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

In chromaffin cells the number of large dense-core vesicles (LDCVs) which c an be released by brief, intense stimuli represents only a small fraction o f the 'morphologically docked' vesicles at the plasma membrane. Recently, i t 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 flas h photolysis of caged calcium. Both components of the exocytotic burst, whi ch represent the fusion of release-competent vesicles, were increased by a factor of three. The sustained component, which represents vesicle maturati on and subsequent fusion, was increased by the same factor. The response to a second flash, however, was greatly reduced, indicating a depletion of re lease-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 a ccelerating the rate constant of vesicle transfer from a pool of docked, bu t unprimed vesicles to a pool of release-competent, primed vesicles.