The role of calcium in neurotransmitter release: existing models and new approaches to evaluate possible mechanisms

Calcium signalling in bovine adrenal chromaffin cells: additive effects of histamine and nicotine

In a previous report, we described the ability of two secretogogues, histamine and nicotine, to stimulate additive effects on catecholamine (CA) release and synapsin II phosphorylation in bovine adrenal chromaffin cells (BACC) [Firestone and Browning (1992), J. Neurochem., 58:441-447]. We hypothesized that these results were due to the combined effects on cytosolic Ca++ of the two distinct signalling pathways. We therefore examined the intracellular Ca++ signals stimulated by histamine and nicotine, alone and together. In Ca(++)-deficient medium, nicotine-stimulated signals were abolished, whereas histamine-stimulated signals were maintained, demonstrating that nicotine depended entirely on Ca++ influx for its effects. Indeed, the nicotine-stimulated signal could also be prevented using a Ca++ channel blocker, nicardipine. Further, the observation that exposure of BACC to thapsigargin reduced histamine-stimulated Ca++ signals verified that histamine mobilizes Ca++ from intracellular stores. Thus, the two secretogogues mobilize Ca++ from distinct pools. When BACC were stimulated with the two secretogogues together, the resulting Ca++ signal was greater than that from either alone. These data are consistent with a model in which two distinct sources of Ca++ can summate within the cell, producing a greater Ca++ signal and, hence, a greater effect on neurotransmitter release.

Divalent cations effectively replace Ca2+ and support bradykinin induced noradrenaline release

Bradykinin (BK), a nonapeptide acting at the B2-type BK-receptor, and depolarization with high KCl (50 mM), induce catecholamine secretion in pheochromocytoma cells (PC-12). The mechanism underlying the BK-induced release, which is absolutely Ca2(+)-dependent, is not yet understood. Alkaline metals, barium (Ba2+), strontium (Sr2+) and other metal cations, manganese (Mn2+) or lanthanum (La3+), support BK-induced [3H]noradrenaline ([3H]NA) release. The extent of supporting transmitter release is dependent upon the specificity of the extracellular cation, with rank order potency of: Ba2+ greater than Sr2+ greater than Ca2+ greater than Mn2+La3+. The same rank order potency was observed for supporting both BK- and K(+)-induced release. [3H]NA release in the presence of Ba2+ or Sr2+ was much greater than in the presence of Ca2+, and unlike with Ca2+ was not saturable at the highest concentration measured. La3+ and Mn2+ were significantly less effective than Ca2+ at supporting release. These results strongly suggest that extracellular Ca2+ entry is essential for release, and that BK mediates release via a receptor-operated Ca2+ channel.