SHAPE CHANGE AND PHYSICAL-PROPERTIES OF GIANT PHOSPHOLIPID-VESICLES PREPARED IN THE PRESENCE OF AN AC ELECTRIC-FIELD

Giant unilamellar vesicles with diameters ranging from 10 to 60 mu m w ere obtained by the swelling of phospholipid bilayers in water in the presence of an AC electric field. This technique leads to a homogeneou s population of perfectly spherical and unilamellar vesicles, as revea led by phase-contrast optical microscopy and freeze-fracture electron microscopy. Freshly prepared vesicles had a high surface tension with no visible surface undulations. Undulations started spontaneously afte r several hours of incubation or were triggered by the application of a small osmotic pressure. Partially deflated giant vesicles could unde rgo further shape change if asymmetrical bilayers were formed by addin g lyse compounds to the external leaflet or by imposing a transmembran e pH gradient that selectively accumulates on one leaflet phosphatidyl glycerol. Fluorescence photobleaching with 7-nitrobenz-2-oxa-1,3-diazo l-4-yl-labeled phospholipids or labeled dextran trapped within the ves icles enabled the measurement of the membrane continuity in the dumbbe ll-shaped vesicles. In all instances phospholipids diffused from one l obe to the other, but soluble dextran sometimes was unable to traverse the neck. This suggests that the diameter of the connecting neck may be variable.