[Characterization of an endogenous factor with digitalis-like activity in the heart of the guinea pig (I): Inhibition of Na,K-ATPase)].

The purpose of this work is to study the presence of an endogenous inhibitor of the sodium pump in mammals. A water extract was prepared from guinea pig heart and partially purified by liquid chromatography. The extract inhibited the activity of Na,K-ATPase but had no effect on Mg-ATPase nor on Ca-ATPase. The inhibition was similar to that produced by cardiac glycosides since it was dose-dependent, antagonised by potassium and the Na,K-ATPase prepared from brain was more sensitive than the one from heart. These results support the hypothesis on an endogenous factor exhibiting digitalis-like activity that could be a physiological regulator of the sodium pump.