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 Call, but estimates of the half-maximal activating concentration have varied over a wide range. In Chinese barrister ovary cells expressing the cardiac Na+/Ca2+ exchanger, the time course of exchange-mediated Call 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 (K-h). After correcting for the effects of mitochondrial Ca2+ Uptake and fura-2 buffering, K, values of similar to300 nM were obtained. After an increase in [(Ca2+](i), the activated state of the exchanger persisted following a subsequent reduction in [Ca2+], 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 Call pump, suggesting that local Ca2+ gradients contribute to maintaining exchanger activation after the return of global [Ca2+](i) to low values.