Zinc inhibits Ca2+ transport by rat brain Na+/Ca2+ exchanger

Calcium transport by the Na+/Ca2+ exchanger was measured in plasma membranes vesicles purified from rat brain and in primary rat cortical cell culture. Sodium-loaded vesicles rapidly accumulate Ca2+ via Na+/Ca2+ exchange (Na+(i)-dependent Ca2+ uptake). Extravesicular zinc inhibited Na+/Ca2+ exchange as evidenced by a reduction of the initial velocity of Ca2+ uptake. Significant inhibition of Ca2+ uptake was seen at concentrations of zinc as low as 3 microM. Lineweaver-Burk analysis of the data was consistent with noncompetitive inhibition with respect to extravesicular Ca2+ concentration. The Ki for zinc inhibition of Ca2+ uptake determined from a Dixon plot was 14.5 microM. This is within the range of zinc concentrations thought to be obtained extracellularly after excitation. When vesicles were preloaded with Ca2+, extravesicular zinc also inhibited reversal of Na+/Ca2+ exchange (Na+(i)-dependent Ca2+ release) although its potency was much less: concentrations of > or = 30 microM zinc were required. Zinc inhibition of Ca2+ release was not Na+ dependent. Na+(i)-dependent calcium uptake by rat cortical cells in primary culture also was inhibited by zinc. The extent of inhibition was similar to that seen for inhibition of Na+(i)-dependent Ca2+ uptake in membrane vesicles, but the potency was less. The results suggest that Ca2+ transport by the Na+/Ca2+ exchanger is inhibited by concentrations of zinc thought to be attained extracellularly after excitation.