Feedback inhibition of sodium/calcium exchange by mitochondrial calcium accumulation

Chinese hamster ovary cells expressing the bovine cardiac Na+/Ca2+ exchange r were subjected to two periods of 5 and 3 min, respectively, during which the extracellular Na+ concentration ([Na+](o)) was reduced to 20 mM; these intervals were separated by a 5-min recovery period at 140 mM Na-o(+). The cytosolic Ca2+ concentration ([Ca2+](i)) increased during both intervals du e to Na+-dependent Ca2+ influx by the exchanger. However, the peak rise in [Ca2+](i) during the second interval was only 26% of the first. The reduced rise in [Ca2+](i) was due to an inhibition of Na+/Ca2+ exchange activity r ather than increased Ca2+ sequestration since the influx of Ba2+, which is not sequestered by internal organelles, was also inhibited by a prior inter val of Ca2+ influx. Mitochondria accumulated Ca2+ during the first interval of reduced [Na+](o). as determined by an increase in fluorescence of the C a2+-indicating dye rhod-2, which preferentially labels mitochondria. Agents that blocked mitochondrial Ca2+ accumulation (uncouplers, nocodazole) elim inated the observed inhibition of exchange activity during the second perio d of low [Na+](o). Conversely, diltiazem, an inhibitor of the mitochondrial Na+/Ca2+ exchanger, increased mitochondrial Ca2+ accumulation and also inc reased the inhibition of exchange activity. We conclude that Na+/Ca2+ excha nge activity is regulated by a feedback inhibition process linked to mitoch ondrial Ca2+ accumulation.