Sodium Dependence of the Rate of Onset of the Ouabain-induced Positive Inotropic Effect on Cardiac Muscle

Abstract

WORK on squid axon¹ has shown a requirement for external Na for the ouabain-induced Na efflux inhibition, for when Na was replaced by choline chloride the rate of Na efflux inhibition was greatly reduced. It is possible that the positive inotropic effect of ouabain on cardiac muscle is connected with its action on the Na + K activated ATPase involved in the transport of Na^2,3. It has been shown⁴ that the extent of the positive inotropic effect on guinea-pig heart is reduced by lowering the concentration of Na and, in atrial muscle⁵, this reduces the rate of onset of the ouabain-induced toxic action. I have therefore investigated the effect of decreasing the external concentration of Na on the rate of onset of the ouabain-induced positive inotropic effect. Because the experiments were carried out in conditions which keep the contraction strength far from maximum⁶, the effect was particularly marked.