Effect of ouabain on sodium movement in cardiac cells.

A computer simulation indicates that intracellular sodium concentration within a space near the inner surface of sarcolemma fluctuates during a cycle of myocardial function. The sodium transient (a transient increase in sodium ion concentration associated with membrane excitation) is enhanced by the inhibition of (Na+, K+)-ATPase by ouabain, but an accumulation of myocardial sodium does not occur until the inhibition exceeds a critical point. The critical magnitude of sodium pump inhibition that causes a progressive sodium accumulation is dependent on the heart rate.