Sarcolemmal calcium kinetics in the neonatal heart.

We characterized sarcolemmal enzymes, Na+-Ca2+ exchange, Ca2+ efflux, and Ca2+ binding in the neonatal rabbit heart. Sarcolemmal vesicles were isolated by differential and sucrose gradient centrifugation. The sarcolemmal purification factor for K+-p-nitro-phenylphosphatase (pNPPase) was 38.8 and 34.5 in the newborn and the adult, respectively. Contamination by mitochondria and sarcoplasmic reticulum was minimal. Specific activities of Na+-K+ ATPase and K+-pNPPase in the newborn were significantly lower than those in the adult. In the newborn, maximal Na+-Ca2+ exchange was 24.6 +/- 1.1 nmol Ca2+/mg/1.5 s, passive Ca2+ efflux rate was 14.6 +/- 2.2 nmol Ca2+/mg/min and Ca2+ binding was 69.4 +/- 3.3 nmol Ca2+/mg/4 min. These values were not significantly different from the adult values. At pH 6.0, Na+-Ca2+ exchange and Ca2+ binding were approximately 60% of control values (pH = 7.4) in two age groups. These values were stimulated at pH 8.0 and 9.0. In conclusion, Na+-Ca2+ exchange, Ca2+ efflux, passive Ca2+ binding and the effect of pH on Na+-Ca2+ exchange and Ca2+ binding in the newborn were similar to those in the adult.