ELECTROPHYSIOLOGICAL EFFECTS OF STROPHANTHIN IN THE HEART

¹ Department of Medicine, University of Oregon Medical School, Portland, Oregon

The electrophysiological effects of strophanthin were studied in excised ventricular muscle and Purkinje fibers of the sheep heart. In ventricular muscle stimulated at a slow rate, repolarization was prolonged initially, and later accelerated. With more rapid stimulation, repolarization was only accelerated. Contractile tension increased steadily in both circumstances—initially, in the absence of changes in electrical activity.

Strophanthin had a biphasic effect on the current-voltage relationship in Purkinje fibers; initially, membrane resistance was increased (reduced potassium permeability); later, the resistance was decreased (increased potassium permeability). Such Permeability changes serve to explain the initially delayed, and later accelerated repolarization. When the direction of change in repolarization was only one of acceleration, it is likely that the influences of the increased rate of stimulation overrode the effect of the early reduction in permeability produced by strophanthin.

A late effect of strophanthin is the reduction of sodium conductance, the action being similar to that of the local anesthetics on the sodium carrier system. The decreased rate of rise of the action potential may underlie the reduced conduction velocity produced by digitalis.

It is unlikely that a direct, causal relationship exists between the permeability effects and the inotropic action of strophanthin in view of the development of the latter in the absence of electrical changes, and apparently irrespective of the direction and degree of the permeability alterations.