It has long been known that the inotropic response of the cardiac muscle to vanadate is dependent on the species and the area of the heart, in contrast to the inotropic action of cardiac glycosides. In order to study the effect of vanadate on the amphibian heart, changes in contractility and the rate of spontaneous beating were determined following superfusion of the isolated frog atrium or ventricle with vanadate solutions (0.0005%-0.05%). The results obtained were as follows:
1. When atrial myocardium was superfused with vanadate solution cardiac contractility decreased and increased in 33 cases and 29 cases, respectively, and in 24 cases a negative inotropic effect was produced after a transient weak positive inotropic effect.
2. Following superfusion of frog atrium with vanadate solution the positive inotropic responses peaked 5-7 min while negative inotropic responses peaked 7-10 min after starting superfusion. Generally inotropic responses to vanadate were dose-dependent.
3. In contrast to a negative chronotropic response to intravenously administered vanadate, the rate of spontaneous beating of the frog atrium generally increased after superfusion with vanadate solution.
4. Vanadate (0.005%-0.05%) invariably produced a dose-dependent positive inotropic effect in ventricular muscle of the frog heart.
These results indicate that vanadate produces both negative and positive inotropic actions on the same area of the frog heart and also produces, in the isolated frog atrium, a positive chronotropic effect, in contrast to negative chronotropic response to intravenously administered vanadate.
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