TRANSPORT, DOCKING AND EXOCYTOSIS OF SINGLE SECRETORY GRANULES IN LIVE CHROMAFFIN CELLS

Neurons maintain a limited pool of synaptic vesicles which are docked at active zones and are awaiting exocytosis(1-4). By contrast, endocri ne cells releasing large, dense-core secretory granules have no active zones, and there is disagreement about the size(5) and even the exist ence(6) of the docked pool. It is not known how, and how rapidly, secr etory vesicles are replaced at exocytic sites in either neurons or end ocrine cells. By using electron microscopy, we have now been able to i dentify a pool of docked granules in chromaffin cells that is selectiv ely depleted when cells secrete. With evanescent-wave fluorescence mic roscopy(7), we observed single granules undergoing exocytosis and leav ing behind patches of bare plasmalemma. Fresh granules travelled to th e plasmalemma at a top speed of 114 nm s(-1), taking an average of 6 m in to arrive. On arrival, their motility diminished 4-food, probably a s a result of docking. Some granules detached and returned to the cyto sol. We conclude that a large pool of docked granules turns over slowl y, that granules move actively to their docking sites, that docking is reversible, and that the 'rapidly releasable pool' measured electroph ysiologically represents a small subset of docked granules.