Two receptors for cardiac glycosides in the heart

Specific binding of 3H-ouabain to rat cardiac cell membranes revealed a high affinity and a low affinity site. In order to test the pharmacological significance of these different types of binding sites, specific 3H-ouabain binding, force of contraction and 86Rb+-uptake were measured simultaneously in contracting cat papillary muscles and in guinea pig atria. The results in the digitalis sensitive cat shows one type of cardiac glycoside receptors with high affinity (KD approximately 10(-7) M) for ouabain. The occupation of this receptor runs parallel with an increase in force of contraction and an inhibition of 86Rb+-uptake. In the rather digitalis insensitive guinea pig, 3H-ouabain binding also runs parallel with increased force of contraction, 86Rb+-uptake, however, is only inhibited at toxic glycoside concentrations. Thus, in rat and guinea pig heart there exist at least two different digitalis receptors--the high affinity receptor seems to be coupled to inotropic effects, the low affinity receptor is linked to inhibition of (Na+ + K+)-ATPase.