Intracellular Ca2+ microdomain-triggered exocytosis in neuroendocrine cells.

Colocalization of voltage-gated Ca2+ channels and exocytotic sites at the active zones of nerve terminals underlies 'synchronous' action potential discharge and synaptic vesicle exocytosis, thus allowing fast interneuronal signalling. Such a demand for a rapid release is not expected in neuroendocrine cells whose secretory products act throughout the entire organism. Nevertheless, by using evanescent field imaging of near-membrane Ca2+ concentrations and fluorescently labelled vesicles, Becherer et al. have recently reported exocytosis of individual large dense-core vesicles triggered by Ca2+ microdomains formed around clusters of open L-type Ca2+ channels in chromaffin cells from the adrenal medulla. This finding, besides illustrating the power of new microscopy imaging techniques, directly demonstrates in neuroendocrine cells a functional interaction between Ca2+ channels and secretory vesicles very much reminiscent of that in neurons.