R.J. Marshall

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.

THE BENEFICIAL ACTIONS OF BEPRIDIL IN ACUTE MYOCARDIAL INFARCTION IN ANAESTHETIZED DOGS

1 When administered intravenously shortly before acute coronary ligation in dogs anaesthetized with chloralose, bepridil (5 mg/kg) produced immediate and transient falls in coronary and systemic vascular resistance which were accompanied by marked decreases in myocardial oxygen extraction. These effects were followed by sustained decreases in heart rate and myocardial oxygen consumption. 2 This dose of bepridil reduced the number of premature ventricular beats and abolished fibrillation induced by coronary artery ligation without modifying the haemodynamic or metabolic consequences (lactate production) of myocardial ischaemia. 3 When administered 1.5‐2 h after ligation, bepridil did not compromise the critical perfusion of the acutely ischaemic zone but reduced the lactate production and ST‐segment elevation in the ischaemic zone. 4 These results suggest that bepridil may be a useful drug in the chronic treatment of angina pectoris and in this respect may possess advantages over β‐adrenoceptor antagonists.

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.

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.

The Effects of Sodium Channel Inhibitors on Early Arrhythmias Associated with Acute Myocardial Ischaemia

The initial discovery that a compound possesses antiarrhythmic activity is usually made in simple screening models involving the use of arrhythmogenic chemicals like aconitine, chloroform or ouabain. If promising results are obtained in these tests then the potential antiarrhythmic drug may be tested (usually in conscious dogs) for its ability to suppress ventricular ectopic activity ensuing 1–2 days after the production of myocardial ischaemia. Historically this sequence of events holds especially for the group of drugs variously described as ‘membrane stabilisers’, ‘sodium channel blockers’ or ‘class 1 agents’ (Vaughan Williams, 1970). Consequently, there is comparatively little literature describing the effects of sodium channel blocking agents on the early experimental ventricular arrhythmias and fibrillation which occur within minutes of the onset of myocardial ischaemia. The reasons for this are many but surely include the difficulties involved in producing a consistent quantitative model in which to assess the protective actions of putative antiarrhythmic drugs. For instance, our own experiences have shown that one-stage coronary artery ligation does not produce ventricular ectopic activity in every species — rabbits and guineapigs being particularly resistant. In addition even in those species which demonstrate early ventricular arrhythmias (for example, the dog), ligation of the same coronary artery can be associated with a range of incidence of ventricular fibrillation (VF) of between zero and 100 per cent depending on the laboratory (see Stephenson et al., 1960, for review of early literature, and chapter 6 of this book).

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 antidysrhythmic and haemodynamic effects of orally or intravenously administered mexiletine and ORG 6001 in the anaesthetized rat.

1 The antidysrhythmic and haemodynamic effects of the aminosteroid, Org 6001, were studied in the rat anaesthetized with pentobarbitone. Mexiletine was used for comparison 2 Both Org 6001 (2–10 mg/kg) and mexiletine (1 mg/kg) given intravenously antagonized the development of dysrhythmias evoked by acute coronary artery ligation in rats 3 In antidysrhythmic doses, Org 6001 and mexiletine exerted only moderate and transient hypotension and depression of cardiac contractility (assessed from LV dP/dtmax). Org 6001 did, however, induce a more sustained bradycardia 4 Effective oral doses of Org 6001 (20–100 mg/kg) were similar to those of mexiletine, disopyramide and propafenone 5 Oral Org 6001 (100 mg/kg) was effective for 18 h whereas mexiletine (100 mg/kg) failed to protect against evoked dysrhythmias 3 h after dosing 6 Org 6001 and mexiletine differed in their actions on ventricular fibrillation threshold (VFT). Org 6001 (100 mg/kg orally 12 h before ligation) prevented the decrease in VFT produced by coronary ligation whereas mexiletine (100 mg/kg orally) had no effect. When administered intravenously, mexiletine (but not Org 6001) increased VFT in normal ventricular muscle.

Effects of selective channel blocking agents on contractions and action potentials in K+‐depolarized guinea‐pig atria

1 Contractions and transmembrane action potentials were induced by 1 μM isoprenaline in K+‐depolarized guinea‐pig left atria driven at 0.5 Hz. 2 The stability of these responses was significantly increased by doubling the extracellular glucose concentration to 22 mM. 3 Action potential overshoot increased by 28 mV per ten fold increase in extracellular calcium concentration suggesting that the inward current in this preparation is carried by Ca2+. 4 In depolarized driven preparations, nanomolar concentrations of nifedipine and nisoldipine reduced contractility, maximum rate of depolarization (dV/dt max) and action potential height, whereas the fast channel blocking agents tetrodotoxin and mexiletine (in micromolar concentrations) produced little change. Nifedipine also rendered spontaneously beating depolarized right atrial preparations quiescent. 5 In concentrations which reduced dV/dt of normal action potentials, the sodium channel blocking agents quinidine and Org 6001 reduced the amplitude of contractions and reduced the maximum rate of phase 0 depolarization (dV/dt) of action potentials in depolarized tissue. These actions were reversed by Ca2+ and suggest calcium antagonistic activity. However action potential height was not reduced. Like bepridil, both drugs also reduced the frequency of spontaneous contractions in depolarized right atrial preparations. 6 Unlike Org 6001, quinidine failed to produce a shift in calcium log dose‐response curves in driven depolarized preparations and induced positive inotropy in the presence of functional sodium channels. 7 Bepridil inhibited contractions in depolarized atria in the absence of a reduction in dV/dt suggesting that any calcium antagonistic action in atrial tissue is mainly located at an intracellular site. 8 In conclusion, action potentials elicited by isoprenaline in potassium‐depolarized atria bathed in high glucose appear to be Ca2+ mediated. In concentrations which inhibit the inward Na+ current, both quinidine and Org 6001 exhibit calcium channel blocking properties.