The route of Ca2+ entry during reloading of the intracellular Ca2+ pool in pancreatic acini.

To trace the route of Ca2+ entry and the role of the cytosolic Ca2+ pool in reloading of the internal stores of pancreatic acinar cells, Mn2+ influx into Fura 2-loaded cells and the effect of 1,2-bis(2-aminophenoxyethane-N,N,N',N'-tetraacetic acid (BAPTA) on Ca2+ storage in intracellular stores and reloading were examined. Treatment of acini suspended in Ca2(+)-free medium with carbachol (cell stimulation) or carbachol and atropine (reloading period) resulted in 2-fold increase in the rate of Mn2+ influx. Increasing Ca2+ permeability of the plasma membrane by elevation of extracellular pH from 7.4 to 8.2 further increased the rate of Mn2+ influx observed during cell stimulation and the reloading period. Loading the acini with BAPTA by incubation with 50 microM of the acetomethoxy form of BAPTA (BAPTA/AM) was followed by a transient reduction in free cytosolic Ca2+ concentration ((Ca2+]i). To compensate for the increased Ca2+ buffering capacity in the cytosol the acini incorporated Ca2+ from the external medium. Although BAPTA prevented changes in free cytosolic Ca2+ concentration during carbachol and atropine treatment, it had no apparent effect on Ca2+ content of the internal stores or the ability of agonists to release Ca2+ from these stores. Loading the cytosol with BAPTA considerably reduced the rate of Ca2+ reloading. These observations are not compatible with direct communication between the medium and the inositol 1,4,5-trisphosphate releasable pool and provide direct evidence for Ca2+ entry into the cytosol prior to its uptake into the intracellular pool, both during cell stimulation and the Ca2+ reloading.