Reduced sodium pump alpha1, alpha3, and beta1-isoform protein levels and Na+,K+-ATPase activity but unchanged Na+-Ca2+ exchanger protein levels in human heart failure.

Cardiac glycosides initiate an increase in force of contraction by inhibiting the sarcolemmal sodium pump (Na+, K+-ATPase), thereby decreasing Ca2+ extrusion by the Na+-Ca2+ exchanger, which increases the cellular content of Ca2+. In patients with heart failure the sensitivity toward cardiac glycosides is enhanced. Because the inotropic effect of cardiac glycosides may be a function of the sodium pump and Na+-Ca2+ exchanger (NCE) expression levels, the present study aimed to investigate protein expression of both transporters (immunoblot with specific antibodies against the sodium pump catalytic alpha1-, alpha2-, alpha3-, and glycoprotein beta1-isoforms and against NCE) in left ventricle from failing (heart transplantations, New York Heart Association class IV, n=21) compared with nonfailing (donor hearts, NF, n=22) human myocardium. The density of 3H-ouabain-binding sites (Bmax) and the Na+,K+-ATPase activity were also measured. In NYHA class IV, protein levels of Na+,K+-ATPase alpha1- (0.62+/-0.06 of control), alpha3- (0.70+/-0.09), and beta1- (0.61+/-0.04) but not alpha2-isoforms were significantly reduced (P<0.01), whereas levels of NCE (0.92+/-0.13 of control) and calsequestrin (0.98+/-0.06) remained unchanged. Both Na+,K+-ATPase activity (NF: 1.9+/-0.29; NYHA class IV: 1.1+/-0.17 micromol ATP/min per milligram of protein) and the 3H-ouabain binding sites (Bmax NF: 15.9+/-1.9 pmol/mg protein; NYHA class IV: 9.7+/-1.5) were reduced in NYHA class IV and correlated significantly to each other (r2=0. 73; P<0.0001), as did beta1-subunit expression. In left ventricular papillary muscle strips from NYHA class IV compared with nonfailing tissue the Na+-channel modulator BDF 9198 exerted an increase in force of contraction with unchanged effectiveness but enhanced potency. The enhanced sensitivity of failing human myocardium toward cardiac glycosides may be, at least in part, attributed to a reduced protein expression and activity of the sarcolemmal Na+,K+-ATPase without a change in Na+-Ca2+ exchanger protein expression.