On the Mechanism of Cardiac Glycoside Action: Stimulation of Myosin B Superprecipitation by Ouabain and Digoxin

A study has been made of the effect of ouabain and other cardiac glycosides on a faithful model of contraction, viz. the “superprecipitation” which occurs when ATP is added to a suspension of purified myosin B. Structural changes during superprecipitation were followed by a turbidity method, and rates of ATP utilization with a pH stat. At “optimal” concentrations (ca. 10 M and 10 M) these substances approximately double the structural and the ATP utilization rates of the myosin B system, but at no concentration do they affect the ATPase of myosin A. In the one case studied, cardioactivity of the substance appeared to parallel its effect in our test system.