Allosteric Activation of Sodium–Calcium Exchange Activity by Calcium: Persistence at Low Calcium Concentrations

The activity of the cardiac Na(+)/Ca(2+) exchanger is stimulated allosterically by Ca(2+), but estimates of the half-maximal activating concentration have varied over a wide range. In Chinese hamster ovary cells expressing the cardiac Na(+)/Ca(2+) exchanger, the time course of exchange-mediated Ca(2+) influx showed a pronounced lag period followed by an acceleration of Ca(2+) uptake. Lag periods were absent in cells expressing an exchanger mutant that was not dependent on regulatory Ca(2+) activation. We assumed that the rate of Ca(2+) uptake during the acceleration phase reflected the degree of allosteric activation of the exchanger and determined the value of cytosolic Ca(2+) ([Ca(2+)](i)) at which the rate of Ca(2+) influx was half-maximal (K(h)). After correcting for the effects of mitochondrial Ca(2+) uptake and fura-2 buffering, K(h) values of ∼300 nM were obtained. After an increase in [Ca(2+)](i), the activated state of the exchanger persisted following a subsequent reduction in [Ca(2+)](i) to values <100 nM. Thus, within 30 s after termination of a transient increase in [Ca(2+)](i), exchange-mediated Ca(2+) entry began without a lag period and displayed a linear rate of Ca(2+) uptake in most cells; a sigmoidal time course of Ca(2+) uptake returned 60–90 s after the transient increase in [Ca(2+)](i) was terminated. Relaxation of the activated state was accelerated by the activity of the endoplasmic reticulum Ca(2+) pump, suggesting that local Ca(2+) gradients contribute to maintaining exchanger activation after the return of global [Ca(2+)](i) to low values.