Canrenone as a partial agonist at the digitalis receptor site of (Na+-K+)ATPase

Canrenone, a spironolactone metabolite, was tested for its possible effects on (Na+-K+) adenosine triphosphatase (ATPase) activity [Mg++-dependent, (Na+-K+)-activated ATP phosphohydrolase (E.C.3.6.1.3) and ouabain interaction with the enzyme. Canrenone competitively antagonized the binding of [3H]ouabain to (Na+-K+)ATPase and inhibited (Na+-K+)ATPase activity. The multiple inhibition technique was used to demonstrate that canrenone is a partial inhibitor of (Na+-K+)ATPase, mutually exclusive with respect to ouabain. Comparative studies of the effects of ouabain and canrenone on potassium-dependent p-nitrophenylphosphatase activity (E.C.9.6.1.7) and potassium activation of (Na+-K+)ATPase confirmed that ouabain and canrenone interacted with the same receptor site. The finding that canrenone is a partial agonist may explain the results of previous in vivo studies showing that spironolactone and the allied drug to potassium conrenoate have either a positive inotropic action or an antagonistic effect against digitalis toxicity.