Eileen Winslow

Development of a severe model of early coronary artery ligation-induced dysrhythmias in the anaesthetized rat.

1 The potential use of catecholamines to increase the severity of dysrhythmias evoked by coronary artery ligation in the anaesthetized rat was investigated. Drugs were given intravenously prior to ligation. 2 Pressor doses of adrenaline (5 μg/kg) noradrenaline (1 μg/kg) phenylephrine (5–10 μg/kg), and angiotensin (0.25 μg/kg) conferred protection against the development of dysrhythmias. 3 Atropine (1 mg/kg) increased mortality from ventricular fibrilloflutter (VF) and abolished the protective effects of phenylephrine (10 μg/kg). 4 Administration of isoprenaline (10 μg/kg) significantly increased the incidence of and the mortality from VF. 5 The order of antidysrhythmic drug potency of Org 6001 (1–10 mg/kg), disopyramide (2–10 mg/kg) and practolol (2–10 mg/kg) was similar in both the standard (without isoprenaline) and modified (with isoprenaline) models. 6 Use of the modified method for antidysrhythmic screening purposes allows demonstration of statistically meaningful results with the use of relatively few animals. 7 Comparison of the pattern of VF in the rat heart induced by various means suggests that the diagnosis of ventricular fibrillation can be made with more confidence in the modified method compared to the standard method.

Effects of antiarrhythmic drugs on ventricular fibrillation thresholds of normal and ischaemic myocardium in the anaesthetized rat

1 The effects of agents which produce membrane stabilization (class I), β1‐adrenoceptor blockade (class II), prolongation of the cardiac action potential (class III) or inhibition of the slow inward current (class IV) were investigated for their ability to increase the ventricular fibrillation threshold (VFT) or to modify the fall in VFT consequent upon coronary artery ligation in the anaesthetized rat. 2 The class I agent, Org 6001, increased VFT of normal myocardium and in lower doses reduced the postligation fall in VFT. 3 The class II agent, metoprolol, failed to increase VFT of normal myocardium but reduced the postligation fall. 4 The class III agent, melperone, increased VFT of both normal and ischaemic myocardium whereas the class IV agent, nifedipine failed to influence VFT in either region. 5 Bepridil (class I and IV) was similar to Org 6001 and sotalol (class II and III) in that it increased VFT of normal myocardium and in lower doses reduced the postligation fall in VFT. 6 Measurement of VFT before and after coronary artery ligation in the rat constitutes a rapid and reproducible screen to detect antifibrillatory activity. 7 The results also suggest that in the rat, the low currents used (˜400 μA) do not release substantial quantities of catecholamines whereas these may be released by coronary artery ligation.

EVALUATION OF ANTAGONISM OF ACONITINE‐INDUCED DYSRHYTHMIAS IN MICE AS A METHOD OF DETECTING AND ASSESSING ANTIDYSRHYTHMIC ACTIVITY

1 Antagonism of aconitine‐induced dysrhythmias in mice as a method of detecting and assessing antidysrhythmic activity was evaluated. 2 Aconitine‐induced dysrhythmias in mice appear to be selectively sensitive to antidysrhythmic agents (administered intraperitoneally) which reduce the inward sodium current in cardiac cells. 3 Antidysrhythmic agents whose mechanism of action is thought to depend on β‐adrenoceptor blockade, prolongation of cardiac monophasic action potentials or calcium antagonism are ineffective in delaying the onset of aconitine‐induced dysrhythmias in mice. The inactive drugs were practolol, sotalol, bretylium, amiodarone and verapamil. 4 Comparisons of anti‐dysrhythmic activities of test drugs should be based on more than one ED value and should take account of efficacy as well as potency. 5 The mouse aconitine test is a useful and rapid method of evaluating oral antidysrhythmic activity in terms of potency, efficacy and duration of action. 6 With respect to potency, efficacy, oral activity, duration of action and safety, 3α‐amino‐5α‐androstan‐2β‐ol‐17‐one hydrochloride (Org 6001) offered the most satisfactory overall profile of the active drugs tested (Org 6001, aprindine, quinidine, disopyramide, lignocaine, mexiletine, procainamide and propranolol).

Comparative antiarrhythmic and electrophysiological effects of drugs known to inhibit calmodulin (TFP, W7 and bepridil)

1 The potential antiarrhythmic and electrophysiological actions of drugs known to inhibit calmodulin, i.e. trifluoperazine (TFP) and N‐(6‐aminohexyl)‐5‐chloro‐1‐naphthalene sulphonamide (W7) have been compared with bepridil, whose antiarrhythmic actions have previously been ascribed to blockade of the fast inward sodium current in cardiac tissue. 2 Like bepridil, both TFP and W7 reduced the severity of arrhythmias evoked by 30 min of coronary artery occlusion in tha anaesthetized rat. 3 TFP (2.5–10 mg kg−1, i.v.), W7 (2.5–10 mg kg−1, i.v.) and bepridil (1–5 mg kg−1, i.v.) also antagonized the development of ventricular fibrillation induced by 5 min of occlusion followed by reperfusion. All three drugs also reduced mortality. TFP and bepridil also reduced the incidence of reperfusion‐induced ventricular tachycardia whilst all 3 drugs reduced its duration. 4 Although TFP was shown to possess α‐adrenoceptor blocking properties, the classical α‐blocker, phentolamine, failed to reduce significantly the incidence or severity of reperfusion arrhythmias. 5 In contrast to bepridil (2–20 μM), which markedly reduced the maximum rate of depolarization (Vmax) of guinea‐pig isolated papillary muscle, W7 (5–50 μM) showed only weak effects on Vmax and was at least 10 times less potent than bepridil whilst TFP only reduced Vmax in high concentrations (40–100 μM) which lowered resting membrane potential. 6 Unlike bepridil, neither TFP (4–40 μM) nor W7 prolonged the absolute refractory period. 7 The results suggest that drugs which inhibit calmodulin confer protection against both ischaemia ‐ and reperfusion‐induced arrhythmias in the rat. Although the electrophysiological actions of bepridil would adequately account for its antiarrhythmic activity, the same cannot be said of W7 and especially TFP. 8 In conclusion, calmodulin antagonism may constitute a mechanism of antiarrhythmic activity.

Effects of amiodarone on cardiac electrophysiology and inducibility of arrhythmias in chronically infarcted dogs: late arrhythmias, haemodynamics, and sympatholytic actions.

Summary: The electrophysiological and antiarrhythmic effects of acute (2 and 10 mg/kg i.v.) and chronic (400 mg/day p.o. for 28 days) amiodarone (AM) treatment were compared in anaesthetised dogs with 5–6-day-old myocardial infarcts. Intravenous AM prolonged the RR interval, sinus node recovery time, the PR interval, and atrial to His conduction time by 36, 33, 25, and 36%, respectively. Corresponding increases after oral amiodarone were 50, 57, 12, and 26%. Atrial and His–Purkinje conduction times were unchanged. Atrial and ventricular refractory periods were increased especially after oral treatment. Oral AM additionally prolonged QRS, QT, and paced QT (by 4, 34, and 19%, respectively). Effects of oral AM on ventricular repolarisation and on the fast inward sodium current were confirmed in vitro. Both modes of AM administration protected against inducible arrhythmias, an effect that was more marked during normal sinus rhythm than during pacing in orally treated dogs. Oral amiodarone failed to protect against spontaneous late arrhythmias 24 h after infarction whilst both modes of administration noncompetitively inhibited isoprenaline-induced tachycardia. Oral AM reduced blood pressure (13%) and LV dP/dt/P (24%) whereas cardiac output was maintained by an increase in stroke volume. It was concluded that oral AM is haemodynamically well tolerated and that prolonged ventricular repolarisation enhanced by bradycardia together with sympatholytic actions may be important mechanisms for antiarrhythmic efficacy, whereas the mechanisms involved in i.v. efficacy are less clear but may depend, at least partly, on sympatholytic actions and perhaps (tentatively) on sodium channel block in Purkinje tissue.

The effects of bepridil compared with calcium antagonists on rat and rabbit aorta

The vasodilator (relaxant) action of the antianginal agent bepridil was compared with that of drugs known to either block membrane calcium channels, to be calmodulin antagonists or to inhibit intracellular calcium flux using rat and rabbit aortic strips. IC50 values were obtained for relaxation of tonic contractions induced by either potassium (K+) or phenylephrine (PE). The order of relaxant specificity against K+ compared with PE in both rabbit and rat tissue preparations was nisoldipine = nifedipine greater than diltiazem greater than verapamil greater than bepridil greater than PrMDI greater than W7 greater than TFP greater than phentolamine. The ratio for flunarizine equalled that of bepridil in rabbit and that of verapamil in rat. The calmodulin antagonist TFP showed additional alpha-adrenoceptor blocking properties. Analysis of concentration response curves to the antagonists together with PE/K+ ratios revealed a different profile for each drug tested on rabbit aorta. The range of PE/K+ ratios was much wider in rabbit compared to rat. The results suggest that, with the method used, rabbit aortic strips offer a simple technique for drug profiling and allows a clearer differentiation between membrane active and intracellularly acting drugs than does rat aorta. The profile of bepridil resembled more that of intracellularly acting drugs suggesting that intracellular actions (possibly calmodulin inhibition) may play a substantial role in its dilator actions on vascular smooth muscle.

Comparative effects of bepridil and verapamil on isolated coronary and systemic vascular and cardiac muscle

The relaxant effects of the antianginal agent bepridil, a drug with calcium antagonistic actions, and verapamil on isolated pig coronary (C) and rabbit aortic (A) smooth muscle were compared. Both drugs competitively antagonised calcium in Tris buffered but not in bicarbonate buffered medium. Both agents relaxed K+-depolarised C and inhibited contractile responses to noradrenaline plus anoxia. Bepridil was approximately 10 fold less potent than verapamil. The two drugs exerted similar calcium antagonistic effects in guinea pig papillary muscle but showed dissimilar profiles in atrial muscle in that responses in the presence of bepridil appeared to be frequency dependent. It is concluded that bepridil inhibits responses to anoxia and competitively antagonises calcium in vascular smooth muscle. On atrial tissue, bepridil, unlike verapamil, appears to exert additional actions possibly involving other ion channels.

Peptidyl aminosteroids as potential new antiarrhythmic agents

The synthesis of peptidyl derivatives of the aminosteroid, amafalone (Am), is described. Six analogs were synthesized: the hydrochloride salts of Gly-Am (2) Ala-Gly-Am (3), D-Ala-Gly-Am (4), Pro-Am (6), Pro-Pro-Am (7), and D-Ala-Pro-Am (8). The peptide bonds were formed by the polymeric reagent method using polymeric hydroxybenzotriazole as the activating polymer. Peptidyl aminosteroids 2, 6, 7, and 8, when administered to rats intravenously, had protective antiarrhythmic effects similar to those of amafalone. By the oral route, less marked protection, in comparison to amafalone, was observed with 6, while 7 and 8 were disappointingly inactive.

Comparative electrophysiological effects of disopyramide and bepridil on rabbit atrial, papillary, and Purkinje tissue: modification by reduced extracellular potassium

The electrophysiological actions of bepridil were compared with those of disopyramide in rabbit atrial, Purkinje, and papillary muscles in both normokalaemic (5.6 mM K +) and low K+ medium. In normokalaemic medium, the major electrophysiological effects of bepridil (10–40 μ M) were a reduction in the maximum rate of rise of phase 0 of the action potential (MRD) of atrial and Purkinje fibres and prolongation of the atrial action potential duration (APD). Higher concentrations (40–100 μ M) reduced MRD of papillary muscle without markedly affecting APD. Disopyramide (40–100 μM) reduced MRD and prolonged APD in similar concentrations in all three tissues. Reducing extracellular (EC) K + to 2.8 mM decreased the effects of bepridil on MRD in all tissues but only significantly reduced the response to disopyramide in papillary muscle. Enhancement of APD prolongation by disopyramide in all tissues resulted when EC K + was reduced, whereas only papillary muscle APD showed a marked prolongation in response to bepridil. These results demonstrate that the class I effect of bepridil is highly K+ dependent. It may be expected that QTc prolongation produced by both these drugs will be more marked in patients with low serum potassium levels.

Comparative Frequency-Dependent Effects of Three Class Ic Agents, Org 7797, F]ecanide, and Proprafenone, on ventricular Action Potential Duration

The frequency-dependent electrophysiological effects of three class lc agents, Org 7797 (0.5–5 μM), flecainide (1–20 μM), and propafenone (1–10 μM), were investigated using isolated guinea pig papillary muscle. Transmemhrane cellular action potentials were recorded using conventional microelectrode techniques at driving frequencies of 0.3, 1.0, and 2.0 Hz. In concentrations inducing similar frequency-dependent decreases in Vmax both flecainide and propafenone shortened action potential duration (AM at the two lower stimulation frequencies whereas increasing the driving frequency to 2 Hz attenuated drug-induced APD shortening: In contrast. Org 7797 did not shorten APD at any stimulation frequency, and indeed APD lengthening was observed at 2.0 Hz. Increases in the effective refractory period (ERP) were seen at 1.0 and 2.0 Hz in the presence of Org 7797 and at 2.0 Hz in the presence of flecainide, but ERP was either unchanged (2.0 Hz) or shortened by propafenone. The rate of onset of sodium channel block was faster in response to propafenone compared to Org 7797 and tlecainide. It was concluded that all three drugs may inhibit outward repolarising potassium currents, especially at higher stimulation frequencies: However, the concentrations of Org 7797 necessary to influence repolarisation may be closer to those required to induce sodium channel block compared to tlecainide and especially propafenone. These differences may influence drug-induced effects on wavelength resulting in differences in antilibrillatory efficacy.