The role of catecholamines in the production of ischaemia-induced ventricular arrhythmias in the rat in vivo and in vitro.

The role of catecholamines in the production of ischaemia-induced ventricular arrhythmias in vivo and in vitro was studied using coronary artery ligation in the rat. Increases in plasma catecholamine concentrations during coronary artery ligation in pentobarbitone-anaesthetized animals were prevented by either acute adrenalectomy or chronic adrenal demedullation, but these procedures did not protect against the occurrence of ventricular arrhythmias. Thus plasma catecholamines were not obligatory mediators of arrhythmogenesis. Three protocols were used in vitro to evaluate the possible influence of intramyocardial release of noradrenaline, produced by the local conditions of ischaemia, on the production of ventricular arrhythmias. During coronary artery ligation in isolated perfused hearts, no enhanced output of 3H could be detected from [3H]-noradrenaline loaded hearts, even in the presence of inhibitors of catecholamine uptake processes, although washout of lactate from ischaemic regions was readily demonstrable. Both optical isomers of propranolol were equally effective in reducing the incidence of arrhythmias, implying a non-specific effect, since the (+)-isomer possesses considerably less beta-adrenoceptor blocking activity. The equipotency of optical isomers of propranolol combined with a lack of effect of atenolol suggested that arrhythmia production was not a consequence of beta-adrenoceptor stimulation. The alpha-adrenoceptor blockers phentolamine and prazosin, both exerted antiarrhythmic actions of similar potency, but phenoxybenzamine and trimazosin had no significant effects. An evaluation of the pharmacological properties of the alpha-adrenoceptor blockers showed that those drugs which had demonstrable local anaesthetic properties also exerted significant antiarrhythmic effects. No relationship was found between potency of alpha-adrenoceptor blockade and antiarrhythmic efficacy. The overall conclusion from these multifaceted approaches was that catecholamines were not necessary mediators of the early phase of ventricular arrhythmias in the rat.