Osmotic Forces and the Fusion of Biomembranes

Abstract

The possibility that osmotic forces are involved in the molecular mechanisms that govern the fusion of biomembranes has been of wide interest since it was observed that solutions of high osmolarity can inhibit the exocytosis of chromaffin granules in adrenal medullary cells (Hampton and Holz, 1983; Pollard et al., 1984). This work followed that of Finkelstein and his colleagues who had previously demonstrated, in an experimental model for exocytosis, that under appropriate conditions phospholipid vesicles will fuse with a planar phospholipid bilayer under the influence of an osmotic gradient (Cohen et al., 1980). It is, however, less well known that Racker and his co-workers had shown in an earlier interesting paper that unilamellar proteoliposomes containing phosphatidylserine, which ceased to fuse in the presence of calcium ions after the liposomes had grown to diameters of about 100 nm, could be induced to fuse further by an osmotic gradient (internal osmotic pressure higher than external) and form single-walled liposomes with diameters exceeding 1 micron as well as multilayered vesicles (Miller et al., 1976).