Kinetic studies on sodium-dependent calcium uptake by myocardial cells and neuroblastoma cells in culture

Abstract

Kinetic analyses were made on intracellular Na +-dependent Ca 2+ 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 Ca 2+-free medium containing various concentrations of Na +. Cells pre-loaded with various concentrations of Na + were incubated in medium containing Ca 2+ 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 Ca 2+ uptake against the external concentration of Ca 2+ fitted well to straight lines obtained by linear regression ( r > 0.95). This result shows that Ca 2+ uptake by the two types of cell was achieved by a carrier-mediated transport system. This Na +-dependent Ca 2+ uptake was accompanied by Na + release and the ratio of Na + release to Ca 2+ 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 Ca 2+ (0.1 mM); (3) the similarities of the K i values for Co 2+, Sr 2+ and Mg 2+ (Co 2+ < Sr 2+ < Mg 2+) 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 Ca 2+ uptake by non-excitable cells was much lower than that by myocardial cells.