Sodium dependence of the positive inotropic effect of cardiac glycosides.

We have investigated the inotropic effects of digoxin, digitoxin and ouabain in cat ventricular muscle under conditions of reduced Na influx in an effort to determine the role of Na in the positive inotropic effect of these cardiac glycosides. When the normal, Na-dependent action potential is inactivated by potassium depolarization, these glycosides retain a positive inotropic effect. In contrast, when muscles are bathed in Na-poor or Na-free solutions, these glycosides do not influence contraction. This suggests that the cardiac glycosides are dependent on Na for their positive inotropic effects.