The electrophysiological effects of disopyramide phosphate on canine ventricular muscle and Purkinje fibers in normal and low potassium.

We studied the effect of lowering the extracellular potassium concentration ([K+]o) on the electrophysiological actions of disopyramide phosphate, a new antiarrhythmic drug. At low [K+]o, therapeutic concentrations of disopyramide phosphate caused significantly less depression of action potential amplitude and maximum upstroke velocity of both Purkinje fiber and ventricular muscle action potentials. The drug shifted the membrane responsiveness curve along the voltage axis to more negative membrane potentials regardless of [K+]o. However, a greater shift occurred when [K+]o was normal. Disopyramide phosphate prolonged both action potential duration and effective refractory period in all fibers but there was consistently greater prolongation of these parameters at low [K+]o. More importantly, disopyramide phosphate altered repolarization time course of action potentials in such a way that action potentials with dissimilar durations throughout the ventricular conducting system became more equal. The drug was less effective in decreasing this disparity in action potential durations throughout the ventricles in the presence of low [K+]o. These modifications of the electrophysiological actions of disopyramide by low [K+]o suggest that a therapeutic concentration of disopyramide might have less of an antiarrhythmic effect in the presence of hypokalemia.