Effect of vanadate on myocardial force of contraction

Ammonium vanadate (NH4VO3; 50–1000 μM) increased the force of contraction of isolated electrically driven cat papillary muscles in a concentration-dependent manner. The positive inotropic effect (PIE) of NH4VO3 became significant at 50 μM and was maximal at 500 to 1000 μM. It was accompanied by an increase in the rate of force development, in the rate of relaxation and in relaxation time of the isometric contraction. Similar results as with NH4VO3 were obtained with NaVO3 and with Na3VO4. The effects of NH4VO3 were also observed in the presence of 1 μM propranolol, 5 μM phentolamine or after reserpine-pretreatment (5 mg/kg i.p.). These results indicate that vanadate produces a direct PIE inventricular cardiac muscle which is unlikely to be mediated by alpha-or beta-adrenoceptor stimulation. In cat left atrial strips, however, vanadate ions produced a negative inotropic effect through a hitherto unknown mechanism. Vanadate effects similar to those observed in the cat heart were obtained in ventricular and atrial preparatins from bovine hearts.