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 J(H6.9)) 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 micromol/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 (J(H6.9 = 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.