TENSION-STABILIZED PORES IN GIANT VESICLES - DETERMINATION OF PORE-SIZE AND PORE LINE TENSION

We present the first observations of giant, long-existing, stabilized pores in vesicle membranes. Using a new experimental technique for stu dying the electropermeabilization of lipid membranes, giant liposomes (from 25 to 56 mum in diameter) were subjected to single, square, elec tric pulses (duration 150 mus and electric field strength from 63 to 1 26 kV/m). The liposomes were held by a micropipet and small membrane t ensions were created by controlling the pipet suction pressure. The li posomes were loaded with media having different refractive index from the outside solution, and, under these conditions, the formation of po res in the pressurized liposome could be visualized by the jet of insi de solution that flowed out from the membrane pore. By adjusting the m embrane tension, pores were kept open, and pore lifetimes could be var ied from tenths of a second to several seconds. The pore size was dete rmined from the volumetric flow in the pore region and the measured pr essure differences across the bilayer. It was clear from the experimen ts that only one pore remained opened after the pulse. The estimated p ore radii were on the order of one micrometer. The pores were in a qua si-stationary state and when they closed they did so spontaneously in a quick process (in milliseconds). The isotropic membrane tension was determined for the same measurements and from determinations of both p ore size and dynamic membrane tension the pore line tension was found. The line tension of the pore region was determined for two lipid comp ositions, stearoyloleoylphosphatidylcholine and stearoyloleoylphosphat idylcholine with 50 mol% cholesterol, and the obtained values for sing le bilayers were (0.92 +/- 0.07) - 10(-11) N and (3.05 +/- 0.12) . 10( -11) N, respectively.