Stimulatory effect of vanadate on (Na+ + K+)-ATPase activity and on 3H-ouabain-binding in a cat heart cell membrane preparation

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VANADATE has been reported to inhibit (Na+ + K+)-ATPase activity in membrane preparations from dog kidney1 and in red cells2. This inhibition, which occurs in the nanomolar concentration range in the presence of high Mg2+ concentrations and in the micromolar range at physiological Mg2+ concentrations2,3 is specific as other ATPase enzymes are not affected in these conditions1. Therefore vanadate has been suggested to be ‘an ideal specific regulator’ of (Na+ + K+)-ATPase1. We have shown4 that vanadate has a distinct positive inotropic effect in cat papillary muscle. It has been claimed that the positive inotropic action of cardiac glycosides, which are specific inhibitors of (Na+ + K+)-ATPase activity5,6, is associated with the inhibition of that enzyme. If that is so, vanadate would be expected to inhibit (Na+ + K+)-ATPase, in a concentration-dependent manner, in parallel with its effect on force of contraction. To test this hypothesis, we measured the effect of vanadate on (Na+ + K+)-ATPase activities and on 3H-ouabain-binding in identical conditions in a membrane preparation taken from cat hearts that were also used for experiments on contraction. We report here, however, that vanadate may stimulate (Na+ + K+)-ATPase activity transiently and 3H-ouabain-binding permanently in these conditions.