Experimental Estimation of Membrane Tension Induced by Osmotic Pressure

Osmotic pressure (II) induces the stretching of plasma membranes of cells or lipid membranes of vesicles, which plays various roles in physiological functions. However, there have been no experimental estimations of the membrane tension of vesicles upon exposure to II In this report, we estimated experimentally the lateral tension of the membranes of giant unilamellar vesicles (GUVs) when they were transferred into a hypotonic solution. First, we investigated the effect of II on the rate constant, k(p), of constant-tension (sigma(ex))-induced rupture of dioleoylphosphatidylcholine (DOPC)-GUVs using the method developed by us recently. We obtained the sigma(ex) dependence of k(p) in GUVs under II and by comparing this result with that in the absence of II, we estimated the tension of the membrane due to II at the swelling equilibrium, sigma(eq)(osm) Next, we measured the volume change of DOPC-GUVs under small II. The experimentally obtained values of sigma(eq)(osm) and the volume change agreed with their theoretical values within the limits of the experimental errors. Finally, we investigated the characteristics of the sigma(ex)-induced pore formation in GUVs. The sigma(eq)(osm) corresponding to the threshold II at which pore formation is induced is similar to the threshold tension of the gex-induced rupture. The time course of the radius change of GUVs in the II -induced pore formation depends on the total membrane tension, sigma(t); for small at, the radius increased with time to an equilibrium one, which remained constant for a long time until pore formation, but for large at, the radius increased with time and pore formation occurred before the swelling equilibrium was reached. Based on these results, we discussed the sigma(eq)(osm) and the II-induced pore formation in lipid membranes.