Differences in the potassium dependence of the myocardial action of some semisynthetic cardiotonic steroids.

Experiments on guinea pig isolated papillary muscle indicated different dependence of the functional actions of cardiac aglycones and some semisynthetic derivatives on the external K+, concentration. Whereas toxicity of the aglycones and their acetates was increased by low K+, that of the "alkylating" compounds remained unchanged; drug-induced contractility was increased uniformly. These differences are explained on the basis of the ATPase reaction scheme, including a carrier-mediated transport of cardiac glycosides.