Sarcolemmal Na+/H+ exchanger activity and expression in human ventricular myocardium

OBJECTIVES To determine sarcolemmal Na+/H+ exchanger (NHE) activity and expression in human ventricular myocardium. BACKGROUND Although the sarcolemmal NHE has been implicated in various physiological and pathophysiological phenomena in animal studies, its activity and expression in human myocardium have not been studied. METHODS Ventricular myocardium was obtained from unused donor hearts with acute myocardial dysfunction (n = 5) and recipient hearts with chronic end stage heart failure (n = 11) through a transplantation program. Intracellular pH (pHi) was monitored in enzymatically isolated single ventricular myocytes by microepifluorescence. As the index of sarcolemmal NHE activity, the rate of H+ efflux at a pHi of 6.90 (JH6.9) was determined after the induction of intracellular acidosis in bicarbonate-free medium. Na+/H+ exchanger isoform 1 (NHE1) expression in ventricular myocardium was determined by immunoblot analysis. RESULTS Human ventricular myocytes exhibited readily detectable sarcolemmal NHE activity after the induction of intracellular acidosis, and this activity was suppressed by the NHE1-selective inhibitor HOE-642 (cariporide) at 1 μmol/L. Sarcolemmal NHE activity of myocytes was significantly greater in recipient hearts (JH6.9 = 1.95 ± 0.18 mmol/L/min) than it was in unused donor hearts (JH6.9 = 1.06 ± 0.15 mmol/L/min). In contrast, NHE1 protein was expressed in similar abundance in ventricular myocardium from both recipient and unused donor hearts. CONCLUSIONS Sarcolemmal NHE activity of human ventricular myocytes arises from the NHE1 isoform and is inhibited by HOE-642. Sarcolemmal NHE activity is significantly greater in recipient hearts with chronic end-stage heart failure than it is in unused donor hearts, and this difference is likely to arise from altered posttranslational regulation.