Influence of derivation on the lipophilicity and inhibitory actions of cardiac glycosides on myocardial Na+-K+-ATPase.

Lipophilicity and inhibitory actions on guinea-pig heart Na+-K+-ATPase of twenty-six digitalis and six strophanthus glycosides comprising the aglycones, mono-, bis-, tris-sugar, alkylated (acylated) tris-sugar, acyl steroid derivatives and three cardanolides were investigated. Their octanol/water partition coefficients (P), reversed phase thin layer (r.t.l.c.) and reversed phase high performance liquid chromatography (r.h.p.l.c.) were determined and the viability of these methods as a measure of the lipophilicity of the cardiotonic steroids evaluated. The influence of lipophilicity and so also structural changes on the inhibitory effects of the cardiac glycosides on myocardial Na+-K+-ATPase was then examined. It is concluded that (a) r.t.l.c. and r.h.p.l.c. are just as effective as the conventional shake-flask method for estimation of the lipophilicity of cardiac glycosides and (b) the inhibitory potencies of cardiotonic steroids on the myocardial Na+-K+-ATPase increase with growing lipophilicity. The relationship between these two parameters is, however, governed by the influence of substitution or derivation of structural components on their inhibitory potencies on the myocardial Na+-K+-ATPase.