Allosteric activation of sodium-calcium exchange activity by calcium: persistence at low calcium concentrations.
The activity of the cardiac Na+/Ca2+ exchanger is stimulated allosterically by Ca2+, but estimates of the half-maximal activating concentration have varied over a wide range. In Chinese hamster ovary cells expressing the cardiac Na+/Ca2+ exchanger, the time course of exchange-mediated Ca2+ influx showed a pronounced lag period followed by an acceleration of Ca2+ uptake. Lag periods were absent in cells expressing an exchanger mutant that was not dependent on regulatory Ca2+ activation. We assumed that the rate of Ca2+ uptake during the acceleration phase reflected the degree of allosteric activation of the exchanger and determined the value of cytosolic Ca2+ ([Ca2+]i) at which the rate of Ca2+ influx was half-maximal (Kh). After correcting for the effects of mitochondrial Ca2+ uptake and fura-2 buffering, Kh values of approximately 300 nM were obtained. After an increase in [Ca2+]i, the activated state of the exchanger persisted following a subsequent reduction in [Ca2+]i to values <100 nM. Thus, within 30 s after termination of a transient increase in [Ca2+]i, exchange-mediated Ca2+ entry began without a lag period and displayed a linear rate of Ca2+ uptake in most cells; a sigmoidal time course of Ca2+ uptake returned 60-90 s after the transient increase in [Ca2+]i was terminated. Relaxation of the activated state was accelerated by the activity of the endoplasmic reticulum Ca2+ pump, suggesting that local Ca2+ gradients contribute to maintaining exchanger activation after the return of global [Ca2+]i to low values.
Reeves
John P
JP
Department of Pharmacology and Physiology, UMDNJ - NJ Medical School, Newark, NJ 07101-1709, USA. reeves@umdnj.edu
Condrescu
Madalina
M
D000494:Allosteric Regulation / Q000187:drug effects / Q000502:physiology; D000818:Animals; D016466:CHO Cells; D002118:Calcium / Q000008:administration & dosage / Q000502:physiology*; D002417:Cattle; D006224:Cricetinae; D006801:Humans; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*
D019831:Sodium-Calcium Exchanger; D002118:Calcium