title: Calcium-dependent regulation of calcium efflux by the cardiac sodium/calcium exchanger.
abstract: Allosteric regulation by cytosolic Ca2+ of Na(+)/Ca2+ exchange activity in the Ca2+ efflux mode has received little attention because it has been technically difficult to distinguish between the roles of Ca2+ as allosteric activator and transport substrate. In this study, we used transfected Chinese hamster ovary cells to compare the Ca2+ efflux activities in nontransfected cells and in cells expressing either the wild-type exchanger or a mutant, Delta(241-680), that operates constitutively; i.e., its activity does not require allosteric Ca2+ activation. Expression of the wild-type exchanger did not significantly lower the cytosolic Ca2+ concentration ([Ca2+](i)) compared with nontransfected cells. During Ca2+ entry through store-operated Ca2+ channels, Ca2+ efflux by the wild-type exchanger became evident only after [Ca2+](i) approached 100-200 nM. A subsequent decline in [Ca2+](i) was observed, suggesting that the activation process was time dependent. In contrast, Ca2+ efflux activity was evident under all experimental conditions in cells expressing the constitutive exchanger mutant. After transient exposure to elevated [Ca2+](i), the wild-type exchanger behaved similarly to the constitutive mutant for tens of seconds after [Ca2+](i) had returned to resting levels. We conclude that Ca2+ efflux activity by the wild-type exchanger is allosterically activated by Ca2+, perhaps in a time-dependent manner, and that the activated state is briefly retained after the return of [Ca2+](i) to resting levels.
authors.affiliation: Dept. of Pharmacology and Physiology, Graduate School of Biomedical Sciences, New Jersey Medical School, University of Medicine and Dentistry of New Jersey, 185 South Orange Avenue, Room H649, Newark, NJ 07103, USA.
authors.forename: Olga
authors.initials: O
authors.lastname: Chernysh
authors.forename: Madalina
authors.initials: M
authors.lastname: Condrescu
authors.forename: John P
authors.initials: JP
authors.lastname: Reeves
mesh_terms: D000494:Allosteric Regulation; D000818:Animals; D016466:CHO Cells; D002118:Calcium / Q000378:metabolism*; D006224:Cricetinae; D003593:Cytoplasm / Q000737:chemistry / Q000378:metabolism; D016257:Fura-2; D006801:Humans; D009124:Muscle Proteins / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D009206:Myocardium / Q000378:metabolism; D062554:NAV1.5 Voltage-Gated Sodium Channel; D015222:Sodium Channels / Q000235:genetics / Q000378:metabolism*; D014162:Transfection
chemical_list: D009124:Muscle Proteins; D062554:NAV1.5 Voltage-Gated Sodium Channel; C568320:SCN5A protein, human; D015222:Sodium Channels; D002118:Calcium; D016257:Fura-2