Cardiac actions of vanadium.

Vanadate produces positive or negative inotropic effects in isolated cardiac muscle preparations, depending on species, type of the muscle, and experimental conditions. In intact animals, vanadate decreases the force of left ventricular contraction, presumably because of a coronary vasoconstriction, and decreases the heart rate. The blood pressure may be elevated or lowered by vanadate. Although vanadate is a potent inhibitor of isolated Na,K-ATPase, the above actions of vanadate are not associated with sodium pump inhibition, because vanadate is incapable of inhibiting the sodium pump in intact myocardial or vascular smooth muscle cells. In addition to inhibiting isolated Na,K-ATPase, vanadate is capable of affecting many, but not all, biochemical reactions involving formation of phosphorylated intermediates. Whether these effects occur in intact cells, however, should be carefully evaluated. In cardiac muscle preparations, vanadate seems to affect a superficial Ca2+ pool; its augmentation and inhibition corresponding to the positive and negative inotropic effects, respectively. The mechanism for the actions of vanadate on such a superficial Ca2+ pool, however, is presently unknown.