CONCENTRATION-DEPENDENCE OF THE BINDING OF OUABAIN TO ISOLATED GUINEA-PIG ATRIA

The influence of eight concentrations of ouabain (6 × 10-9 to 3 × 10-6 M) on force of contraction and on cellular Na, K contents of isolated guinea pig left atria was determined and compared to he binding of the drug. Rate and amount of ouabain binding were found to be concentration dependent. At a frequency of 1 Hz ouabain was bound but remained ineffective with regard to contractile force up to a concentration of 3 × 10-8 M, corresponding to an occupation of about 20% of available binding sites. From 1 to 4 × 10-7 M the occupied binding sites ranged between 20 and 80%, respectively. Over this range the drug exerted positive inotropic effects, yet cellular Na and K concentrations remained unaltered for as long as the inotropic effect was maintained. Greater than 80% occupation of binding sites at concentrations between 4 × 10-7 and 3 × 10-6 M was followed by an intoxication of the muscle, the ionic gradients deteriorated, and the binding process was counteracted by a spontaneous release of ouabain. Under the prevailing conditions a total amount of 9 × 10-7 moles of binding sites were determined per kilogram of tissue. The observation that the number of binding sites varies with different frequencies results from different overall pump rates. The concentration of Na, K-ATPase, however, available for mediating the positive inotropic effect decreased with increasing frequency of contraction. The results suggest that constant Na and K gradients are prerequisites for a maintained positive inotropic effect by cardiac glycosides and that the positive inotropic effect is not the consequence of an inhibition of the Na, K transport. © 1979 Raven Press, New York.