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, of constant-tension (σ)-induced rupture of dioleoylphosphatidylcholine (DOPC)-GUVs using the method developed by us recently. We obtained the σ dependence of k 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. Next, we measured the volume change of DOPC-GUVs under small Π. The experimentally obtained values of σ 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 corresponding to the threshold Π at which pore formation is induced is similar to the threshold tension of the σ-induced rupture. The time course of the radius change of GUVs in the Π-induced pore formation depends on the total membrane tension, σ; for small σ, the radius increased with time to an equilibrium one, which remained constant for a long time until pore formation, but for large σ, the radius increased with time and pore formation occurred before the swelling equilibrium was reached. Based on these results, we discussed the σ and the Π-induced pore formation in lipid membranes.