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1. A clone of the rat phaeochromocytoma cell line (PC12) was treated with nerve growth factor (NGF) for 4-6 days and used to study caffeine- and bradykinin-induced Ca2+ release from intracellular Ca2+ stores. The caffeine-sensitive store can be depleted by Ca2+-induced Ca2+ release (CICR), while the bradykinin-induced release is mediated by inositol 1,4,5-trisphosphate (IP3). The effect of Ca2+ release from these Ca2+ stores on cytosolic free Ca2+ ([Ca2+]i) was measured by means of fura-2 single cell microfluorimetry. 2. Caffeine application caused no or only a small Ca2+ release in untreated cells in normal culture medium. The caffeine-sensitive pool could be filled by Ca2+ entry into cells through either voltage-activated Ca2+ channels or ligand-gated cation channels. 3. Bradykinin application produced substantial Ca2+ release in untreated cells in normal culture medium. The response was enhanced after K+-depolarization of the cells. The bradykinin-induced release of Ca2+ also caused depletion of the caffeine-sensitive pool by CICR. However, Ca2+ released from the IP3-sensitive store was not sequestered into the caffeine-sensitive Ca2+store. 4. The caffeine-induced rise in [Ca2+]i was blocked by ryanodine in a use-dependent manner. In addition, a substantial use-dependent ryanodine block resulted from the bradykinin-induced rise of [Ca2+]i and subsequent CICR. By contrast, the K+-induced rise of [Ca2+]i caused only a marginal use-dependent ryanodine inhibition of Ca2+ release. 5. Our results suggest an enhancement of the IP3-induced [Ca2+]i rise in the cytoplasm by CICR from the caffeine-sensitive pool. 6. A mathematical model adequately simulates our experimental data.
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