Developmental changes of sarcolemmal Na+-H+ exchange.

ABSTRACT

We previously demonstrated that the effect of respiratory acidosis on cardiac contractility in the newborn was less than in the adult rabbit, and these data suggested a higher [Na+]i and [Na+]i-[Ca2+]o exchange in the newborn as compared to the adult. In this study, we investigated developmental changes of Na+-H+ exchange in isolated sarcolemmal vesicles. Sarcolemmal purification for Na+-K ATPase was 61.9 and 67.1 fold in the newborn and the adult rabbit heart, respectively. In the presence of an outwardly directed proton gradient across the vesicular membrane, sarcolemmal 22Na uptake rate in the newborn (0.22 +/- 0.01 nmol Na+/mg prot/s) was significantly higher than than in the adult (0.16 +/- 0.01 nmol Na+/mg prot/s). 1.0 mM amiloride inhibited 22Na uptake by 75% and 80% in the newborn and the adult, respectively. In the absence of a pH gradient, vesicular 22Na uptake in the newborn and the adult were not significantly different. In conclusion, the higher Na+-H+ exchange in the newborn may lead to a higher [Na+]i and subsequent calcium influx via Na+-Ca2+ exchange as compared with the adult during acidosis. This may explain the greater recovery of mechanical function in the newborn heart as compared to the adult heart during acidosis.