Fusion pores or depressions in pancreatic acinar or GH-secreting cells are cone-shaped structures at the plasma membrane, with a 100- to 150-nm-diameter opening. Membrane-bound secretory vesicles ranging in size from 0.2 to 1.2 µm in diameter dock and fuse at depressions to release vesicular contents. Following fusion of secretory vesicles at depressions, only a 20–35% increase in depression diameter is demonstrated. It is therefore reasonable to conclude that secretory vesicles "transiently" dock and fuse at depressions. In contrast to accepted belief, if secretory vesicles were to completely incorporate at depressions, the fusion pore would distend much wider than what is observed. Furthermore, if secretory vesicles were to completely fuse at the plasma membrane, there would be a loss in vesicle number following secretion. Examination of secretory vesicles within cells before and after secretion demonstrates that, although the total number of secretory vesicles remains unchanged following secretion, the number of empty and partially empty vesicles increases significantly, supporting the occurrence of transient fusion (6). Earlier studies on mast cells also demonstrated an increase in the number of spent and partially spent vesicles following stimulation of secretion, without any demonstrable increase in cell size (12). Other supporting evidence favoring transient fusion is the presence of neurotransmitter transporters at the synaptic vesicle membrane. These vesicle-associated transporters would be of little use if vesicles were to fuse completely at the plasma membrane to be endocytosed at a later time. Although the fusion of secretory vesicles at the cell plasma membrane occurs transiently, complete incorporation of membrane at the cell plasma membrane takes place when cells need to incorporate signaling molecules like receptors, second messengers, and ion channels.
I wish to thank Sang-Joon Cho for help in preparation of the figures and David M. Lawson for valuable comments and suggestions.
Studies performed in my laboratory were supported by Grants DK-56212 and NS-39918 from the National Institutes of Health.