Saturable adenosine 5'-triphosphate-independent binding of [3H]-ouabain to brain and cardiac tissue in vitro.

1. Several investigators have proposed that membrane Na+, K+-adenosine 5'-triphosphatase (Na+, K+-ATPase) is a mechanism for the transmembrane transport of cardiac glycosides, rather than the receptor for pharmacological actions of these agents. This implies that the glycosides bind to an intracellular constituent (receptor) other than Na+, K+-ATPase. 2. In search for such a receptor site, saturable ATP-independent [3H]-ouabain binding was studied in rat brain and dog and guinea-pig heart homogenates. The binding of the glucoside to this site results in a relatively unstable complex which is stabilized by K+ to a lesser extent than is the complex formed with the ATP-dependent binding to Na+, K+-ATPase. 3. The ATP-independent ouabain binding sites are more abundant in rat brain tissue than in cardiac tissue, and have a lower ouabain affinity compared to the binding sites on Na+, K+-ATPase. 4. These results do not support the contention that there are intracellular inotropic receptors for digitalis.