Canrenone competes for the same site of ouabain on Na+/K+-ATPase

Abstract

Recently several studies suggested the existence in human plasma of an endogenous digitalis-like factors (EDLF) similar to ouabain which could have a role in the pathogenesis of some forms of hypertension. Canrenone is a metabolic product of the antialdosteronic drug spironolactone, used in hypertensive therapy and recently described as a blocker of ouabain effects. The aim of this study was to evaluate the effect of canrenone on Na+/K+-ATPase in relation to ouabain investigating its interaction with the ouabain receptor and with the sodium-potassium pump activity. For this purpose we employed a3H ouabain receptor assay in human placental membranes and a 86Rb uptake assay in human erythrocytes. Increasing concentration of canrenone (0-180 μM) partially inhibited the ouabain sensitive 86Rb uptake in red blood cells until 40%. Moreover canrenone partially competed for 30% with 3H ouabain binding in placental membrane receptors in absence and in presence of 5 mM K+, respectively at a concentration of 350 μM and 70 μM. When the displacement of ouabain (10-7M) by canrenone (180 μM) was valuated, measuring ouabain sensitive 86Rb uptake, a recovery of 86Rb uptake was obtained only in presence of 5 mM K+. Finally Scatchard plot from radioreceptor assay showed that ouabain in absence and presence of canrenone intersected at a common point on the abscissal axis. These results provide evidence that canrenone is a partial competitive agonist of ouabain and interacts with the same receptor site suggesting that it could be used in hypertensive states with high levels of endogenous ouabain.