Cardiac Na+-H+ exchanger during postnatal development in the rat: changes in mRNA expression and sarcolemmal activity

We examined Na+-H+ exchanger isoform 1 (NHE-1) mRNA expression in ventricular myocardium and its correlation with sarcolemmal NHE activity in isolated ventricular myocytes, during postnatal development in the rat. The expression of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA did not change in ventricular myocardium between 2 and 42 days after birth. Therefore, at seven time points within that age range. GAPDH expression was used to normalize NHE-1 mRNA levels, as determined by reverse transcription polymerase chain reaction analysis. There was a progressive five-fold reduction in NHE-1 mRNA expression in ventricular myocardium from 2 days to 42 days of age. As an index of NHE activity, acid efflux rates (J(H)) were determined in single neonatal (2-4-day-old) and adult (42-day-old) ventricular myocytes (n=16/group) loaded with the pH fluoroprobe carboxy-seminaphthorhodafluor-1. In HEPES-buffered medium, basal intracellular pH (pH(i)) was similar at 7.28+/-0.02 in neonatal and 7.31+/-0.02 in adult myocytes, but intrinsic buffering power was lower in the former age group. The rate at which pH(i) recovered from a similar acid load was significantly greater in neonatal than in adult myocytes (0.36+/-0.07 v 0.16+/-0.02 pH units/min at pH(i)=6.8). This was reflected by a significantly greater J(H) (22+/-4 v 9+/-1 pmol/cm2/s at pH(i)=6.8), indicating greater sarcolemmal NHE activity in neonatal myocytes. The concomitant reductions in tissue NHE-1 mRNA expression and sarcolemmal NHE activity suggest that myocardial NHE-1 is subject to regulation at the mRNA level during postnatal development.