A New Method for the Biological Assay of Cardiotonic Steroids Based on the Potentiation of Potassium-Contracture of the Frog Ventricular Muscle

Abstract

So far there seems to have been no practical method for the assay of cardiotonic ste roids which measures their positive inotropic activity per se directly on the cardiac muscle preparation. A series of studies on the structure-activity relationship of cardenolides conducted in this laboratory have brought about much newer knowledge on this problem, including dispensability of 319-hydroxy group or 14β-hydroxy group for cardiotonic ac tion and inactiveness of 15α-hydroxydigitoxigenin (1-4). In the course of the study, it was felt necessary to describe the potencies of important compounds in terms of numeri cal values. This led us to develop a new assay method which is based on the potentiation of K-contracture of frog ventricular muscle by these compounds, and it has been applied successfully at least to the aglycones. The present paper will report on the new method and the results of the assay of seven aglycones. In addition, the relative potencies of the compounds were compared with those obtained by another method with Straub's isolated frog heart preparation. MATERIALS AND METHODSThe isolated ventricular strip preparation and the isolated heart preparation (Straub's preparation) of the frog (Rana nigroinaculata) were used. Experiments were performed from May through September.