Increased functional importance of the Na,Ca-exchanger in contracting failing human myocardium but unchanged activity in isolated vesicles.

The present study aimed to investigate the hypothesis that the function of the Na,Ca-exchanger (NCX) is of higher importance for contractility and Ca(2+)-homeostasis in left ventricle from terminally failing than from nonfailing human hearts. The effect of decreasing extracellular [Na](e) (140 to 25 mmol/L) on force of contraction in isolated left ventricular papillary muscle strips was studied as a reflection of NCX function in multicellular preparations (terminally failing, DCM, dilated cardiomyopathy, NYHA IV, n = 13; nonfailing, NF, donor hearts, n = 10). Decreasing [Na](e) has previously been shown to increase contractility in vitro secondary to a decreased Ca(2+)-extrusion by the NCX. In addition, the NCX activity was measured as Na(+)-dependent (45)Ca(2+)-uptake into isolated myocardial vesicles as a function of time and Ca(2+)-concentration (DCM n = 8, NF n = 8). Decreasing [Na](e) enhanced the contractility of papillary muscle strips in both DCM and NF, but the contractility of DCM was increased at smaller reductions of [Na](e) than NF. The NCX activity in isolated myocardial vesicles was unchanged as a function of time (T(1/2): DCM 2.4 +/- 0.3 s versus NF 2.5 +/- 0.3 s) and as a function of Ca(2+) (DCM 0.99 +/- 0.08 versus NF 0.96 +/- 0.07 nmol/mg protein x 3 s, K(1/2): DCM 39.2 microM versus NF 38.3 microM). These results demonstrate a higher sensitivity of the failing human myocardium towards Na,Ca-exchanger mediated positive inotropic effects, suggesting a higher significance of the Na,Ca-exchanger for the extrusion of Ca(2+)-ions in intact failing versus nonfailing human myocardium. Since the activity and the Ca (2+)-affinity of the Na,Ca-exchanger in isolated vesicles was unchanged, we propose that alterations in Ca(2+)-and Na(+)-homeostasis (due to impaired function of the sarcoplasmic reticulum and the Na(+), K(+)-ATPase) or the prolonged action potential are the reason for this observation.