DYNAMIC PROPERTIES OF AN INOSITOL 1,4,5-TRISPHOSPHATE AND THAPSIGARGIN-INSENSITIVE CALCIUM POOL IN MAMMALIAN-CELL LINES

The functional characteristics of a nonacidic, inositol 1,4,5-trisphos phate- and thapsigargin-insensitive Ca2+ pool have been characterized in mammalian cells derived from the rat pituitary gland (GH3, GC, and GH3B6), the adrenal tissue (PC12), and mast cells (RBL-1). This Ca2+ p ool is released into the cytoplasm by the Ca2+ ionophores ionomycin or A23187 after the discharge of the inositol 1,4,5-trisphosphate-sensit ive store with an agonist coupled to phospholipase C activation and/or thapsigargin, The amount of Ca2+ trapped within this pool increased s ignificantly after a prolonged elevation of intracellular Ca2+ concent ration elicited by activation of Ca2+ influx, This pool was affected n either by caffeine-ryanodine nor by mitochondrial uncouplers. Probing mitochondrial Ca2+ with recombinant aequorin confirmed that this pool did not coincide with mitochondria, whereas its homogeneous distributi on across the cytosol, as revealed by confocal microscopy, and its ins ensitivity to brefeldin A make localization within the Golgi complex u nlikely. A proton gradient as the driving mechanism for Ca2+ uptake wa s excluded since ionomycin is inefficient in releasing Ca2+ from acidi c pools and Ca2+ accumulation/release in/from this store was unaffecte d by monensin or NH4Cl, drugs known to collapse organelle acidic pH gr adients. Ca2+ sequestration inside this pool, thus, may occur through a low-affinity, high-capacity Ca2+-ATPase system, which is, however, d istinct from classical endosarcoplasmic reticulum Ca2+-ATPases. The cy tological nature and functional role of this Ca2+ storage compartment are discussed.