Negative and positive inotropic action of vanadate on atrial and ventricular myocardium

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 concentrations^2–4. 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.