The highly conserved cardiac glycoside binding site of Na,K-ATPase plays a role in blood pressure regulation

Abstract

The Na,K-ATPase contains a binding site for cardiac glycosides, such as ouabain, digoxin, and digitoxin, which is highly conserved among species ranging from Drosophila to humans. Although advantage has been taken of this site to treat congestive heart failure with drugs such as digoxin, it is unknown whether this site has a natural function in vivo. Here we show that this site plays an important role in the regulation of blood pressure, and it specifically mediates adrenocorticotropic hormone (ACTH)-induced hypertension in mice. We used genetically engineered mice in which the Na,K-ATPase ␣2 isoform, which is normally sensitive to cardiac glycosides, was made resistant to these compounds. Chronic administration of ACTH caused hypertension in WT mice but not in mice with an ouabain-resistant ␣2 isoform of Na,K-ATPase. This finding demonstrates that the cardiac glycoside binding site of the Na,K-ATPase plays an important role in blood pressure regulation, most likely by responding to a naturally occurring ligand. Because the ␣1 isoform is sensitive to cardiac glycosides in humans, we developed mice in which the naturally occurring ouabain-resistant ␣1 isoform was made ouabain-sensitive. Mice with the ouabainsensitive ''human-like'' ␣1 isoform and an ouabain-resistant ␣2 isoform developed ACTH-induced hypertension to greater extent than WT animals. This result indicates that the cardiac glycoside binding site of the ␣1 isoform can also mediate ACTH-induced hypertension. Taken together these results demonstrate that the cardiac glycoside binding site of the ␣ isoforms of the Na,K-ATPase have a physiological function and supports the hypothesis for a role of the endogenous cardiac glycosides. adrenocorticotropic hormone ͉ hypertension ͉ endogenous cardiac glycosides

How does salt retention raise blood pressure?

“…In transgenic mice with mutated ␣1 and ␣ 2 Na ϩ pumps, in which ␣ 1 is rendered ouabain sensitive and ␣ 2 is ouabain resistant, ACTH induces a profound, Digibindsensitive hypertension within 24 h (20). The implication is that acute inhibition of ␣ 1 , which increases tone and contractility in isolated arteries (120), can elevate BP.…”

“…But the NCX and ␣ 2 Na ϩ pumps are colocalized in PM microdomains at PM-SER junctions (Fig. 2) (56,106) where these two transport systems appear to function cooperatively (4,30 (20), and this increases myogenic tone (50,120) by a direct effect on the contractile machinery.…”

“…Moreover, the plasma from mice with ACTHinduced hypertension had significantly elevated levels of a water-soluble substance that cross-reacted with antidigoxinspecific Fab fragments (Digibind) (20). Digibind, which binds ouabain with high affinity (91), abolished the hypertensinogenic effect of ACTH (20,66). Interestingly, plasma from patients with pregnancy-induced hypertension have elevated levels of a ouabain-like compound and marinobufagenin (70).…”

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

“…However, it is still difficult to draw definitive conclusions regarding the specific function of these molecules in pathophysiology. Although the studies by Dostanic-Larson et al (2) strongly support the suggestion that endogenous cardiac glycoside-like compounds are the naturally occurring ligand, they do not eliminate the possibility that other steroid-like (or, indeed, nonsteroidal) molecules, either previously identified or novel, may play this functional role. Nevertheless, this important work does clearly demonstrate that a naturally occurring regulator must exist and that its interaction with the cardiac glycoside binding site of the Na,K-ATPase plays a role in blood pressure regulation.…”

“…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.…”