Vesicles of variable sizes produced by a rapid extrusion procedure.

Previous studies from this laboratory have shown that large unilamellar vesicles can be efficiently produced by extrusion of multilamellar vesicles through polycarbonate filters with a pore size of 100 nm (Hope, M.J., Bally, M.B., Webb, G. and Cullis, P.R. (1985) Biochim. Biophys. Acta 812, 55-65). In this work it is shown that similar procedures can be employed for the production of homogeneously sized unilamellar or plurilamellar vesicles by utilizing filters with pore sizes ranging from 30 to 400 nm. The unilamellarity and trapping efficiencies of these vesicles can be significantly enhanced by freezing and thawing the multilamellar vesicles prior to extrusion. This procedure is particularly applicable when very high lipid concentrations (400 mg/ml) are used, where extrusion of the frozen and thawed multilamellar vesicles through 100 and 400 nm filters results in trapping efficiencies of 56 and 80%, respectively. Freeze-fracture electron microscopy revealed that vesicles produced at these lipid concentrations exhibit size distributions and extent of multilamellar character comparable to systems produced at lower lipid levels. These results indicate that the freeze-thaw and extrusion process is the technique of choice for the production of vesicles of variable sizes and high trapping efficiency.