Vesicular Ca2+ participates in the catalysis of exocytosis

Effects of vesicular monoamine transporter inhibitors on catecholamine release from bovine chromaffin cells have been examined at the level of individual exocytotic events. As expected for a depletion of vesicular stores, release evoked by depolarizing agents was decreased following 15-min incubations with reserpine and tetrabenazine, as evidenced by a decrease in exocytotic frequency and amount released per event. In contrast, two reserpine derivatives, methyl reserpate and reserpic acid, were much less effective. Surprisingly, the incubations also decreased the accompanying rise in intracellular Ca2+ evoked by depolarizing agents. Subcellular studies revealed that reserpine and tetrabenazine at concentrations near their Ri values not only could increase cytoplasmic catecholamines but also could displace Ca2+ from vesicles. Furthermore, transient exposure to tetrabenazine and reserpine, but not methyl reserpate and reserpic acid, induced exocytotic release of catecholamines, Reserpine induced a rise in intracellular Ca2+, as detected by whole-cell measurements with Fura-2, It could induce exocytosis, albeit at a lower frequency, in Ca2+-free solutions, supporting an internal Ca2+ source. Depletion of endoplasmic reticulum and mitochondrial Ca2+ pools did not eliminate the reserpine-activated release. These results indicate that vesicular Ca2+ can play an important role in exocytosis and under some conditions may be involved in initiating this process.