Ca2+ microdomains have been implicated in triggering hormone and transmitter release, but until now researchers have not been able to track individual vesicles and measure localized microdomain activity in the membrane to confirm the one-on-one dynamics. Oheim's team used dual-color evanescent field imaging with low affinity Ca2+ dyes to image near-membrane Ca2+ concentration and watch the fusion of individual fluorescently filled vesicles. The team finds that voltage-gated Ca2+ entry evokes spatially confined spikes of Ca2+ that trigger release of a vesicle's contents into the extracellular space.
The docked vesicles only respond to Ca2+ microdomain spikes within a narrow region. “If a granule is located more than 300 nm away from the Ca2+ spike, it is completely blind to the event, as if there was no microdomain,” said Oheim. This limited region of influence agrees with previous mathematical models of diffusion that predict that Ca2+ microdomains would have a very narrow range of action.
More surprising is the observation that only about half of the vesicles docked at the membrane and exposed to a Ca2+ microdomain spike fuse with the plasma membrane. The researchers speculate that these nonfusing vesicles have just arrived at the membrane and are not yet fully primed for release. When such vesicles are exposed to elevated Ca2+, they move toward the Ca2+ entry site, which may increase their chances of release during the next spike. ▪