Vesicular Ca super(2+) Participates in the Catalysis of Exocytosis

• Published in: The Journal of biological chemistry 2000-03, Vol.275 (13), p.9136-9142
• Main Authors: Mundorf, M L, Troyer, K P, Hochstetler, SE, Near, JA, Wightman, R M
• Format: Article
• Language: English
• Subjects: monoamine transporter; tetrabenazine
• ISSN: 0021-9258

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 Ca super(2+) evoked by depolarizing agents. Subcellular studies revealed that reserpine and tetrabenazine at concentrations near their K sub()i values not only could increase cytoplasmic catecholamines but also could displace Ca super(2+) 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 Ca super(2+), as detected by whole-cell measurements with Fur-a2. It could induce exocytosis, albeit at a lower frequency, in Ca super(2+)-free solutions, supporting an internal Ca super(2+) source. Depletion of endoplasmic reticulum and mitochondrial Ca super(2+) pools did not eliminate the reserpine-activated release. These results indicate that vesicular Ca super(2+) can play an important role in exocytosis and under some conditions may be involved in initiating this process.