Osmotic Effects in Membrane Fusion during Exocytosis

This chapter assesses the chemiosmotic hypothesis of secretion in the light of the properties exhibited by isolated secretory granules and by intact and permeabilized secretory cells. The molecular events associated with membrane fusion are far from understood, as are most aspects of the exocytotic process. One hypothesis proposed to explain the membrane fusion event in exocytosis is the chemiosmotic hypothesis of secretion. The original chemiosmotic hypothesis explains the coupling of mitochondrial electron transport to ATP synthesis. The chemiosmotic hypothesis of secretion was formulated following the discovery of the chemiosmotic properties of isolated chromaffin granules. However, subsequent results obtained with intact and permeabilized cells revealed large differences between the predicted and observed secretory properties of these cells. Although the chemiosmotic hypothesis as originally proposed is no longer valid, several alternative hypotheses have been advanced according to which the osmotic swelling of secretory vesicles constitutes the force that drives the fission step in exocytosis.