Cassaine: mechanism of inhibition of Na+ +K+ -ATPase and relationship of this inhibition to cardiotonic actions.

The erythrophleum alkaloid cassaine shares many of the pharmacological actions of the cardiac glycosides but lacks the structural characteristics typical of cardiac glycosides. To further investigate the relationship between Na+ +K+ -ATPase inhibition and the cardiotonic actions of these drugs we investigated the interaction of cassaine with the Na+ +K+ -ATPase. Cassaine inhibited rat brain Na+ +K+ -ATPase with about one quarter of the apparent affinity of ouabain for this enzyme. This inhibition was non-competitive with respect to K+. Cassaine also inhibited this enzyme in the presence of Mg2+ and this inhibition was enhanced by Pi and antagonized by Na+. In the presence of Na+, Mg2+ and (gamma-32P)-ATP cassaine acted to stabilize the phosphorylated intermediate of Na+ +K+ -ATPase. Cassaine also acted to displace specifically bound (3H)-ouabain from this enzyme. These observations suggested that cassaine inhibited the Na+ +K+ -ATPase by interacting at the cardiotonic steroid binding sites of Na+ +K+ -ATPase. Consistent with this hypothesis, dog, guinea pig and rat heart Na+ +K+ -ATPase showed differing sensitivities to cassaine paralleling their differing sensitivities to ouabain. The principal difference between the interaction of cassaine and ouabain with Na+ +K+ -ATPase appeared to be the more rapid dissociation of cassaine from the cardiotonic steroid binding site(s) of Na+ +K+ -ATPase. In keeping with this the rates of offset of cassaine-induced inotropy in Langendorff perfused dog and guinea pig hearts were several times faster than those of ouabain-induced inotropy.