Termination of cytosolic Ca2+ signals: Ca2+ reuptake into intracellular stores is regulated by the free Ca2+ concentration in the store lumen

The mechanism by which agonist‐evoked cytosolic Ca 2+ signals are terminated has been investigated. We measured the Ca 2+ concentration inside the endoplasmic reticulum store of pancreatic acinar cells and monitored the cytoplasmic Ca 2+ concentration by whole‐cell patch‐clamp recording of the Ca 2+ ‐sensitive currents. When the cytosolic Ca 2+ concentration was clamped at the resting level by a high concentration of a selective Ca 2+ buffer, acetylcholine evoked the usual depletion of intracellular Ca 2+ stores, but without increasing the Ca 2+ ‐sensitive currents. Removal of acetylcholine allowed thapsigargin‐sensitive Ca 2+ reuptake into the stores, and this process stopped when the stores had been loaded to the pre‐stimulation level. The apparent rate of Ca 2+ reuptake decreased steeply with an increase in the Ca 2+ concentration in the store lumen and it is this negative feedback on the Ca 2+ pump that controls the Ca 2+ store content. In the absence of a cytoplasmic Ca 2+ clamp, acetylcholine removal resulted in a rapid return of the elevated cytoplasmic Ca 2+ concentration to the pre‐stimulation resting level, which was attained long before the endoplasmic reticulum Ca 2+ store had been completely refilled. We conclude that control of Ca 2+ reuptake by the Ca 2+ concentration inside the intracellular store allows precise Ca 2+ signal termination without interfering with store refilling.