Physiological role of the α₁- and α₂-isoforms of the Na⁺-K⁺-ATPase and biological significance of their cardiac glycoside binding site

Abstract

An interesting feature of Na+-K+-ATPase is that it contains four isoforms of the catalytic alpha-subunit, each with a tissue-specific distribution. Our laboratory has used gene targeting to define the functional role of the alpha1- and alpha2-isoforms. While knockout mice demonstrated the importance of the alpha1- and alpha2-isoforms for survival, the knockin mice, in which each isoform can be individually inhibited by ouabain and its function determined, demonstrated that both isoforms are regulators of cardiac muscle contractility. Another intriguing aspect of the Na+-K+-ATPase is that it contains a binding site for cardiac glycosides, such as digoxin. Conservation of this site suggests that it may have an in vivo role and that a natural ligand must exist to interact with this site. In fact, cardiac glycoside-like compounds have been observed in mammals. Our recent study demonstrates that the cardiac glycoside binding site of the Na+-K+-ATPase plays a role in the regulation of blood pressure and that it mediates both ouabain-induced and ACTH-induced hypertension in mice. Whereas chronic administration of ouabain or ACTH caused hypertension in wild-type mice, it had no effect on blood pressure in mice with a ouabain-resistant alpha2-isoform of Na+-K+-ATPase. Interestingly, animals with the ouabain-sensitive alpha1-isoform and a ouabain-resistant alpha2-isoform develop ACTH-induced hypertension to a greater extent than wild-type animals. Taken together, these results demonstrate that the cardiac glycoside binding of the Na+-K+-ATPase has a physiological role and suggests a function for a naturally occurring ligand that is stimulated by administration of ACTH.

The cardiotonic steroid hormone marinobufagenin induces renal fibrosis: implication of epithelial-to-mesenchymal transition

“…*P Ͻ 0.05, **P Ͻ 0.001 vs. control (n ϭ 9 from 3 independent experiments). tracellular signal through the same molecule, the ␣-subunit of Na ϩ -K ϩ -ATPase (16,21). MBG and ouabain belong to two different subclasses of CTSs: bufadienolides, containing a five-membered lactone ring, and cardienolides with six-membered lactone, respectively.…”

“…Moreover, elevated levels of endogenous CTSs including marinobufagenin (MBG) have been associated with various pathological conditions: essential hypertension (4,24,73), preeclampsia (3,45,83), experimental diabetes (7), uremic cardiomyopathy (33), and cardiac failure (20,48,49,66,74). MBG, like other CTSs, binds to the extracellular domain of the ␣-subunit of Na ϩ -K ϩ -ATPase (16,21), which, in addition to its well-known function of maintaining cellular electrochemical balance through pumping sodium and potassium ions, can act as a typical membrane receptor (14,28,35,55,56,73,74,84,85,88).…”

Selective association of endogenous ouabain with subclinical organ damage in treated hypertensive patients

“…5,6 The increased level of EO may have direct and indirect (via marinobufagenin) vasoconstrictor (hypertensive) effects by blocking sodium/potassium pump (Na þ /K þ -ATPase), consecutively increasing intracellular calcium level by the activation of the sodium-calcium exchanger, but this effect of EO is not homogenous, that is different Na þ /K þ -ATPase isoforms respond differently to EO. [7][8][9] Experimental use of low doses of ouabain increased arterial blood pressure in rats. 10 Beyond the classical effects of EO, the digitalis-like factors have genomic effects inducing hypertrophy in myocardial and vascular smooth muscle cells because of the activation of numerous known intracellular signaling pathways.…”

Stabilization of the α2 Isoform of Na,K-ATPase by Mutations in a Phospholipid Binding Pocket

“…Similarly, there is good evidence that ␣2 plays an important role in contractility of vascular smooth muscle and control of blood pressure and regulation of Ca 2ϩ handling (23,24). Experiments with mice engineered to have ouabain-sensitive ␣1 and -insensitive ␣2 isoforms (␣1 S/S ␣2 R/R ) or ouabain-insensitive ␣1 and ␣2 (␣1 R/R ␣2 R/R ), instead of the wild-type with ouabain-insensitive ␣1 and ouabain-sensitive ␣2 (␣1 R/R ␣2 S/S ), have shown that ␣2 plays a predominant role in cardiac glycoside-induced positive inotropy (17,25) and also provide strong evidence for endogenous mammalian cardiac glycosides (25,26). ␣2 also plays the major role in cardiac glycoside-induced positive inotropy in rat cardiomyocytes (18).…”