Porocytosis: secretion from small and medium-diameter vesicles and vesicle arrays without membrane fusion

We have recently proposed a mechanism to describe secretion, a fundament process in all cells. That hypothesis, called porocytosis, embodies all available data, and encompasses both forms of secretion, i.e., vesicular and constitutive. The current accepted view of exocytotic secretion involves the physical fusion of vesicle- and plasma membranes. However, that hypothesized mechanism does not fit all available physiological data (Silver et al., 2001; Kriebel et al., 2001). Energetics of apposed lipid bilayers do not favor unfacilitated fusion. Calcium ion levels are elevated in microdomains at levels of 10(-4)-10(-3)M for 1 ms or less, with the calcium ions showing limited lateral mobility at the site of secretion (Llinas et al., 1992, Silver et al., 1994). We consider that calcium ions, whose mobility is restricted in space and time, establish "salt-bridges" among adjacent lipid molecules, and establishes transient pores that span the vesicle and plasma membrane lipid bilayers; the lifetime of that transient pore being completely dependent on duration of sufficient calcium ion levels.