[The effect of potassium on ouabain-binding to human cardiac cell membranes (author's transl)]

The concentration of cardiac glycosides in the serum of an individual patient does not necessarily correlate strictly with the known effects or side effects of these drugs. It is good clinical experience for instance that an acutely decreased serum potassium concentration due to diuretic therapy or dialysis may cause severe digitalis-induced arrhythmias in spite of an unchanged serum glycoside level. On the other hand digitalis intoxication often is treated successfully by the application of potassium. These observations prompted us to investigate the specific binding of tritiated g-strophanthin to isolated human cardiac cell membranes in the presence of different potassium concentrations. Potassium in a concentration-dependent manner displaces ouabain from its binding sites by decreasing the affinity of the receptor for the drug. However, the number of receptor sites remains constant. When analysing the association and dissociation rates of the ouabain-receptor complex it was found that potassium inhibits the binding process only, without affecting the dissociation. At least up to a concentration of 5 mM the potassium induced displacement of ouabain from its binding sites seems to be competitive. These in vitro measurements of the strophanthin--receptor interactions agree with clinical observations and are helpful in their explanation.