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.
Becherer
Ute
U
Max-Planck Institute for Experimental Medicine, Molecular Biology of Neuronal Signals, Hermann-Rein Str. 3, D-37075 Göttingen, Germany.
Moser
Tobias
T
Stühmer
Walter
W
Oheim
Martin
M
D000200:Action Potentials; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D015220:Calcium Channels / Q000737:chemistry / Q000502:physiology*; D002478:Cells, Cultured; D019439:Chromaffin Cells / Q000378:metabolism; D005089:Exocytosis / Q000502:physiology*; D009994:Osmolar Concentration; D017434:Protein Structure, Tertiary / Q000502:physiology; D022142:Secretory Vesicles / Q000378:metabolism*
D015220:Calcium Channels; D002118:Calcium