The Na,K-ATPase is a ubiquitous transmembrane pump and a specific receptor for cardiac glycosides such as ouabain and digoxin, which are used in the management of congestive heart failure (CHF). A potential role for these so-called endogenous cardiotonic steroids (CS) has been explored, and it has become apparent that such compounds are elevated and may play an important role in a variety of physiological and pathophysiological conditions such as hypertension and CHF. Recent evidence suggests that the Na,K-ATPase may act as a signal transducer upon CS binding and induce nonproliferative cardiac growth, implicating a role for endogenous CS in the development of cardiac hypertrophy and progressive failure of the heart. In the present study, we tested whether hypertrophic responses to pressure overload would be altered in mutant mice that specifically express ouabain-sensitive or ouabain-resistant α1- and α2-Na,K-ATPase subunits, as follows: α1-resistant, α2-resistant (α1(R/R)α2(R/R)); α1-sensitive, α2-resistant (α1(S/S)α2(R/R)); and α1-resistant, α2-sensitive (α1(R/R)α2(S/S), wild-type). In α1(S/S)α2(R/R) mice, pressure overload by transverse aortic coarctation induced severe left ventricular (LV) hypertrophy with extensive perivascular and replacement fibrosis at only 4 wk. Responses in α1(R/R)α2(S/S) and α1(R/R)α2(R/R) mice were comparatively mild. Mutant α1(S/S)α2(R/R) mice also had LV dilatation and depressed LV systolic contractile function by 4 wk of pressure overload. In separate experiments, chronic Digibind treatment prevented the rapid progression of cardiac hypertrophy and fibrosis in α1(S/S)α2(R/R) mice. These data demonstrate that mice with a ouabain-sensitive α1-Na,K-ATPase subunit have a dramatic susceptibility to the development of cardiac hypertrophy, and failure from LV pressure overload and provide evidence for the involvement of endogenous CS in this process.

Endogenous Na(+) pump inhibitors are thought to play important (patho)physiological roles and occur in two different chemical forms in the mammalian circulation: cardenolides, such as ouabain, and bufadienolides, such as marinobufagenin (MBG). Although all alpha Na(+)-K(+)-ATPase isoforms (alpha(1-4)) are sensitive to ouabain in most species, in rats and mice the ubiquitously expressed alpha(1) Na(+)-K(+)-ATPase is resistant to ouabain. We have previously shown that selective modification of the putative ouabain binding site of either the alpha(1) or alpha(2) Na(+)-K(+)-ATPase subunit in mice substantially alters the cardiotonic influence of exogenously applied cardenolides. To determine whether the ouabain binding site also interacts with MBG and if this interaction plays a functional role, we evaluated cardiovascular function in alpha(1)-resistant/alpha(2)-resistant (alpha(1)(R/R)alpha(2)(R/R)), alpha(1)-sensitive/alpha(2)-resistant (alpha(1)(S/S)alpha(2)(R/R)), and alpha(1)-resistant/alpha(2)-sensitive mice (alpha(1)(R/R)alpha(2)(S/S), wild type). Cardiovascular indexes were evaluated in vivo by cardiac catheterization at baseline and during graded infusions of MBG. There were no differences in baseline measurements of targeted mice, indicating normal hemodynamics and cardiac function. MBG at 0.025, 0.05, and 0.1 nmol*min(-1)*g body wt(-1) significantly increased cardiac performance to a greater extent in alpha(1)(S/S)alpha(2)(R/R) compared with alpha(1)(R/R)alpha(2)(R/R) and wild-type mice. The increase in LVdP/dt(max) in alpha(1)(S/S)alpha(2)(R/R) mice was greater at higher concentrations of MBG compared with both alpha(1)(R/R)alpha(2)(R/R) and alpha(1)(R/R)alpha(2)(S/S) mice (P < 0.05). These results suggest that MBG interacts with the ouabain binding site of the alpha(1) Na(+)-K(+)-ATPase subunit and can thereby influence cardiac inotropy.

Endogenous Na+ pump inhibitors are thought to play important (patho)physiological roles, and occur in two different chemical forms in the mammalian circulation: cardenolides, such as ouabain, and bufadienolides, such as marinobufagenin (MBG). While all αNKA isoforms are sensitive to ouabain in most species, in rats and mice the ubiquitously expressed α1NKA is resistant to ouabain. We have previously shown that selective modification of the putative ouabain‐binding site of either the α1 or α2 NKA subunit in mice substantially alters the cardiotonic influence of exogenously applied cardenolides. To determine whether the ouabain‐binding site also interacts with MBG and if this interaction plays a functional role, we evaluated cardiovascular function in α1‐resistant/α2‐resistant (α1R/Rα2R/R), α1‐sensitive/α2‐resistant (α1S/Sα2R/R), and α1‐resistant/α2‐sensitive mice (α1R/Rα2S/S, wild type). Cardiovascular indices were evaluated in vivo by cardiac catheterization at baseline and during graded infusions of MBG. There were no differences in baseline measurements of targeted mice, indicating normal hemodynamics and cardiac function. As shown in the graph, MBG increased cardiac performance to a greater extent in α1S/Sα2R/R compared to α1R/Rα2R/R and wild type mice. These data indicate that MBG interacts with the ouabain‐binding site of the α1NKA subunit and can thereby influence cardiac inotropy.

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