SARCOLEMMAL NA+-CA-2+ EXCHANGE ACTIVITY AND EXCHANGER IMMUNOREACTIVITY IN DEVELOPING RABBIT HEARTS

It has been postulated that as a consequence of an underdeveloped sarcoplasmic reticulum, sarcolemmal Na+-Ca2+ exchange assumes relatively greater importance in modulating Ca2+ fluxes in the developing heart. To explore this concept, cardiac sarcolemmal vesicles were prepared from late fetal (28-day gestation), newborn (24-48 h), immature (14-16 days), and adult New Zealand White rabbits. Na+-dependent Ca2+ uptake was measured by diluting Na+-loaded (140 mM) vesicles into Na+-free buffer and measuring Ca-45(2+) uptake (40 muM Ca2+) by timed quenching and rapid filtration. Vesicles from all four age groups demonstrated Ca2+ uptake curves characteristic of Na+-Ca2+ exchange with stimulation by valinomycin and inhibition by amiloride. Initial uptake velocity (measured at 2 s and corrected for the fraction of competent vesicles) was significantly higher in fetal (23.2 +/- 5.5 nmol/mg) and newborn (26.2 +/- 5.9 nmol/mg) than in adult sarcolemmal preparations (7.3 +/- 1.2 nmol/mg). Uptake was intermediate in the 2-wk-old group (13.7 +/- 1.7 nmol/mg). The relative amounts of exchanger protein were compared by quantitating immunoreactivity using a polyclonal antibody to the Na+-Ca2+ exchanger. Densitometric scanning of protein slot blots demonstrated approximately 2.5 times more exchanger protein in fetal and newborn sarcolemma than in adult preparations. The relative amount of exchanger protein detected immunologically corresponded with the age-related differences observed in exchanger activity. Thus the cardiac sarcolemmal Na+-Ca2+ exchanger is abundant and functionally well-developed in the late fetal/early newborn rabbit heart and appears to decline postnatally. These results support the hypothesis that Na+-Ca2+ exchange plays a predominant role in the regulation of cardiac myocellular Ca2+ during perinatal maturation.