title: Feedback inhibition of sodium/calcium exchange by mitochondrial calcium accumulation.
abstract: Chinese hamster ovary cells expressing the bovine cardiac Na(+)/Ca(2+) 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 Ca(2+) concentration ([Ca(2+)](i)) increased during both intervals due to Na(+)-dependent Ca(2+) influx by the exchanger. However, the peak rise in [Ca(2+)](i) during the second interval was only 26% of the first. The reduced rise in [Ca(2+)](i) was due to an inhibition of Na(+)/Ca(2+) exchange activity rather than increased Ca(2+) sequestration since the influx of Ba(2+), which is not sequestered by internal organelles, was also inhibited by a prior interval of Ca(2+) influx. Mitochondria accumulated Ca(2+) during the first interval of reduced [Na(+)](o), as determined by an increase in fluorescence of the Ca(2+)-indicating dye rhod-2, which preferentially labels mitochondria. Agents that blocked mitochondrial Ca(2+) 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(+)/Ca(2+) exchanger, increased mitochondrial Ca(2+) accumulation and also increased the inhibition of exchange activity. We conclude that Na(+)/Ca(2+) exchange activity is regulated by a feedback inhibition process linked to mitochondrial Ca(2+) accumulation.
authors.affiliation: Department of Pharmacology and Physiology, University of Medicine and Dentistry of New Jersey-New Jersey Medical School, Graduate School of Biomedical Sciences, Newark, New Jersey 07103, USA.
authors.forename: K
authors.initials: K
authors.lastname: Opuni
authors.forename: J P
authors.initials: JP
authors.lastname: Reeves
mesh_terms: D000818:Animals; D016466:CHO Cells; D002118:Calcium / Q000378:metabolism*; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D002417:Cattle; D006224:Cricetinae; D003600:Cytosol / Q000378:metabolism; D004110:Diltiazem / Q000494:pharmacology; D005246:Feedback; D007700:Kinetics; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism*; D009206:Myocardium / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*; D014162:Transfection
chemical_list: D011994:Recombinant Proteins; D019831:Sodium-Calcium Exchanger; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D012964:Sodium; D004110:Diltiazem; D002118:Calcium