Diffusion-controlled receptor occupancy determines the rate of inotropic action of some cardioactive steroids

In guinea-pig myocardium the rates of onset and offset of the inotropic effects of digitoxigenin-monodigitoxoside, digitoxin, ouabain, digoxin, digoxigenin, and dihydroouabain correlated negatively, and independently of the lipophilicity of the steroids, with their intropic and Na-K-ATPase inhibitory potencies. The intropic potency of ouabain was considerably higher than that of dihydroouabain whereas its inotropic effect developed much more slowly (T50 17 min v. 3 min). By contrast, the inhibition of sarcolemmal Na-K-ATPase by equieffective concentrations of these steroids appeared at similar rates within a few minutes. The homogeneity of the effects, as shown by parallel concentration-effect curves with similar order of potencies of the inotropic and the Na-K-ATPase inhibitory effects, excluded the different activation of inhomogeneous receptor populations. The correlation between the geometry of the papillary muscles and the rates of onset of the inotropic ouabain effect or tissue/medium ratios of 3H-ouabain indicated the relevance of diffusion. The discrepancy between the rates of action in sarcolemmal preparations and in papillary muscles could thus be related to the different diffusion distances. The apparently contradictory results in which receptor occupation (dependence of T50 on EC50) appeared as the rate-limiting step are explained by diffusion-controlled receptor occupancy: the reduced rate of diffusion due to an affinity-dependent reduction of the free drug concentration (receptor occupancy as a temporal 'site of loss') accounted for the slow onset of the inotropic effect of the highly potent cardioactive steroids.