Intracellular Ca2+ pools in PC12 cells. Three intracellular pools are distinguished by their turnover and mechanisms of Ca2+ accumulation, storage, and release.

Three, non-cytosolic Ca2+ pools were characterized in intact PC12 cells. The first pool, sensitive to both inositol 1,4,5-trisphosphate and caffeine (Zacchetti, D., Clementi, E., Fasolato, C., Zottini, M., Grohovaz, F., Fumagalli, G., Pozzan, T., and Meldolesi, J. (1991) J. Biol. Chem. 266, 20152-20158) accounts for approximately equal to 200 microM of Ca2+/liter of cell water (less than 30% of total exchangeable Ca2+) and takes up Ca2+ from the cytosol via a Ca(2+)-ATPase, blocked by thapsigargin. A second pool, approximately equal to 400 microM/liter, is insensitive to both inositol 1,4,5-trisphosphate, caffeine, and thapsigargin and is released by the Ca2+ ionophore ionomycin. This pool is probably heterogeneous and its intracellular localization and physiological roles remain undefined. The third pool, approximately equal to 170 mumoles of Ca2+/liter, was discharged by the combination of ionomycin together with a substance that collapsed intracellular pH gradients, such as monensin or NH4Cl. This indicates that the pool is acidic, at variance with the first two. When exocytosis was stimulated, the size of this pool declined, indicating its primary residence within secretory granules. In the conditions of our experiments no major transfer of Ca2+ among the pools seemed to occur. This is the first comprehensive description of non-cytosolic Ca2+ pools investigated in intact neurosecretory cells by non-invasive procedures.