was 30 rain. Atropine and quinidine concentrations are reported as the molar concentration of drug base. Data are reported as mean + S.E.M. and comparisons between groups were made using t-test for paired or unpaired data as appropriate.
3. Results
3.1. Elevated calcium concentration Increases in left atrial contractile force in response to elevation of CaC12 concentration from 2.2 to 5.0 mM are shown in panel A, fig. 1. The effects of atropine, quinidine and time on these responses are shown in panel B. Successive exposures to 5 mM CaC12 at 30 min intervals did not adversely affect the atrium. Over the 30 min time control, contractile force declined only slightly, and the atrium
Quinidine
TC. 4.0 hr.
1 min. Fig. 1. C o n t r a c t i l e force r e s p o n s e s o f 4 isolated left
atria upon raising CaCl2 concentration from 2.2 to 5 mM (at arrows). Panel A shows control responses; panel B shows additional responses after treatment. Panel B responses are: time control response after 30 rain (no drug); atropine response (30 rain after adding 1.7 x 10-s M atropine); quinidine response (30 min after adding 6.2 x 10-6 M quinidine); time control response after 4 h (no drug). remained fully responsive to calcium stimulation. Contractile force was reduced after 30 min of atropine exposure and calcium stimulation did n o t increase force to the level achieved prior to atropine (panel B, fig. 1). Quinidine effects on contractile force and the response to calcium stimulation were similar to those observed after atropine. In other controls calcium stimulations were separated by 4 h to m o n i t o r responsiveness over a long period of time. Contractile force at normal Ca level was reduced at 4 h and was comparable to forces recorded in atria exposed to atropine or quinidine. However, contractile force responses to calcium stimulation were the same as the initial response. Responses replicating those in fig. 1 were observed in additional atria. Table 1 shows post-treatment contractile force at normal and elevated calcium levels as a percent of respective pretreatment responses. In the 30
426
J.E. H O L L
TABLE 1 E f f e c t o f a t r o p i n e a n d q u i n i d i n e o n c a l c i u m - i n d u c e d c o n t r a c t i l e force r e s p o n s e o f left atria. Experimental treatment
Post-treatment calcium-induced c o n t r a c t i l e force r e s p o n s e 1 (% p r e t r e a t m e n t r e s p o n s e ) 2
No d r u g T i m e lapse, 30 m i n (time control) A t r o p i n e , 1.6 X 10 -s M T i m e lapse, 30 m i n Q u i n i d i n e , 6.2 × 10 -6 M T i m e lapse, 30 m i n No d r u g T i m e lapse, 4 h 1 2 3 4
R e p r e s e n t a t i v e trace s h o w n R e p r e s e n t a t i v e trace s h o w n Significantly d i f f e r e n t f r o m Significantly d i f f e r e n t f r o m
N u m b e r o f atria
2.2 m M CaC12
5.0 m M CaCl2
90 ± 4
97 ± 1
9
64 +_ 6 3
84 ± 3 4
9
70 £ 7
89 ± 5
5
60 _+ 6 3
98 _+ 1
10
in p a n e l B of fig. 1. in p a n e l A o f fig. 1. 30 m i n t i m e c o n t r o l value at 2.2 mM, CaCl2, p < 0.05, u n p a i r e d t-test. b o t h t i m e c o n t r o l values at 5 mM, CaCl2, p < 0.05, u n p a i r e d t-test.
TABLE 2 E v a l u a t i o n o f t h e e f f e c t o f a t r o p i n e a n d q u i n i d i n e o n t h e a u t o m a t ± c i t y o f r a b b i t right atria using overdrive suppression. Experimental treatment
Pretreatment A f t e r n o drug T i m e lapse, 3 0 m i n n = 9 (time c o n t r o l ) Pretreatment After atropine, 1.7 × 10 -6 M Time lapse, 3 0 m i n n=5 Pretreatment After atropine, 1.7 × 10 -s M T i m e lapse, 3 0 m i n n=13 Pretreatment After quinidine, 6.2 × 10 -6 M T i m e lapse, 30 m i n n = 16
Spontaneous rate (beats/min) 85.6 ± 5.6 83.0 +_ 6.1 (--3.0 +_ 0.2) 1
99.0 ± 3.0 94.5 +- 1.5 {--4.5+2.1)
85.3 _+ 3.2 77.7 _+ 2.4 (--9.0+0.3)
82.3 + 3.8 68.2 _+ 3.8 (--17.0 ± 0.9) 2,3
Post-overdrive asystolic t i m e in sec a f t e r drive rates of: 90/min
120/min
150/min
0.93 ± 0.4 0.96 _+ 0.04 (+4.9 ± 0.9)
1.01 + 0.05 1.03 + 0.05 (+3.4 ± 1.6)
1.15 _+ 0.08 1.19 ± 0.09 (+4.3 ± 1.0)
0.68 -+ 0.03 0.67 ± 0.04
0.76 -+ 0.04 0.78 -+ 0.02
0.83 ± 0.06 0.86 _+ 0.02
(--1.5
±1.3)
0.87 ± 0.03 0.95 + 0.03 (+11.7
± 1.0)2
0.88 -+ 0.02 1.16 + 0.03 (+37.4
_+ 3.3) 2,3
(+2.8
± 1.7)
0.90 + 0.04 1.02 _+ 0.05 (+14.1
± 1.8)2
0.91 + 0.04 1.30 _+ 0.04 (+47.8
± 6.8) 2,3
I N u m b e r s in p a r e n t h e s e s i n d i c a t e % increase o r decrease f r o m p r e t r e a t m e n t value. 2 Significantly d i f f e r e n t f r o m t i m e c o n t r o l change, p < 0.05, u n p a i r e d t-test. 3 Significantly d i f f e r e n t f r o m a t r o p i n e - i n d u c e d c h a n g e , p < 0.05, u n p a i r e d t-test.
(+4.2
± 1.5)
0.99 -+ 0.04 1.10 ± 0.05 (+13.9
±2.0)2
0.99 _+ 0.04 1.46 ± 0.05 (+52.9
± 5.9) 2,3
CARDIODEPRESSANT ACTIVITY OF ATROPINE
3.2. Overdrive suppression
TABLE 3 Evaluation of the effect of atropine and quinidine on t h e effective r e f r a c t o r y p e r i o d o f r a b b i t left atria. Experimental treatment
427
Effective refractory period (msec)
Pretreatment A f t e r n o drug T i m e lapse, 30 m i n n = 8 (time control)
154.4 + 10.2 143.9 + 10.1 (--10.0 + 3.1) 1
Pretreatment A f t e r a t r o p i n e , 1.7 × 10 -s M T i m e lapse, 30 m i n n=6
150.7 -+ 6.2 1 7 1 . 5 + 4.3 2 (+20.8 -+ 5 . 4 ) 3
Pretreatment A f t e r q u i n i d i n e , 6.2 × 10 -6 M T i m e lapse, 30 m i n n=6
153.3 _+ 7.2 1 9 8 . 5 + 8.4 2 (+45.2 - 8.0) 3,4
I N u m b e r s in p a r e n t h e s e s i n d i c a t e p e r c e n t increase or decrease f r o m p r e t r e a t m e n t value. 2 Significantly d i f f e r e n t f r o m p r e t r e a t m e n t value, p < 0.05 paired t-test. 3 Significantly d i f f e r e n t f r o m t i m e c o n t r o l change, p < 0 . 0 5 u n p a i r e d t-test. 4 Significantly different from atropine-induced change, p < 0.05 u n p a i r e d t-test.
min time control, atria maintained 90% of the original contractile force and the calcium stimulation response equaled 97% of the maximum force developed initially. Atropine significantly reduced both contractile force at normal calcium level (64% of pretreatment) and the maximum force developed during CaC12 stimulation (84% of pretreatment). The effect of quinidine was similar to that o f atropine. Contractile force decreased when quinidine was added, b u t the change was n o t significant. In the presence of quinidine, the contractile force response to CaC12 stimulation was 89% of that observed previously. In 4 h time controls contractile force dropped to 60% of the original level. However the response to calcium stimulation was 98% of that achieved initially.
The results of overdrive suppression studies are shown in table 2. Data are presented as the asystolic period (in sec) and as the % change induced by treatment. As the drive rate was increased incrementally from 90 to 150 beats/min, the asystolic period after termination increased proportionally. Right atrial preparations were stable; spontaneous rate was essentially unchanged after 30 min, and pacemaker recovery from overdrive did n o t deteriorate more than 3--5%. Spontaneous atrial rate was reduced after 30 min exposure to atropine (1.7 × 10 -s M). Rate reduction was also observed 30 min after 1.7 X 10 -6 M atropine, however this reduction may not have been atropine-induced since this reduction was n o t different from that seen in time control. Automaticity was similarly affected; no change from time control was noted at 1.7 X 10 -6 M atropine, b u t it was significantly reduced at 1.7 × 10 -s M. Asysrole after overdrive was lengthened 11--14% at the higher concentration. Following exposure to quinidine (6.2 × 10 -6 M) spontaneous rate was reduced 17% and the asystole interval following overdrive was prolonged 37--53%. Quinidine was more effective than atropine in reducing automaticity of atrial pacemakers. A quinidine concentration 0.36 that of atropine, induced more than twice the effect.
3.3. Effective refractory period The effects of atropine (1.7 × 10 -s M) and quinidine (6.2 X 10 -6 M) on left atrial effective refractory period are shown in table 3. Time control measurements showed no significant change over 30 min, b u t after exposure to atropine for 30 min the effective refractory period was increased 20% above control. Exposure of other atria to quinidine elicited a 45% increase in effective refractory period, giving approximately the same relationship between atropine and quinidine doses as seen in overdrive studies.
428 4. Discussion The results of this study show that, in addition to its well known anticholinergic properties, atropine affects the myocardium directly. A statistically significant, drug-induced reduction in left atrial contractile force was observed 30 min after atropine (1.7 × 10 -s M) was added to the tissue bath. Additionally, the stimulation contractile force effects by 5 mM CaC12 was significantly reduced in the presence of atropine. Contractile force changes similar to those seen after atropine were also observed after quinidine; however quinidine appeared to be slightly less cardiodepressant. The stimulation of contractile force by 5 mM CaC12 in quinidine treated atria was not significantly different from that of controls. The time control data show that increasing the CaC12 concentration from 2.2 to 5 mM effectively and reproducibly increases left atrial contractile force in the absence of druginduced depression. Raising CaC12 concentration increased contractile force to the initially recorded level regardless of whether 30 min or 4 h had passed, or whether the contractile force at normal CaC12 concentration was significantly reduced. Another report suggests that atropine may depress inotropic responses to other cardiac stimulants more effectively than they were to elevated CaC12 in this study. Koch-Weser (1971) found that atropine pretreatment (5 × 10 -7 M) of guinea pig atrial strips reduced inotropic responsiveness to norepinephrine, tyramine and high intensity electrical stimulation to less than 15% of control response level. Patterson and Hamilton (1970) established the local anesthetic activity of atropine in frog sciatic nerves and found that atropine was a more potent local anesthetic than procalnamide. The reduced contractile force of atria treated with atropine may be related to the drug's local anesthetic properties. Josephson and Sperelakis (1976) showed that local anesthetics block Ca2÷ fluxes into the myocardial cell during activation. Conceivably such
J.E. HOLL blockade should reduce Ca 2+ availability for contraction. Reduced contractile force is normally associated with local anesthetic drugs and quinidine and procainamide, both possess local anesthetic properties (Hoffman et al., 1975). The local anesthetic character of atropine may also have been manifest in altered electrophysiologic properties of the myocardium. Right atrial automaticity and left atrial refractoriness underwent changes after atropine treatment which were characteristic of those induced by quinidine. Atropine and quinidine both reduced myocardial automaticity. The duration of asystole following overdrive was significantly lengthened in the presence of both drugs, although quinidine reduced automaticity more effectively. Simultaneous time control experiments showed only slight change in overdrive suppression in the absence of drugs. These findings confirmed that changes occurring in the presence of atropine and quinidine were due to the presence of the drugs, not deterioration of the preparation. The examination of overdrive suppression in right atrial preparations is useful in evaluating drug effects on automaticity and according to this study may be a more sensitive indicator of automatic tendencies of pacemakers than simple spontaneous rate monitoring. The magnitude of drug-induced changes in the post-overdrive asystolic interval was always greater than changes in spontaneous atrial rate, suggesting that small changes in automatic tendencies of pacemakers may be amplified when overdrive suppression is analysed. Kelliher et al. (1972) also found greater sensitivity of overdrive suppression when studying propranolol effects on cat ventricle. Previous investigators have indicated that acetylcholine released during atrial overdrive is the cause of post-overdrive asystole, since acetylcholinesterase inhibitors enhanced atrial overdrive suppression while atropine antagonized it (West, 1961; Amory and West, 1962). However, other studies found atropine alone did not fully abolish post drive suppression
CARDIODEPRESSANT ACTIVITY OF ATROPINE and Lu et al. (1965) have concluded that other factors such as potassium loss might be involved. Overdrive studies on Purkinje fibers indicate that pacemaker cells lose intracellular potassium and gain intracellular sodium (Vassalle, 1971). The increased sodium load is then thought to activate a temperature sensitive electrogenic sodium pump, the activity of which must subside before re-establishment of normal intracellular ionic milieu and re-establishment of spontaneous rhythm (Krellenstein et al., 1974; Vassalle, 1971). The degree of pump activation, and thus the duration of post-overdrive asystole appear to depend on factors relating directly to the overdrive, namely stimulation frequency and duration of overdrive (Krellenstein et al., 1974; Brooks and Lu, 1972). Brooks and Lu (1972) indicate that Purkinje fiber overdrive suppression is not influenced by acetylcholine release. While there is evidence that overdrive releases acetylcholine in atria, the present data support the findings of Lu et al. (1965) and suggest that other factors affected by atropine and quinidine may also influence overdrive suppression. The duration of asystole in right atria was proportional to overdrive frequency. Since atropine and quinidine both prolonged overdrive suppression without disturbing the relationship between stimulation frequency and asystolic interval, acetylcholine antagonism seems an unlikely mechanism. The quinidine concentration used in this study only weakly inhibits acetylcholine activity (Torchiana and Angelakis, 1964), whereas the atropine concentration exceeds that required for acetylcholine antagonism (Barlow, 1968). The local anesthetic effect of atropine is also evident when effective refractory period data are examined. Again, atropine effects were similar to those of quinidine, except that quinidine induced a significantly greater increase. DiPalma and Mascatello (1951) observed comparable refractory period changes in cat atria, but found atropine less effective in papillary muscle.
429 These results show that atropine has direct activity in vitro myocardium which mimics the antiarrhythmic effects of quinidine. Furthermore, the atropine concentration used is within the effective range reported for procainaminde (Hoffman et al., 1 9 7 5 ) - a n d diphenylhydantoin (Rosen et al., 1976). Whether this atropine concentration is achieved in situ after normal dosage is uncertain. However, Holl et al. (1970) reported diminished chronotropic and inotropic response to epinephrine after atropine in heart block, and Erdelyi and Gyarfas (1965) found that atropine protected heart block dogs from catecholamine-induced arrhythmias while cervical vagotomy did not. Thus the antiarrhythmic properties of atropine defined in the present study may be operable in the intact dog as well. Atropine effects on electrophysiologic parameters were more consistent in this study than in previous reports (Akhtar et al., 1974; G u s s e t al., 1976). In the absence of vagal influences to confound drug effect, atropine, in appropriate concentration, consistently lengthened effective refractory period and reduced automaticity. These findings do not necessarily explain the reduced incidence of arrhythmias reported by Scheinman et al. (1975). However, they do show that atropine has potential antiarrhythmic properties and indicate a need to evaluate whether antiarrhythmic concentrations can be achieved in the heart without systemic atropine toxicity.
Acknowledgement Portions of this paper were presented at the Annual Meetings of the Federation of American Societies for Experimental Biology, Anaheim, California 1976. I would like to thank Ms. Linda S. Howard for her technical assistance. The research was partially funded by the Georgia Heart Association and N.I.H. University of Georgia Biomedical Research Support Grant.
430
References Akhtar, M., A.N. Damato, W.R. Caracta, W.P. Batsford, M.E. Josephson and S.H. Lau, 1974, Electrophysiologic effects of atropine on atrioventricular conduction studied by His bundle electrogram, Amer. J. Cardiol. 3 3 , 3 3 3 . Amory, D.W. and T.C. West, 1962, Chronotropic responses following direct electrical stimulation of the isolated sinoatrial node: A pharmacologic evaluation, J. Pharmacol. Exptl. Therap. 137, 14. Barlow, R.B., 1968, Introduction to Chemical Pharmacology (Methuen and Co. Ltd., London) p. 216. Brooks, C., McC. and H.H. Lu, 1972, The Sinoatrial Pacemaker of the Heart (C.C. Thomas, Springfield, Ill.) p. 111. Chenoweth, M.D. and E.S. Koelle, 1946, An isolated heart perfusion system adapted to the determination of nongaseous metabolites, J. Lab. Clin. Med. 31,600. DiPalma, J.R. and A.V. Mascatello, 1951, Analysis of the actions of acetylcholine, atropine, epinephrine and quinidine on the heart muscle of the cat, J. Pharmacol. Exptl. Therap. 101,243. Erdelyi, A. and I. Gyarfas, 1965, Disturbances of cardiac rhythm caused by acetylcholine and catecholamines in animals with complete heart block, Acta Med. Acad. Sci. Hung. 2 1 , 3 2 5 . Goldstein, R.E., R.B. Karsh, E.R. Smith, M. Orlando, D. Norman, G. Farnham, D.R. Redwood and S.E. Epstein, 1973, Influence of atropine and of vagally mediated bradycardia on the occurrence of ventricular arrhythmias following acute coronary occlusion in closed-chest dogs, Circulation 47, 1180. Govier, W.C., 1965, The mechanism of the atrial refractory period change produced by ouabain, J. Pharmacol. Exptl. Therap. 148, 100. Guss, S.B., J.A. Kastor, M.E. Josephson and D.L. Scharf, 1976, Human ventricular refractoriness. The effects of cycle length, pacing site and atropine, Circulation 53, 450. Hoffman, B.F., M.R. Rosen and A.L. Wit, 1975, Electrophysiology and pharmacology of cardiac arrhythmias. VII Cardiac effects of quinidine and procainamide B, Amer. Heart J. 90, 117. Holl, J.E., E.F. Woods, W.P. Boyd, Jr., J.K. Pruett and H.B. Daniell, 1970, Responses to catecholamines in acute heart block, Federation Proc. 29, ~39.
J.E. HOLL Josephson, I. and N. Sperelakis, 1976, Local Anesthetic blockade of Ca2+-mediated action potentials in cardiac muscle, European J. Pharmacol. 40, 201. Keltiher, G., J. Luoto and J. Roberts, 1972, The effect of certain adrenoceptor antagonists on overdrive suppression, Brit. J. Pharmacol. 4 6 , 3 6 2 . Kent, K.M., S.E. Epstein, T. Cooper and D.M. Jacobowitz, 1974, Cholinergic innervation of the canine and human ventricular conduction system. Anatomic and electrophysiologic correlations, Circulation 50,948. Koch-Weser, J., 1971, ~-Receptor blockade and myocardial effects of cardiac glycosides, Circulation Res. 29,109. Krellenstein, D.J., M.B. Pliam, C.McC. Brooks and M. Vassalle, 1974, On the mechanism of idioventricular pacemaker suppression by fast drive, Circulation Res. 35,923. Lu, H.H., G. Lange and C.McC. Brooks, 1965, Factors controlling pacemaker action in cells of the sinoatrial node, Circulation Res. 17,460. Patterson, J.G. and J.T. Hamilton, 1970, A nervechamber bioassay of some new and clinically used compounds, Arch. Intern. Pharmacodyn. 185,370. Rosen, M.R., P. Danilo, Jr., M.B. Alonso and C.E. Pippenger, 1976, Effects of therapeutic concentrations of diphenylhydantoin on transmembrane potentials of normal and depressed Purkinje fibers, J. Pharmacol. Exptl. Therap. 197,594. Scheinman, M.M., D. Thorburnand and J.A. Abbot, 1975, Use of atropine in patients with acute myocardial infarction and sinus bradycardia, Circulation 52,627. Stanton, H.C. and R.L. Vick, 1968, Cholinergic and adrenergic influences on right ventricular myocardial contractility in the dog, Arch. Intern. Pharmacodyn. 176, 16. Torchinana, M.L. and E.T. Angelakos, 1964, Antagonism of the excitatory effects of acetylcholine on atrial refractoriness by atropine, quinidine, and procaine amide, Arch. Intern. Pharmacodyn. 150, 379. Vassalle, M., 1971, Automaticity and automatic rhythms, Amer. J. Cardiol. 28,245. West, T.C., 1961, Effects of chronotropic influences on subthreshold oscillations in the sino-atrial node, in: The Specialized Tissues of the Heart, eds. A. Paes de Carvalho, W.C. de Mello and B.F. Hoffman (Elsevier Publishing Co., Amsterdam) p. 81.
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© Elsevier/North-Holland Biomedical Press
QUINIDINE-LIKE ACTIVITY OF ATROPINE IN RABBIT ATRIA J.E. HOLL
Department of Pharmacology, School of Pharmacy, University of Georgia, Athens, Georgia 30602, U.S.A. Received 8 July 1977, revised MS received 18 October 1977, accepted 4 November 1977
J.E. HOLL, Quinidine-like activity of atropine in rabbit atria, European J. Pharmacol. 47 (1978) 423--430. Direct effects of atropine and quinidine on contractile force, overdrive suppression, and effective refractory period were studied in rabbit atria. Left atrial contractile force at normal CaCI2 concentration (2.2 raM) and the contractile force response to elevated CaCl2 (5 mM) were unchanged over 30 min. Atropine exposure (1.7 × 10-s M) for 30 min significantly reduced contractile force at both normal and elevated CaCl2 levels. Quinidine exposure (6.2 × 10-6 M) for 30 min produced similar but statistically insignificant changes. At 4 h, control contractile force at 2.2 mM CaCl2 decreased to the post-atropine level, but the response to 5 mM CaC12 was unchanged. The asystolic interval of right atria following overdrive (for 2 rain at 3 rates) was increased after 30 m i n by 4% for controls, 14% after atropine (1.7 × 10 -s M) and 45% after quinidine. The effective refractory period of left atria (evaluated by paired pulse stimulation) was unchanged after 30 min for controls, but increased by 20% after atropine and 45% after quinidine. Effective refractory period Overdrive suppression
Contractile force
1. Introduction The use of atropine for management of bradyarrhythmias following acute myocardial infarction has been evaluated in dogs (Goldstein et al., 1973) and man (8heinman et al., 1975), conflicting results provide no firm conclusion concerning the efficacy or safety of atropine. In these reports atropine perhaps enhanced arrhythmia generation in acutely infarcted dogs, whereas it protected from premature ventricular contractions in humans. The electrophysiologic properties of atropine have been evaluated in humans, again with inconclusive results. According to Akhtar et al. (1974) vagolytic concentration of atropine increased sino-atrial rate and AV conduction, but did not alter His--Purkinje conduction. Atrial refractoriness was increased, decreased and unaltered. Similar changes were found when human ventricular refractoriness
Quinidine
CaCI2
Isolated atria
was evaluated before and after atropine (Guss et al., 1976). Vagal innervation of atria is well documented, ant it is possible that erratic atropine effects on atrial refractoriness may derive from atropine action in relation to background vagal tone. Vagal innervation of the ventricles is less well known. It does exist (Stanton and Vick, 1968; Kent et al., 1974}, but its function is obscure. Therefore attribution of inconsistent atropine effects on ventricular refractoriness to variation in vagal tone is more hypothetical. In view of the unknown contribution of background nervous activity to atropine action, and the contradictory reports of atropine arrhythmic/antiarrhythmic properties, atropine effects on the heart were evaluated in isolated tissue preparations in which nervous activity was eliminated. The influence of atropine on the mechanical and electrophysio-
424
logic properties of the heart were specifically evaluated and compared to those of quinidine to determine whether atropine affects the heart similarly and has potential antiarrythmic properties as suggested by Scheinman et al. (1976).
JE. HOLL
2. Materials and methods
drug. 30 rain or 4 h after the first calcium response the calcium levels were again increased to 5 mM and the parameters remeasured. Time control baths into which no drugs were introduced between calcium responses were included to determine the effect of time lapse and prior exposure to calcium on subsequent responses.
2.1. Contractile force
2.3. Overdrive suppression
New Zealand rabbits of either sex, weighing approximately 2 kg were killed by a blow on the head and their hearts removed and placed in an cooled, aerated (95% 02, 5% CO2)solution containing (mM); NaC1 120, KC1 5.6, MgC12 2.1, NaHCO3 25, CaC12 2.2 and dextrose 10 (Chenoweth and Koelle, 1946). Right and left atria were dissected free, separated and m o u n t e d in 100 ml tissue baths at 30°C. Atria were connected by threads to Grass FT-03 Force Displacement Transducers for contractile force monitoring (developed peak systolic tension) on a Grass Model 7 Polygraph. A 1.0 g diastolic load was imposed and a 1 h equilibration period was allowed. Right atria were connected to a Grass Model 7P44A Tachograph to provide linear recording of frequency changes in beats/min. Left atria were driven at 120 beats/min with a Grass $48 Stimulator set to deliver suprathreshold electrical pulses of 1.0 msec duration through stainless steel electrodes which were embedded in the muscle holders. Voltages employed ranged between O.9--4.5 V.
After a 1 h equilibration and the establishm e n t of constant spontaneous rate, overdrive suppression of the right atrial pacemaker was measured to evaluate automaticity of the tissue. In a variation of the m e t h o d of Kelliher et al. (1972), the right atrium was electrically driven for 2 min. When stimulation was stopped the period of asystole between the last driven beat and the first spontaneous beat was measured with the recorder chart speed set at 50 mm/sec. This procedure was repeated so that three rates of drive were used: 90, 120 and 150 beats/min. In approximately 95% of atria studied the duration of asystole was proportional to the rate of stimulation; as the frequency of stimulation was increased, the asystolic period was lengthened. At least 2 observations were made at each drive rate and 2 min rest periods were allowed between observations. From these control data, a linear regression analysis of asystolic period vs. drive frequency was obtained. Only those tissues with an r > 0.95 were studied further. In the approximately 5% of atria where r ~ 0.95, the atria appeared to contain at least 2 primary pacemakers, one normally latent which become d o m i n a n t at some appropriate stimulation frequency. Therefore, an r > 0.95 was taken to indicate the probability that only one pacemaker was influenced by electrical stimulation, and that drug studies were conducted on a single pacemaker system. Right atria qualifying for further study were exposed either to 1.7 X 10 -6 M or 1.7 X 10 -s M atropine or 6.2 X 10 -6 M quinidine for
2.2. Elevated calcium concentration After equilibration, the calcium concentration in the tissue baths containing left atria was raised to 5 mM and contractile force increases were measured. The excess calcium was then washed o u t by at least four bath fluid exchanges and contractile force at 2.2 mM CaC12 re-established. Atropine (1.7X 10 -s M) or quinidine (6.2 X 10 -6 M) was then added to some baths while others received no
CARDIODEPRESSANT ACTIVITY OF ATROPINE 30 min, and overdrive studies were repeated. Time control atria were retested for overdrive suppression after 30 min with no drug intervention.
425
P,
B T C.0.5 hr.
4'
2.4. Effective refractory period The functional refractory period of left atria was determined by the m e t h o d of Govier (1965). The basic stimulation rate was 60 beats/min and the refractory period was determined by delivering a second stimulus identical to the first. Initially, the second stimulus was delivered within the refractory period of the muscle; then the interval to the second stimulus was increased until a mechanical response to both stimuli and was recorded. At endpoint, the interval between stimuli was measured with Tektronix R5103/ D13 Storage Oscilloscope. After control readings were taken, atria were exposed to 1.7 × 10 -s M atropine or 6.2 × 10 -4 M quinidine for 30 min and refractory period redetermined. In a time control group with no drug intervention, the elapsed time between refractory period determinations was 30 rain. Atropine and quinidine concentrations are reported as the molar concentration of drug base. Data are reported as mean + S.E.M. and comparisons between groups were made using t-test for paired or unpaired data as appropriate.
3. Results
3.1. Elevated calcium concentration Increases in left atrial contractile force in response to elevation of CaC12 concentration from 2.2 to 5.0 mM are shown in panel A, fig. 1. The effects of atropine, quinidine and time on these responses are shown in panel B. Successive exposures to 5 mM CaC12 at 30 min intervals did not adversely affect the atrium. Over the 30 min time control, contractile force declined only slightly, and the atrium
Quinidine
TC. 4.0 hr.
1 min. Fig. 1. C o n t r a c t i l e force r e s p o n s e s o f 4 isolated left
atria upon raising CaCl2 concentration from 2.2 to 5 mM (at arrows). Panel A shows control responses; panel B shows additional responses after treatment. Panel B responses are: time control response after 30 rain (no drug); atropine response (30 rain after adding 1.7 x 10-s M atropine); quinidine response (30 min after adding 6.2 x 10-6 M quinidine); time control response after 4 h (no drug). remained fully responsive to calcium stimulation. Contractile force was reduced after 30 min of atropine exposure and calcium stimulation did n o t increase force to the level achieved prior to atropine (panel B, fig. 1). Quinidine effects on contractile force and the response to calcium stimulation were similar to those observed after atropine. In other controls calcium stimulations were separated by 4 h to m o n i t o r responsiveness over a long period of time. Contractile force at normal Ca level was reduced at 4 h and was comparable to forces recorded in atria exposed to atropine or quinidine. However, contractile force responses to calcium stimulation were the same as the initial response. Responses replicating those in fig. 1 were observed in additional atria. Table 1 shows post-treatment contractile force at normal and elevated calcium levels as a percent of respective pretreatment responses. In the 30
426
J.E. H O L L
TABLE 1 E f f e c t o f a t r o p i n e a n d q u i n i d i n e o n c a l c i u m - i n d u c e d c o n t r a c t i l e force r e s p o n s e o f left atria. Experimental treatment
Post-treatment calcium-induced c o n t r a c t i l e force r e s p o n s e 1 (% p r e t r e a t m e n t r e s p o n s e ) 2
No d r u g T i m e lapse, 30 m i n (time control) A t r o p i n e , 1.6 X 10 -s M T i m e lapse, 30 m i n Q u i n i d i n e , 6.2 × 10 -6 M T i m e lapse, 30 m i n No d r u g T i m e lapse, 4 h 1 2 3 4
R e p r e s e n t a t i v e trace s h o w n R e p r e s e n t a t i v e trace s h o w n Significantly d i f f e r e n t f r o m Significantly d i f f e r e n t f r o m
N u m b e r o f atria
2.2 m M CaC12
5.0 m M CaCl2
90 ± 4
97 ± 1
9
64 +_ 6 3
84 ± 3 4
9
70 £ 7
89 ± 5
5
60 _+ 6 3
98 _+ 1
10
in p a n e l B of fig. 1. in p a n e l A o f fig. 1. 30 m i n t i m e c o n t r o l value at 2.2 mM, CaCl2, p < 0.05, u n p a i r e d t-test. b o t h t i m e c o n t r o l values at 5 mM, CaCl2, p < 0.05, u n p a i r e d t-test.
TABLE 2 E v a l u a t i o n o f t h e e f f e c t o f a t r o p i n e a n d q u i n i d i n e o n t h e a u t o m a t ± c i t y o f r a b b i t right atria using overdrive suppression. Experimental treatment
Pretreatment A f t e r n o drug T i m e lapse, 3 0 m i n n = 9 (time c o n t r o l ) Pretreatment After atropine, 1.7 × 10 -6 M Time lapse, 3 0 m i n n=5 Pretreatment After atropine, 1.7 × 10 -s M T i m e lapse, 3 0 m i n n=13 Pretreatment After quinidine, 6.2 × 10 -6 M T i m e lapse, 30 m i n n = 16
Spontaneous rate (beats/min) 85.6 ± 5.6 83.0 +_ 6.1 (--3.0 +_ 0.2) 1
99.0 ± 3.0 94.5 +- 1.5 {--4.5+2.1)
85.3 _+ 3.2 77.7 _+ 2.4 (--9.0+0.3)
82.3 + 3.8 68.2 _+ 3.8 (--17.0 ± 0.9) 2,3
Post-overdrive asystolic t i m e in sec a f t e r drive rates of: 90/min
120/min
150/min
0.93 ± 0.4 0.96 _+ 0.04 (+4.9 ± 0.9)
1.01 + 0.05 1.03 + 0.05 (+3.4 ± 1.6)
1.15 _+ 0.08 1.19 ± 0.09 (+4.3 ± 1.0)
0.68 -+ 0.03 0.67 ± 0.04
0.76 -+ 0.04 0.78 -+ 0.02
0.83 ± 0.06 0.86 _+ 0.02
(--1.5
±1.3)
0.87 ± 0.03 0.95 + 0.03 (+11.7
± 1.0)2
0.88 -+ 0.02 1.16 + 0.03 (+37.4
_+ 3.3) 2,3
(+2.8
± 1.7)
0.90 + 0.04 1.02 _+ 0.05 (+14.1
± 1.8)2
0.91 + 0.04 1.30 _+ 0.04 (+47.8
± 6.8) 2,3
I N u m b e r s in p a r e n t h e s e s i n d i c a t e % increase o r decrease f r o m p r e t r e a t m e n t value. 2 Significantly d i f f e r e n t f r o m t i m e c o n t r o l change, p < 0.05, u n p a i r e d t-test. 3 Significantly d i f f e r e n t f r o m a t r o p i n e - i n d u c e d c h a n g e , p < 0.05, u n p a i r e d t-test.
(+4.2
± 1.5)
0.99 -+ 0.04 1.10 ± 0.05 (+13.9
±2.0)2
0.99 _+ 0.04 1.46 ± 0.05 (+52.9
± 5.9) 2,3
CARDIODEPRESSANT ACTIVITY OF ATROPINE
3.2. Overdrive suppression
TABLE 3 Evaluation of the effect of atropine and quinidine on t h e effective r e f r a c t o r y p e r i o d o f r a b b i t left atria. Experimental treatment
427
Effective refractory period (msec)
Pretreatment A f t e r n o drug T i m e lapse, 30 m i n n = 8 (time control)
154.4 + 10.2 143.9 + 10.1 (--10.0 + 3.1) 1
Pretreatment A f t e r a t r o p i n e , 1.7 × 10 -s M T i m e lapse, 30 m i n n=6
150.7 -+ 6.2 1 7 1 . 5 + 4.3 2 (+20.8 -+ 5 . 4 ) 3
Pretreatment A f t e r q u i n i d i n e , 6.2 × 10 -6 M T i m e lapse, 30 m i n n=6
153.3 _+ 7.2 1 9 8 . 5 + 8.4 2 (+45.2 - 8.0) 3,4
I N u m b e r s in p a r e n t h e s e s i n d i c a t e p e r c e n t increase or decrease f r o m p r e t r e a t m e n t value. 2 Significantly d i f f e r e n t f r o m p r e t r e a t m e n t value, p < 0.05 paired t-test. 3 Significantly d i f f e r e n t f r o m t i m e c o n t r o l change, p < 0 . 0 5 u n p a i r e d t-test. 4 Significantly different from atropine-induced change, p < 0.05 u n p a i r e d t-test.
min time control, atria maintained 90% of the original contractile force and the calcium stimulation response equaled 97% of the maximum force developed initially. Atropine significantly reduced both contractile force at normal calcium level (64% of pretreatment) and the maximum force developed during CaC12 stimulation (84% of pretreatment). The effect of quinidine was similar to that o f atropine. Contractile force decreased when quinidine was added, b u t the change was n o t significant. In the presence of quinidine, the contractile force response to CaC12 stimulation was 89% of that observed previously. In 4 h time controls contractile force dropped to 60% of the original level. However the response to calcium stimulation was 98% of that achieved initially.
The results of overdrive suppression studies are shown in table 2. Data are presented as the asystolic period (in sec) and as the % change induced by treatment. As the drive rate was increased incrementally from 90 to 150 beats/min, the asystolic period after termination increased proportionally. Right atrial preparations were stable; spontaneous rate was essentially unchanged after 30 min, and pacemaker recovery from overdrive did n o t deteriorate more than 3--5%. Spontaneous atrial rate was reduced after 30 min exposure to atropine (1.7 × 10 -s M). Rate reduction was also observed 30 min after 1.7 X 10 -6 M atropine, however this reduction may not have been atropine-induced since this reduction was n o t different from that seen in time control. Automaticity was similarly affected; no change from time control was noted at 1.7 X 10 -6 M atropine, b u t it was significantly reduced at 1.7 × 10 -s M. Asysrole after overdrive was lengthened 11--14% at the higher concentration. Following exposure to quinidine (6.2 × 10 -6 M) spontaneous rate was reduced 17% and the asystole interval following overdrive was prolonged 37--53%. Quinidine was more effective than atropine in reducing automaticity of atrial pacemakers. A quinidine concentration 0.36 that of atropine, induced more than twice the effect.
3.3. Effective refractory period The effects of atropine (1.7 × 10 -s M) and quinidine (6.2 X 10 -6 M) on left atrial effective refractory period are shown in table 3. Time control measurements showed no significant change over 30 min, b u t after exposure to atropine for 30 min the effective refractory period was increased 20% above control. Exposure of other atria to quinidine elicited a 45% increase in effective refractory period, giving approximately the same relationship between atropine and quinidine doses as seen in overdrive studies.
428 4. Discussion The results of this study show that, in addition to its well known anticholinergic properties, atropine affects the myocardium directly. A statistically significant, drug-induced reduction in left atrial contractile force was observed 30 min after atropine (1.7 × 10 -s M) was added to the tissue bath. Additionally, the stimulation contractile force effects by 5 mM CaC12 was significantly reduced in the presence of atropine. Contractile force changes similar to those seen after atropine were also observed after quinidine; however quinidine appeared to be slightly less cardiodepressant. The stimulation of contractile force by 5 mM CaC12 in quinidine treated atria was not significantly different from that of controls. The time control data show that increasing the CaC12 concentration from 2.2 to 5 mM effectively and reproducibly increases left atrial contractile force in the absence of druginduced depression. Raising CaC12 concentration increased contractile force to the initially recorded level regardless of whether 30 min or 4 h had passed, or whether the contractile force at normal CaC12 concentration was significantly reduced. Another report suggests that atropine may depress inotropic responses to other cardiac stimulants more effectively than they were to elevated CaC12 in this study. Koch-Weser (1971) found that atropine pretreatment (5 × 10 -7 M) of guinea pig atrial strips reduced inotropic responsiveness to norepinephrine, tyramine and high intensity electrical stimulation to less than 15% of control response level. Patterson and Hamilton (1970) established the local anesthetic activity of atropine in frog sciatic nerves and found that atropine was a more potent local anesthetic than procalnamide. The reduced contractile force of atria treated with atropine may be related to the drug's local anesthetic properties. Josephson and Sperelakis (1976) showed that local anesthetics block Ca2÷ fluxes into the myocardial cell during activation. Conceivably such
J.E. HOLL blockade should reduce Ca 2+ availability for contraction. Reduced contractile force is normally associated with local anesthetic drugs and quinidine and procainamide, both possess local anesthetic properties (Hoffman et al., 1975). The local anesthetic character of atropine may also have been manifest in altered electrophysiologic properties of the myocardium. Right atrial automaticity and left atrial refractoriness underwent changes after atropine treatment which were characteristic of those induced by quinidine. Atropine and quinidine both reduced myocardial automaticity. The duration of asystole following overdrive was significantly lengthened in the presence of both drugs, although quinidine reduced automaticity more effectively. Simultaneous time control experiments showed only slight change in overdrive suppression in the absence of drugs. These findings confirmed that changes occurring in the presence of atropine and quinidine were due to the presence of the drugs, not deterioration of the preparation. The examination of overdrive suppression in right atrial preparations is useful in evaluating drug effects on automaticity and according to this study may be a more sensitive indicator of automatic tendencies of pacemakers than simple spontaneous rate monitoring. The magnitude of drug-induced changes in the post-overdrive asystolic interval was always greater than changes in spontaneous atrial rate, suggesting that small changes in automatic tendencies of pacemakers may be amplified when overdrive suppression is analysed. Kelliher et al. (1972) also found greater sensitivity of overdrive suppression when studying propranolol effects on cat ventricle. Previous investigators have indicated that acetylcholine released during atrial overdrive is the cause of post-overdrive asystole, since acetylcholinesterase inhibitors enhanced atrial overdrive suppression while atropine antagonized it (West, 1961; Amory and West, 1962). However, other studies found atropine alone did not fully abolish post drive suppression
CARDIODEPRESSANT ACTIVITY OF ATROPINE and Lu et al. (1965) have concluded that other factors such as potassium loss might be involved. Overdrive studies on Purkinje fibers indicate that pacemaker cells lose intracellular potassium and gain intracellular sodium (Vassalle, 1971). The increased sodium load is then thought to activate a temperature sensitive electrogenic sodium pump, the activity of which must subside before re-establishment of normal intracellular ionic milieu and re-establishment of spontaneous rhythm (Krellenstein et al., 1974; Vassalle, 1971). The degree of pump activation, and thus the duration of post-overdrive asystole appear to depend on factors relating directly to the overdrive, namely stimulation frequency and duration of overdrive (Krellenstein et al., 1974; Brooks and Lu, 1972). Brooks and Lu (1972) indicate that Purkinje fiber overdrive suppression is not influenced by acetylcholine release. While there is evidence that overdrive releases acetylcholine in atria, the present data support the findings of Lu et al. (1965) and suggest that other factors affected by atropine and quinidine may also influence overdrive suppression. The duration of asystole in right atria was proportional to overdrive frequency. Since atropine and quinidine both prolonged overdrive suppression without disturbing the relationship between stimulation frequency and asystolic interval, acetylcholine antagonism seems an unlikely mechanism. The quinidine concentration used in this study only weakly inhibits acetylcholine activity (Torchiana and Angelakis, 1964), whereas the atropine concentration exceeds that required for acetylcholine antagonism (Barlow, 1968). The local anesthetic effect of atropine is also evident when effective refractory period data are examined. Again, atropine effects were similar to those of quinidine, except that quinidine induced a significantly greater increase. DiPalma and Mascatello (1951) observed comparable refractory period changes in cat atria, but found atropine less effective in papillary muscle.
429 These results show that atropine has direct activity in vitro myocardium which mimics the antiarrhythmic effects of quinidine. Furthermore, the atropine concentration used is within the effective range reported for procainaminde (Hoffman et al., 1 9 7 5 ) - a n d diphenylhydantoin (Rosen et al., 1976). Whether this atropine concentration is achieved in situ after normal dosage is uncertain. However, Holl et al. (1970) reported diminished chronotropic and inotropic response to epinephrine after atropine in heart block, and Erdelyi and Gyarfas (1965) found that atropine protected heart block dogs from catecholamine-induced arrhythmias while cervical vagotomy did not. Thus the antiarrhythmic properties of atropine defined in the present study may be operable in the intact dog as well. Atropine effects on electrophysiologic parameters were more consistent in this study than in previous reports (Akhtar et al., 1974; G u s s e t al., 1976). In the absence of vagal influences to confound drug effect, atropine, in appropriate concentration, consistently lengthened effective refractory period and reduced automaticity. These findings do not necessarily explain the reduced incidence of arrhythmias reported by Scheinman et al. (1975). However, they do show that atropine has potential antiarrhythmic properties and indicate a need to evaluate whether antiarrhythmic concentrations can be achieved in the heart without systemic atropine toxicity.
Acknowledgement Portions of this paper were presented at the Annual Meetings of the Federation of American Societies for Experimental Biology, Anaheim, California 1976. I would like to thank Ms. Linda S. Howard for her technical assistance. The research was partially funded by the Georgia Heart Association and N.I.H. University of Georgia Biomedical Research Support Grant.
430
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