REGULATION OF NA+ CA2+ EXCHANGE ACTIVITY BY CYTOSOLIC CA2+ IN TRANSFECTED CHINESE-HAMSTER OVARY CELLS/

Transfected Chinese hamster ovary cells stably expressing the bovine c ardiac Na+/Ca2+ exchanger (CK1.4 cells) were used to determine the ran ge of cytosolic Ca2+ concentrations ([Ca2+](i)) that activate Na+/Ca2 exchange activity. Ba2+ influx was measured in fura 2-loaded, ionomyc in-treated cells under conditions in which the intracellular Naf conce ntration was clamped with gramicidin at similar to 20 mM. [Ca2+](i) wa s varied by preincubating ionomycin-treated cells with either the acet oxymethyl ester of EGTA or medium containing 0-1 mM added CaCl2. The r ate of Ba2+ influx increased in a saturable manner with [Ca2+](i), wit h the half-maximal activation value of 44 nM and a Hill coefficient of 1.6. When identical experiments were carried out with cells expressin g a Ca2+-insensitive mutant of the exchanger, Ba2+ influx did not vary with [Ca2+](i). The concentration for activation of exchange activity was similar to that reported for whole cardiac myocytes but approxima tely an order of magnitude lower than that reported for excised, giant patches. The reason for the difference in Ca2+ regulation between who le cells and membrane patches is unknown.