Kinetic Studies on Sodium-dependent Calcium Uptake by Myocardial Cells and Neuroblastoma Cells in Culture

Kinetic analyses were made on intracellular Na+-dependent Ca2+ uptake by myocardial cells and neuroblastoma cells (N-18 strain) in culture. Cells loaded with various concentrations of Na+ could be prepared by incubating them in Ca2+-free medium containing various concentrations of Na+. Cells pre-loaded with various concentrations of Na+ were incubated in medium containing Ca2+ and 45Ca. The resulting 45Ca uptake by the two types of cell depended greatly on the initial intracellular concentrations of Na+. Lineweaver-Burk plots of the initial rate of Ca2+ uptake against the external concentration of Ca2+ fitted well to straight lines obtained by linear regression (r greater than 0.95). This result shows that Ca2+ uptake by the two types of cell was achieved by a carrier-mediated transport system. This Na+-dependent Ca2+ uptake was accompanied by Na+ release and the ratio of Na+ release to Ca2+ uptake was close to 3 : 1. A comparison of the kinetic data between myocardial cells and N-18 cells suggested that N-18 cells possess a carrier showing the same properties as that of myocardial cells, i.e.: (1) a similar dependency on the intracellular concentration of Na+; (2) the coincidence of the apparent Michaelis constants for Ca2+ (0.1 mM); (3) the similarities of the Ki values for Co2+, Sr2+ and Mg2+ (Co2+ less than Sr2+ less than Mg2+) and (4) a similar dependency on pH. However, the maximal initial rate, V, of N-18 cells was about 1/100 that of myocardial cells. The rate of Na+-dependent Ca2+ uptake by non-excitable cells was much lower than that by myocardial cells.