Feedback Inhibition of Sodium/Calcium Exchange by Mitochondrial Calcium Accumulation

Chinese hamster ovary cells expressing the bovine cardiac Na+/Ca2+ exchanger 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 due 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 rather than increased Ca2+ sequestration since the influx of Ba2+, which is not sequestered by internal organelles, was also inhibited by a prior interval of Ca2+ influx. Mitochondria accumulated Ca2+ during the first interval of reduced [Na+]o, as determined by an increase in fluorescence of the Ca2+-indicating dye rhod-2, which preferentially labels mitochondria. Agents that blocked mitochondrial Ca2+ accumulation (uncouplers, nocodazole) eliminated the observed inhibition of exchange activity during the second period of low [Na+]o. Conversely, diltiazem, an inhibitor of the mitochondrial Na+/Ca2+ exchanger, increased mitochondrial Ca2+ accumulation and also increased the inhibition of exchange activity. We conclude that Na+/Ca2+ exchange activity is regulated by a feedback inhibition process linked to mitochondrial Ca2+accumulation.