Abstract
VANADATE (vanadium in the +5 oxidation state) occurs widely in animal tissues1, and has been suggested to be the first regulatory agent of (Na+ + K+)ATPase; it inhibits ATPase preparations from kidney and red blood cells at very low concentrations2–4. Hackbarth et al. have shown that vanadate produces positive inotropic effects in isolated cat papillary muscles5, raising the question of whether this positive inotropic effect is accompanied by an inhibition of myocardial (Na+ + K+)ATPase, and, if so, whether both effects occur at similar concentrations of vanadate. We report here that we have observed a striking difference in the inotropic actions of vanadate on isolated heart preparations. Although the force of contraction was reduced in atria, it was increased in ventricular myocardium. This dissociation correlated well with changes in transmembrane potential but not with inhibition of (Na+ + K+)ATPase isolated from both atrial and ventricular myocardium.
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