Calcium regulates exocytosis at the level of single vesicles.

Ca2+ microdomains that form during the opening of voltage-gated Ca2+ channels have been implicated in regulating the kinetics of hormone and transmitter release. Direct assessment of the interaction between a single Ca2+ microdomain and a single secretory vesicle has been impossible because of technical limitations. Using evanescent field imaging of near-membrane micromolar Ca2+ concentration ([Ca2+]) and fluorescently labeled vesicles, we have observed exocytosis of individual chromaffin dense-core vesicles that was triggered by Ca2+ microdomains. Ca2+ microdomains selectively triggered the release of vesicles that were docked within 300 nm. Not all vesicles exposed to a Ca2+ microdomain were released, indicating that some vesicles are docked but are not ready for release. In addition to its established role as a trigger for release, elevated near-membrane [Ca2+] reduced the distance between docked vesicles and Ca2+ entry sites. Our results suggest a new mechanism for stimulation-dependent facilitation of exocytosis, whereby vesicles are moved closer to Ca2+ entry sites, thereby increasing a Ca2+ microdomain's efficacy to trigger vesicle fusion.