Ca2+ Mobilized by Caffeine from the Inositol 1,4,5‐Trisphosphate‐Insensitive Pool of Ca2+ in Somatic Regions of Sympathetic Neurons Does Not Evoke [3H]Norepinephrine Release

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Abstract
The effects of electrical stimulation, muscarinic and serotonergic agonists, and caffeine on [3H]inositol 1,4,5-trisphosphate ([3H]Ins(1,4,5)P3) content, intracellular free Ca2+ concentration ([Ca2+]i), and release of [3H]norepinephrine ([3H]NE) were studied in cultured sympathetic neurons. Neuronal cell body [Ca2+]i was unaffected by muscarinic or serotonergic receptor stimulation, which significantly increased [3H]Ins(1,4,5)P3 content. Stimulation at 2 Hz and caffeine had no effect on [3H]Ins(1,4,5)P3, but caused greater than two-fold increase in [Ca2+]i. Only 2-Hz stimulation released [3H]NE. Caffeine had no effect on the release. When [Ca2+]i was measured in growth cones, only electrical stimulation produced an increase in [Ca2+]i. The other agents had no effect on Ca2+ at the terminal regions of the neurons. We conclude that Ins(1,4,5)P3-insensitive, but caffeine-sensitive Ca2+ stores in sympathetic neurons are located only in the cell body and are not coupled to [3H]NE release.