The electrophysiological effects of the beta-adrenergic antagonist, alprenolol, were compared in normal and depressed canine myocardium. Both (+) and (+/-)-alprenolol (5 x 10(-6) and 10(-5) mol.litre-1) decreased action potential amplitude and Vmax in Purkinje fibres superfused with Tyrode's solution in tissue bath. These concentrations shortened action potential duration and effective refractory period of Purkinje fibres but prolonged those of ventricular muscle. Alprenolol more markedly decreased Vmax and sometimes prevented action potential propagation in Purkinje fibres overlying infarcted regions. Similar depressant actions were noted in Purkinje fibres depolarised by exposure to 9 mmol.litre-1 K+. Depolarised and diseased myocardium is more sensitive to alprenolol and the drug's membrane depressant actions may be significant in terminating arrhythmias in such tissue.
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