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 ≅200 μM of Ca²⁺/liter of cell water (<30% of total exchangeable Ca²⁺) and takes up Ca²⁺ from the cytosol via a Ca²⁺-ATPase, blocked by thapsigargin. A second pool, ≅400 μM/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, ≅170 μmoles of Ca²⁺/liter, was discharged by the combination of ionomycin together with a substance that collapsed intracellular pH gradients, such as monensin or NH₄C1. 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.