The sodium pump and hypertension: a physiological role for the cardiac glycoside binding site of the Na,K-ATPase.

T he Na,K-ATPase (or Na pump) is an integral membrane protein that transports Na and K across the plasma membrane of almost all animal cells and couples this work to the hydrolysis of the terminal phosphate bond of ATP (1). A significant fraction (up to 30%) of the ATP generated by cell metabolism is dedicated to this active transport process. The electrical gradient created by the Na pump is essential for the excitable activity of muscle and nerve tissue, and the inwardly directed Na gradient maintained by the Na pump is used in most cells and organs to drive the uptake and accumulation of a wide range of essential nutrients and cellular substrates and to reduce cell calcium and proton concentrations. The Na,K-ATPase also serves as the unique binding site for cardiac glycosides, such as ouabain, digoxin, and digitoxin. Plant extracts containing cardiac glycosides have been used therapeutically since the 18th century. The cardiac glycoside site is the pharmacological target for digoxin, which is currently used widely in the treatment of congestive heart failure. However, the strong evolutionary conservation of this binding site among almost all species has suggested that important physiological functions may involve recognition of this site by endogenous agents. The work of Dostanic-Larson et al. in this issue of PNAS (2) demonstrates that the cardiac glycoside binding site of Na,K-ATPase plays an important physiological role in blood pressure regulation. It is reported that ACTH-induced hypertension is abolished when the interaction of cardiac glycosides with the Na,K-ATPase is disrupted. The interaction was abolished by developing genetically engineered knock-in mice in which two amino acids (located in the first extracellular domain of the Na,K-ATPase -subunit) that are critical for ouabain sensitivity were replaced (3, 4). By using this approach, the normally ouabain-sensitive 2 isoform of the Na,K-ATPase was rendered resistant, whereas the normally insensitive 1 isoform was made ouabainresponsive (5, 6). Although the amino acid substitutions altered the sensitivity to cardiac glycosides of the 1 and 2 isoforms, they did not affect the expression of these two isoforms or the total Na,K-ATPase activity in tissues from the knock-in mice. It was noted that chronic administration of ACTH, which caused hypertension in wild-type mice, did not have such an effect in mice in which the 2 isoform was made resistant to cardiac glycosides. In an observation that completes the circle, knock-in animals in which the normally ouabainresistant 1 isoform was made sensitive to cardiac glycosides and the sensitive 2 isoform was made resistant also developed ACTH-induced hypertension. These results provide a clear demonstration that an endogenous ligand must exist that mediates its action by binding to the Na,K-ATPase and is one of the regulators of blood pressure. There has been a considerable controversy during the last two decades over the existence, identity, and physiological significance of a natural ligand that might interact with the cardiac glycoside binding site on the Na,K-ATPase. The identification of cardiac glycoside-like molecules in mammals has led to the