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.