TERMINATION OF CYTOSOLIC CA2- CA2+ REUPTAKE INTO INTRACELLULAR STORESIS REGULATED BY THE FREE CA2+ CONCENTRATION IN THE STORE LUMEN( SIGNALS )

The mechanism by which agonist-evoked cytosolic Ca2+ signals are termi nated has been investigated, We measured the Ca2+ concentration inside the endoplasmic reticulum store of pancreatic acinar cells and monito red the cytoplasmic Ca2+ concentration by whole-cell patch-clamp recor ding of the Ca2+-sensitive currents, When the cytosolic Ca2+ concentra tion was clamped at the resting level by a high concentration of a sel ective Ca2+ buffer, acetylcholine evoked the usual depletion of intrac ellular Ca2+ stores, but without increasing the Ca2+-sensitive current s. Removal of acetylcholine allowed thapsigargin-sensitive Ca2+ reupta ke into the stores, and this process stopped when the stores had been loaded to the pre-stimulation level. The apparent rate of Ca2+ reuptak e decreased steeply with an increase in the Ca2+ concentration in the store lumen and it is this negative feedback on the Ca2+ pump that con trols the Ca2+ store content. In the absence of a cytoplasmic Ca2+ cla mp, acetylcholine removal resulted in a rapid return of the elevated c ytoplasmic Ca2+ concentration to the pre-stimulation resting level, wh ich was attained long before the endoplasmic reticulum Ca2+ store had been completely refilled, We conclude that control of Ca2+ reuptake by the Ca2+ concentration inside the intracellular store allows precise Ca2+ signal termination without interfering with store refilling.