Endogenous digitalis-like activity in the plasma of the toad Bufo marinus

Abstract

CARDIAC GLYCOSIDES derived from plants have a major role in the pharmacotherapy of cardiac disease, and, because they bind to and inhibit (Na+ + K+)ATPase, are important in studies of ion transport mechanisms. In the animal kingdom, structurally related compounds with cardiotonic activity have been shown to exist in the poison glands of bufonid toads1, but this area has received little attention. Recently we developed a sensitive radioreceptor assay for the detection of endogenous ‘digitalis-like’ agonists in animals2. This assay depends on the ability of a substance to compete with 3H-ouabain for binding to (Na+ + K+)ATPase on human red cells. Using this assay, together with assays for inhibition of K transport and (Na+ + K+)ATPase activity, we found high concentrations of ‘digitalis-like’ activity in the skin of several species of Amphibia2. Although this activity is presumably present in skin s aa protective toxin, it is possible that it might, under some circumstances, also function as an endogenous inhibitor of (Na+ + K+)ATPase. We now demonstrate, using both receptor and immunoassays, that high concentrations of digitalis-like activity are present in the plasma of the toad, Bufo marinus. As cellular (Na+ + K+)ATPase would presumably be exposed to this endogenous digitalis-like activity, it may have an important, but as yet undetermined, function in this species.