Sodium, and potassium dependent adenosinetriphosphatase from Cavia cobaya kidney: interaction with very low cardiotonic steroid concentrations

We noticed that very low cardiotonic steroid concentrations activate the Na, K-ATPase in a variety of different preparations. In the present research the effect of three cardioactive steroids on the enzymatic activity was tested. The glycosides activated the Na,K-ATPase, while the aglycone strophantidine does not. Ouabain binding studies on various preparations showed the presence of two binding site classes with different affinities. Purification procedures shift the apparent Kd values, while K+ increase them. Accordingly, the activatory and inhibitory effects may be explained by the cardiotonic steroid binding on different sites of the Na,K-ATPase molecule.