Sodium-calcium exchange in renal epithelial cells: Dependence on cell sodium and competitive inhibition by magnesium

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Abstract
Kinetic properties of Na+−Ca2+ exchange in a renal epithelial cell line (LLC-MK2) were assessed by measuring cytosolic free Ca2+ with fura-2 and45Ca2+ influx. Replacing external Na+ with K+ produced relatively small increases in free Ca2+ and45Ca2+ uptake unless the cells were incubated with ouabain. Ouabain markedly increased cell Na+ and strongly potentiated the effect of replacing external Na+ with K+ on free Ca2+ and45Ca2+ uptake.45Ca2+ influx in 140mm K+ or N-methyl-d-glucamine minus influx in 140mm Na+ was used to quantify Na+−Ca2+ exchange activity of Na+-loaded cells. The dependence of exchange on cell Na+ was sigmoidal; theK 0.5 was 26±3 mmol/liter cell water space, and the Hill coefficient was 3.1±0.2. The kinetic features of the dependence of exchange on cell Na+ partly account for the small increase in Ca2+ influx when all external Na+ is replaced by K+. Besides raising cell Na+ ouabain appears to activate the exchanger. Magnesium competitively inhibited exchange activity. The potency of Mg2+ was 8.2-fold lower with potassium instead of N-methyl-d-glucamine or choline as the replacement for external Na+. Potassium also increased theV max of exchange by 86% and had no effect on theK m for Ca2+. The exchanger does not cause detectable22Na+−Mg2+ exchange and does not appear to require K+ or transport86Rb+. Although exchange activity was plentiful in the epithelial cells from monkey kidney, others from amphibian, canine, opossum, and porcine kidney had no detectable exchange activity. All of the measured kinetic properties of Na+−Ca2+ exchange in the renal epithelial cells are very similar to those of the exchanger in rat aortic myocytes.