Effects of vanadate on Na +,K +-ATPase and on the force of contraction in guinea-pig hearts

Abstract

Vanadate has been shown to be a potent inhibitor of isolated Na +,K +-ATPase. Since the inhibition of this enzyme system has been implicated in a mechanism for the positive inotropic action of cardiac glycosides, the cardiac actions of vanadate were examined in connection with its action on Na +,K +-ATPase. Vanadate inhibited isolated Na +,K +-ATPase obtained from various tissues. The differences in the vanadate sensitivity due to enzyme source were relatively small. K +-stimulated phosphatase activity was more sensitive than Na +,K +-stimulated ATP hydrolysis. The compounds was more potent than phosphate in supporting [ 3H] oubain binding in the presence of Mg 2+, indicating a higher affinity of the enzyme for vanadate. It, however, failed to inhibit oubain sensitive 86Rb uptake in electrically stimulated atrial muscle of guinea-pig hearts in concentrations which would inhibit isolated Na +,K +-ATPase. These latter concentrations of vanadate also failed to produce positive inotropic effects in electrically stimulated left atrial preparations of guinea-pig hearts. Higher concentrations produced marked negative inotropic effects associated with a shortening of the action potential duration. These results indicate that vanadate is a potent inhibitor of isolated Na +,K +-ATPase, but cannot inhibit the enzyme in intact myocardial cells or produce positive inotropic effects when applied extracellularly. Inhibitory sites on the enzyme are probably located at the internal surface of the cell membrane which are normally inaccessible to vanadate in intact tissue.