Dihydroouabain is an antagonist of ouabain inotropic action

The Na+, K+-pump controls a wide variety of cellular systems and its inhibition by cardiac glycosides modifies important physiological functions and evokes several pharmacological effects (refs 1, 2 and refs therein). However, not all the actions of cardiac glycosides can be attributed to Na+, K+-pump inhibition and several observations show that, at low doses, cardiac glycosides stimulate the pump. It has been proposed that their positive inotropic effect could be the sum of two processes: the inhibition of the pump and a still unknown additional inotropic mechanism. In guinea pig heart, low doses of ouabain interact with high-affinity binding sites, which differ from the lower-affinity sites responsible for Na+, K+-pump inhibition. It has been suggested that ouabain interaction with these high-affinity sites could be responsible for the additional inotropic mechanism. The existence of two classes of ouabain-binding sites has been documented not only in guinea pig heart, but also in dog, rat and human heart. Dihydroouabain, a derivative of ouabain in which the lactone ring is saturated, is about 50-fold less potent than ouabain as an inhibitor of Na+, K+-pump and does not stimulate the pump at low doses. Its inotropic effect can be entirely accounted for by the inhibition of the pump. We have examined the pharmacological action of ouabain in the presence of dihydroouabain and report here that dihydroouabain reduces ouabain inotropic action but not Na+, K+-pump inhibition.