Low nanomolar concentrations of ouabain may induce higher activity of the Na+/K+-ATPase in human erythrocytes

The inhibitor g-Strophantin, also known as ouabain, is a specific inhibitor of the Na+/K+-ATPase. In this work ouabain has been used for the inhibition of the Na+/K+-ATPase in human erythrocytes and isolated enzyme from pig kidney. Enzymatic activity of the Na+/K+-ATPase has been measured in a broad concentration range of ouabain in two different in vitro investigations. As a potent inhibitor of the sodium pump the cardiotonic steroid ouabain inhibits its enzymatic activity on both isolated human red blood cells and on the purified enzyme from pig kidney. Na+/K+-ATPase activity in erythrocyte can be determined by measuring the ouabain-sensitive uptake of 86Rb (as a congener for potassium). This work provides evidence that very low concentrations of ouabain in the nM range can stimulate increase of Na+/K+-ATPase activity in human erythrocytes, in contrast to inhibition of the purified enzyme from pig kidney, where such an effect was not observed.