Sodium-potassium pump and sodium-calcium exchange in adult and neonatal canine cardiac sarcolemma.

The purpose of this study was to determine whether the Na(+)-K+ pump and the Na(+)-Ca2+ exchanger, two systems thought to be important in excitation-contraction coupling in the heart, change during postnatal development. In adult and neonatal canine cardiac sarcolemmal preparations, Na(+)-K(+)-adenosine-triphosphatase (ATPase) activity and specific [3H]ouabain binding were found to be higher in the adult compared with the neonate, although Na(+)-K(+)-ATPase turnover numbers were not significantly different. Furthermore, ouabain association and dissociation rate constants, examined under a variety of conditions, were the same for both the neonate and adult preparations. These results suggest that the number of pump units per milligram sarcolemmal protein changes during postnatal development. In contrast, the initial rate of Na(+)-dependent-45Ca2+ influx, an index of Na(+)-Ca2+ exchange activity, was not significantly different in the two age groups. Additionally, the sensitivity of the exchanger to extravesicular Ca2+ and K+ was the same. In summary, during postnatal development sarcolemmal Na(+)-K(+)-ATPase and [3H]ouabain binding increase from neonate to adult, whereas Na(+)-Ca2+ exchange activity remains the same. This may result in a greater importance of Ca2+ flux through the Na(+)-Ca2+ exchange system in the neonate compared with the adult.