Association of the positive inotropic action of ouabain with a second species of digitalis receptors

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Abstract
Studies were conducted to determine whether Na–K ATPase or a second species of digitalis receptors in canine cardiac sarcolemma membrane preparations is associated with the positive inotropic action of nontoxic concentrations of ouabain. [3H]ouabain association and dissociation experiments using highly enriched sarcolemma preparations from canine ventricle indicate the presence of two species of ouabain binding receptors. Ouabain binding to Na–K ATPase of the sarcolemma preparation requires supporting ligands and is characterized by fast association and very slow dissociation in vitro. The second species of digitalis receptor does not require supporting ligands for ouabain binding and is characterized by slow association and fast dissociation. To determine which species of digitalis receptor is associated with the positive inotropic action of digitalis, ouabain washout experiments were conducted using various isolated canine myocardial preparations. Washout of the positive inotropic effects of 1.2–2.4 × 10−7 M ouabain gave half-life values of 1.5–2.0 h for the various myocardial preparations. [3H]ouabain dissociation from the second species of digitalis receptors gave half-life values of 1.7–1.8 h, whereas dissociation from the sarcolemma Na–K ATPase gave half-life values of 8.9–9.3 h for the various sarcolemma preparations utilized. Therefore, based on similarities in half-life values between ouabain inotropy and [3H]ouabain dissociation from the second class of digitalis receptors, it is postulated that the positive inotropic action of digitalis glycosides is associated with the second species of digitalis receptors in the sarcolemma and not with the digitalis inhibitory receptor of Na–K ATPase for nontoxic concentrations of digitalis.