Selective release of non-electrolytes from liposomes upon perturbation of bilayers by temperature change or polyene antibiotics.
A new system for assaying the permeability characteristics of liposomes was established using Amicon cells equipped with a membrane filter (pore size, 0.3 micrometer). In this system, damage of liposomes during the assay procedure was negligible. Changes in permeability to non-electrolytes, such as glucose (Mr 180), sucrose (Mr 342), inulin (Mr 5000) and dextran (Mr 75000), induced by perturbation of the bilayers were examined with this system. The following results were obtained on the barrier properties of multilamellar liposomes modified by various treatments. 1. Amphotericin B and nystatin did not cause any change in permeability to glucose of egg yolk phosphatidylcholine liposomes prepared in physiological saline and containing trace amounts of radioactive markers in their aqueous compartments. Both antibiotics, however, induced nonspecific release of glucose, sucrose, inulin and dextran from liposomes that contained 0.3 M glucose in their aqueous compartments. These antibiotics first seem to form pores through which small ions can permeate; Na+ and Cl- can enter the liposomes through these pores, whereas glucose in the liposomes cannot pass out. As a result, the liposomes become swollen with consequent severe disruption of their membranes. 2. Filipin and digitonin disrupted the membrane structures, resulting in release of large molecules such as dextran even in the absence of an osmotic mechanism. 3. Perturbation of the phase equilibrium by temperature change resulted in formation of 'pores'. The penetration of cations and anions through these 'pores' was apparently much faster than that of glucose, since when liposomes swollen in 0.3 M glucose were incubated in salt solution they were disrupted by an osmotic mechanism releasing not only glucose but also dextran. Most of the 'pores' were not large enough to allow passage of large non-electrolytes, such as inulin and dextran, since no appreciable amounts of these markers were released from liposomes under conditions where there should be no osmotic gradient. 4. At a temperature well above the phase transition temperature, egg yolk phosphatidylcholine liposomes exhibited specific release of glucose. This process did not involve an osmotic gradient, indicating that it was mainly due to diffusion of the solutes through the bilayers.