Preparation of giant liposomes in physiological conditions and their characterization under an optical microscope

Unilamellar liposomes with diameters of 25-100 μm were prepared in various physiological salt solutions, e.g., 100 mM KCl plus 1 mM CaCl₂. Successful preparation of the giant liposomes at high ionic strengths required the inclusion of 10-20% of a charged lipid, such as phosphatidylglycerol, phosphatidylserine, phosphatidic acid, or cardiolipin, in phosphatidylcholine or phosphatidylethanolamine. Three criteria were employed to identify unilamellar liposomes, yielding consistent results. Under a phase-contrast microscope those liposomes that showed the thinnest contour and had a vigorously undulating membrane were judged unilamellar. When liposomes were stained with the lipophilic fluorescent dye octadecyl rhodamine B, fluorescence intensities of the membrane of individual liposomes were integer multiples (up to four) of the lowest ones, the least fluorescent liposomes being those also judged unilamellar in the phase-contrast image. Micropipette aspiration test showed that the liposomes judged unilamellar in phase and fluorescence images had an area elastic modulus of ~160 dyn/cm, in agreement with literature values. The giant liposomes were stable and retained a concentration gradient of K⁺ across the membrane, as evidenced in fluorescence images of the K⁺-indicator PBFI encapsulated in the liposomes. Ionophore-induced K⁺ transport and associated volume change were observed in individual liposomes.