Experimental Estimation of Membrane Tension Induced by Osmotic Pressure

Osmotic pressure (Π) 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 Π. 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 Π on the rate constant, k_p, of constant-tension (σ_ex)-induced rupture of dioleoylphosphatidylcholine (DOPC)-GUVs using the method developed by us recently. We obtained the σ_ex dependence of k_p in GUVs under Π and by comparing this result with that in the absence of Π, we estimated the tension of the membrane due to Π at the swelling equilibrium, σ_osm^eq. Next, we measured the volume change of DOPC-GUVs under small Π. The experimentally obtained values of σ_osm^eq and the volume change agreed with their theoretical values within the limits of the experimental errors. Finally, we investigated the characteristics of the Π-induced pore formation in GUVs. The σ_osm^eq corresponding to the threshold Π at which pore formation is induced is similar to the threshold tension of the σ_ex-induced rupture. The time course of the radius change of GUVs in the Π-induced pore formation depends on the total membrane tension, σ_t; for small σ_t, the radius increased with time to an equilibrium one, which remained constant for a long time until pore formation, but for large σ_t, the radius increased with time and pore formation occurred before the swelling equilibrium was reached. Based on these results, we discussed the σ_osm^eq and the Π-induced pore formation in lipid membranes.