Na+/Ca2+ Exchange-mediated Calcium Entry in Human Lymphocytes

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Regulation of cytosolic Ca2+ and cytosolic Na+ is critical for lymphocyte cation homeostasis and function.To examine the influence of cytosolic Na+ on Ca2+ regulation in human peripheral blood lymphocytes, Ca2+ entry and cytosolic Ca2+ (measured with fura-2) were monitored in cells in which cytosolic Na+ was increased and/or the Na+ gradient was decreased by reduction of external Na+ concentration. Ouabain-treated cells (0.1 mM for 30 min at 37 degrees C), suspended in Na+ -free medium, showed a 30-65% increase in Ca2+ uptake compared to cells in 140 mM Na+ medium. Enhanced Ca2+ influx was entirely dependent on ouabain pretreatment and reversal of the Na (+) gradient. Na pump inhibition or Na ionophore addition and subsequent exposure to Na+ -free medium resulted in a sustained elevation of cytosolic Ca2+. As preincubation of cells in Ca2+ -free medium further enhanced the ouabain-dependent increase in cytosolic Ca2+, the effects of the microsomal Ca2+ -ATPase inhibitor thapsigargin on Ca2+ influx and cytosolic Ca2+ were studied. Thapsigargin stimulated Ca (2+) entry following ouabain pretreatment and reversal of the Na+ gradient; the effects of thapsigargin were retained in the presence of LaCl (3), a potent inhibitor of store-dependent calcium influx pathways. These results show lymphocytes demonstrate Na+/Ca2+ exchange activity and suggest the Na+/Ca2+ exchanger modulates cytosolic Ca2+ following intracellular Ca2+ store depletion. (J. Clin. Invest. 1994. 94:2002-2008.) Key words: human T lymphocytes. thapsigargin. Ca2+ uptake. store-dependent Ca2+ influx. intracellular Ca2+ stores

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