CARDIOTONIC ACTION OF RING A-MODIFIED CARDENOLIDES, WITH SPECIAL REFERENCE TO CLEAVAGE OF THE RING

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Cardiotonic activities of six newly synthesized ring A-modified cardenolides, four 3, 5-seco-4-nor-cardenolides and two 4-oxa-cardenolides, were studied in isolated frog hearts and guinea pig atria. These compounds were 14-hydroxy-3, 5-seco-4-nor-5-oxo-l4β-card-20(22)-enolid-3-oic acid (IIa), and its methyl ester (IIb); 5β, 14 and 5α, 14-dihydroxy-3, 5-seco-4-nor-14β, -card-20(22)-enolid-3-ols (IIIa and IIIb); 3-dehydro-4-oxa-uzarigenin (IVa) and 3-dehydro-4-oxa-digitoxigenin (1Vb). In the frog heart (Straubs preparation), positive inotropic effects were demonstrated with all compounds except IIa. The pattern of response was the same as that to digitoxigenin (I). The relative potencies obtained on the basis of the concentration of each compound in which a contracture was brought about, the potency of I being taken as standard, were as follows: I (1.0), IIa (-), IIb (0.03-0.1), IIIa (0.01), IIIb (<0.01), IVa (<0.01), and IVb (0.03-0.1). The positive isotropic effect of IIb was also confirmed in guinea pig atria. IIb and IVb potentiated dose-dependently the K-contracture of the frog ventricular muscle just as was seen with I, indicating that the cardiotonic activity of the compounds is the same in nature as that of digitoxigenin. It was clearly demonstrated that the perhydrocyclopentanophenanthrene nucleus is not an indispensable requirement for cardiotonic activity.
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