Block of Conduction in Partially Depolarized Cardiac Purkinje Fibres induced by an α-Adrenergic Agent

Abstract

IF discrete areas of the ventricular conducting system are exposed to high concentrations of K⁺, conduction through these areas is slowed so much that the impulse may re-enter the normal part of the conducting system after the end of the refractory period, thereby evoking another excitation of the heart (refs. 1 and 2 and our unpublished work). This slow conduction results from a slow response that resembles the slow component of the cardiac action potential³ and the action potential of normal fibres of the atrioventricular node⁴. Adrenaline enhances the response of Purkinje fibres, depressed by exposure to high K⁺ concentration⁵, and we now report that an α-adrenergic agent, methoxamine, depresses the slow response at concentrations that do not affect the normal action potential. This finding is of interest because very few of the actions of adrenergic agents on the heart can be attributed to the stimulation of α-receptors⁶, because selective depression of the slow response is further evidence that it differs qualitatively from the response of normal fibres (our unpublished work), and because it suggests that α-adrenergic agents may be useful in the prevention of certain cardiac arrhythmias.