Effect of canrenone in the digitalis site of Na+-ATPase in human placental membranes and in erythrocytes

It has been reported that canrenone, which is used in hypertensive therapy as an antialdosteronic drug, may also act as a blocker of ouabain effects. Several studies suggest the existence in human plasma of an endogenous ouabain-like factor (OLF) similar to ouabain which may be increased in hypertension, in pregnancy and in neonatal state. This study evaluated: 1) the effect of canrenone on Na+/K+-ATPase in relation to ouabain in human placental membranes and erythrocytes by 3H-ouabain binding assay, 2) the capacity of canrenone (10m__ to reverse the inhibition of Na+/K+-ATPase by ouabain and by OLF (from umbilical cord plasma) in human erythrocytes employing a 86Rb uptake assay. Increasing concentrations of canrenone (0-____m__ partially competed with 3H-ouabain binding in placental membrane (40%) and erythrocytes (60%). Scatchard plot from radioreceptor assay in placental membrane showed that ouabain and canrenone compete for the same binding site. In erythrocytes canrenone completely reversed the inhibition caused by ouabain (5x10-9 M) and OLF (2x10-9 M ouabain equivalents). A reduction of inhibition of about 50% was observed with ouabain and OLF respectively at a concentration 5x10-8 M and 2x10-8 M (ouabain equivalents). Our results thus provide evidence that canrenone is a partial competitive agonist of ouabain and of OLF in human placental membranes and erythrocytes.