Comparison of the affinity of human, beef and cat heart (Na+ + K+)-ATPase for different digitalis derivatives.

The potencies of eight digitalis derivatives, including two new derivatives of digitoxin, were determined on heart (Na+ + K+)-ATPase or erythrocytes from three digitalis-sensitive species, beef, cat and human. Three methods were used: inhibition of 3H-ouabain binding to give the dissociation constant (KD-value), or inhibition of (Na+ + K+)-ATPase activity or 86Rb+-uptake into human erythrocytes to give the IC50-values. The same order of potency was observed with all methods. The slopes of the concentration-response curves were similar for all compounds. For all compounds, the concentrations which inhibited 3H-ouabain binding by 50% caused about a 50% inhibition of (Na+ + K+)-ATPase activity. All three methods are suitable for determining the potency of new semisynthetic digitalis derivatives. The two new derivatives of digitoxin, 3"'-dehydrodigitoxin oxime and 3"'-dehydrodigitoxin methyloxime, were less potent than digitoxin but were of similar potency to ouabain.