Abstract
The influence of various Na+ concentrations on [3H]‐ouabain binding was studied in experiments on a microsomal Na+‐K+‐adenosine triphosphatase (ATPase) from guinea‐pig hearts. The ATP‐independent cardiac glycoside binding was not influenced by increasing Na+ concentrations. However, a good correlation was found between the ATP‐dependent [3H]‐ouabain binding and Na+ concentration. A more detailed analysis of these results according to Hofstee (1952) revealed two distinct processes involved in this interaction: one ouabain binding process was activated at rather low Na+ concentrations, (K0.5 = 4.5 mM); this type of [3H]‐ouabain binding was strongly correlated to the Na+ concentration necessary for half maximum phosphorylation (K0.5 = 1 mM). The other ouabain binding process was predominant at high Na+ concentrations (K0.5 = 69 mM). On the basis of the commonly accepted ATPase reaction cycle a model for the interaction of cardiac glycosides with the Na+‐K+‐ATPase is proposed, assuming two different binding sites for cardiac glycosides (E2‐P and E1‐P) and involving a translocation of these drugs from an outer to an inner compartment of the cell membrane.