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 that human plasma contains an endogenous ouabain-like factor similar to ouabain, which may be increased in hypertension, in pregnancy, and in the 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 (10?M) to reverse the inhibition of Na+/K+-ATPase by ouabain and by ouabain-like factor (from umbilical cord plasma) in human erythrocytes employing a 86Rb uptake assay. Increasing concentrations of canrenone (0-350?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 (5 × 10-9 M) and ouabain-like factor (2 × 10-9 M ouabain equivalents). A reduction of inhibition of about 50% was observed with ouabain and ouabain-like factor respectively at a concentration of 5 × 10-8 M and 2 × 10-8 M (ouabain equivalents). Our results thus provide evidence that canrenone, at therapeutical concentrations, is a partial competitive agonist of ouabain and of ouabain-like factor in human placental membranes and erythrocytes.