Sodium-calcium exchange and store-dependent calcium influx in transfected Chinese hamster ovary cells expressing the bovine cardiac sodium-calcium exchanger - Acceleration of exchange activity in thapsigargin-treated cells

The effects of extracellular Na+ on store-dependent Ca2+ influx were compared for transfected Chinese hamster ovary cells expressing the bovine cardiac Na+-Ca2+ exchanger (CK1.4 cells) and vector-transfected control cells, Store-dependent Ca2+ influx was elicited by depletion of intracellular Ca2+ stores with ionomycin, thapsigargin, or extracellular ATP, a purinergic agonist, In each case, the rise in [Ca2+](i) upon the addition of extracellular Ca2+ was reduced in CK1.4 cells compared with control cells at physiological [Na+](o). When Li+ or NMDG was substituted for Na+, the CK1.4 cells showed a greater rise in [Ca2+](i) than control cells over the subsequent 3 min after the addition of Ca-o(2+). Under Na+-free conditions, SK&F 96365 (50 mu M), a blocker of store-operated Ca2+ channels, nearly abolished the thapsigargin-induced rise in [Ca2+](i) in the control cells but only partially inhibited this response in the CK1.4 cells, We conclude that in the CK1.4 cells, Ca2+ entry through store-operated channels was counteracted by Na-o(+)-dependent Ca2+ efflux at physiological [Na+](o), whereas Ca2+ entry was enhanced through Na-i(+)-dependent Ca2+ influx in the Na+-free medium. We examined the effects of thapsigargin on Ba2+ entry in the CK1.4 cells because Ba2+ is transported by the Na+-Ca2+ exchanger, but it enters these cells only poorly through store operated channels, and it is not sequestered by intracellular organelles, Thapsigargin treatment stimulated Ba2+ influx in a Na+-free medium, consistent with an acceleration of Ba2+ entry through the Na+-Ca2+ exchanger, We conclude that organellar Ca2+ release induces a regulatory activation of Na+-Ca2+ exchange activity.