Na+-Ca2+ exchange and Ca2+ efflux in transfected Chinese hamster ovary cells.
The objective of this study was to assess the contribution of Na+-Ca2+ exchange activity to Ca2+ efflux at various cytosolic Ca2+ concentrations ([Ca2+]i) in transfected Chinese hamster cells expressing the bovine cardiac Na+-Ca2+ exchanger. Ionomycin was added to fura-2 loaded cells and the resulting [Ca2+]i transient was monitored in Ca2+-free media with or without extracellular Na+. The presence of Na+ reduced both the amplitude and duration of the [Ca2+]i transient. Na+ had similar effects when the peak of the [Ca2+]i transient was buffered to 100 nM by cytosolic EGTA, or when Ca2+ was slowly released from internal stores with thapsigargin. Ca2+ efflux following ionomycin addition was directly measured with extracellular fura-2 and followed a biphasic time course (t(1/2) approximately = 10 s and 90s). The proportion of total efflux owing to the rapid phase was increased by Na+ and reduced by EGTA-loading. Na+ accelerated the initial rate of Ca2+ efflux by 65% in unloaded cells but only by 16% in EGTA-loaded cells. In both cases, the stimulation by Na+ was less than expected, given the pronounced effects of Na+ on the [Ca2+]i transient. We conclude that the exchanger contributes importantly to Ca2+ efflux activity at all [Ca2+]i values above 40 nM. We also suggest that Ca2+ efflux pathways may involve non-cytosolic or local routes of Ca2+ traffic.
Fang
Y
Y
Department of Pharmacology and Physiology, University of Medicine and Dentistry, New Jersey Medical School, Newark 07103, USA.
Condrescu
M
M
Reeves
J P
JP
D000818:Animals; D001692:Biological Transport; D016466:CHO Cells / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002417:Cattle; D006224:Cricetinae; D003412:Cricetulus; D004533:Egtazic Acid / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes / Q000378:metabolism; D016257:Fura-2 / Q000378:metabolism; D015759:Ionomycin / Q000494:pharmacology; D007476:Ionophores / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology; D014162:Transfection
D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D007476:Ionophores; D019831:Sodium-Calcium Exchanger; C110374:sodium-calcium exchanger 1; D004533:Egtazic Acid; D015759:Ionomycin; D019284:Thapsigargin; D012964:Sodium; D000252:Calcium-Transporting ATPases; D002118:Calcium; D016257:Fura-2