Cardiac glycoside binding sites in cultured heart muscle cells

Lipophilicity and pharmacodynamics of cardiotonic steroids in guinea-pig isolated heart muscle preparations

1. The inhibitory potency of cardiotonic steroids on myocardial Na+-K+-ATPase increases with increasing lipophilicity. Employing the inotropic action on guinea-pig isolated left atria, the relationship between lipophilicity, chemical structure and the pharmacodynamic properties of cardiac glycosides was further examined. Nineteen digitoxigenin- and digoxigenin-derivatives, whose lipophilic nature and inhibitory effects on the myocardial Na+-K+-ATPase have been previously investigated, were tested. 2. All steroids exhibited positive inotropic effects which varied with the lipophilicity of these drugs. The dependence of the relationship between these two parameters on structural transformations of the steroids showed qualitatively very close parallelism to that between lipophilicity and their inhibitory effects on myocardial Na+-K+-ATPase. 3. The positive inotropic effects and the inhibitory effects on myocardial Na+-K+-ATPase also correlated very well, exhibiting similar patterns in their respective correlations with the individual lipophilicity parameters. 4. It is inferred that (a) the positive inotropic effects of the cardiac glycosides vary with their lipophilicity, exhibiting trends similar to those shown for their inhibitory effects on myocardial Na+-K+-ATPase, (b) this interdependence is secondary to the influence of structural changes, particularly on the steroid nucleus, and (c) both the lipophilic nature and pharmacodynamic behaviour of the cardiac glycosides almost exclusively depend on the steroid nucleus itself.

Decreased digitalis receptor activity in acute rejecting canine transplanted heart

In the course of acute rejection, myocardial tissue undergoes massive transformation and we hypothetized that for digitalis-like substances, receptor binding characteristics might be altered. Ten canine heterotopic cardiac allografts were carried out and were harvested once rejection had developed (8-10 days post-transplant). Microsomal membrane fractions of those grafts and of native hearts were isolated. Radioligand binding studies were carried out in a medium containing 5 mM Tris PO4, 50 mM Tris HCl, 5 mM MgCl2, pH 7.4 at 37 degrees C, using 3H-ouabain as the ligand. Saturation experiments (n = 10) indicate the presence of one homogeneous population of high affinity binding sites with an affinity constant (Kd) of 8-13 nM and a maximum binding capacity (Bmax) of 47 +/- 3.5 pmol/mg protein. Both saturation and competition binding studies illustrate the fact that acute rejection resulted in a significant decrease in Bmax (43%) without significant alteration in Kd value. These studies indicate that digitalis-like substances might not exert significant inotropic activity during rejection, but this hypothesis must be confirmed by in vivo haemodynamic experiments.