Munc13-1 acts as a priming factor for large dense-core vesicles in bovine chromaffin cells

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Abstract
In chromaffin cells the number of large dense‐core vesicles (LDCVs) which can be released by brief, intense stimuli represents only a small fraction of the ‘morphologically docked’ vesicles at the plasma membrane. Recently, it was shown that Munc13‐1 is essential for a post‐docking step of synaptic vesicle fusion. To investigate the role of Munc13‐1 in LDCV exocytosis, we overexpressed Munc13‐1 in chromaffin cells and stimulated secretion by flash photolysis of caged calcium. Both components of the exocytotic burst, which represent the fusion of release‐competent vesicles, were increased by a factor of three. The sustained component, which represents vesicle maturation and subsequent fusion, was increased by the same factor. The response to a second flash, however, was greatly reduced, indicating a depletion of release‐competent vesicles. Since there was no apparent change in the number of docked vesicles, we conclude that Munc13‐1 acts as a priming factor by accelerating the rate constant of vesicle transfer from a pool of docked, but unprimed vesicles to a pool of release‐competent, primed vesicles.