Bilayer membranes exhibit an elastic resistance to changes in curvature. This resistance depends both on the intrinsic stiffness of the constituent monolayers and on the curvature-induced expansion or compression of the monolayers relative to each other. The monolayers are constrained by hydrophobic forces to remain in contact, but they are capable of independent lateral redistribution to minimize the relative expansion or compression of each leaflet. Therefore, the magnitude of the expansion and compression of the monolayers relative to each other depends on the integral of the curvature over the entire membrane capsule. The coefficient characterizing the membrane stiffness resulting from relative expansion is the nonlocal bending modulus kr. Both the intrinsic (local) bending modulus (kc) and the nonlocal bending modulus (kr) can be measured by the formation of thin cylindrical membrane strands (tethers) from giant phospholipid vesicles. Previously, we reported measurements of kc based on measurements of tether radius as a function of force (Song and Waugh, 1991, J. Biomech. Engr. 112:233). Further analysis has revealed that the contribution from the nonlocal bending stiffness can be detected by measuring the change in the aspiration pressure required to establish equilibrium with increasing tether length. Using this approach, we obtain a mean value for the nonlocal bending modulus kr of approximately 4.1 x 10(-19)J. The range of values is broad (1.1-10.1 x 10(-19)J) and could reflect contributions other than simple mechanical equilibrium. Inclusion of the nonlocal bending stiffness in the calculation of kc results in a value for that modulus of approximately 1.20 +/- 0.17 x 10(-19)J, in close agreement with values obtained by other methods.

Bilayer membranes in the fluid state exhibit a large resistance to changes in surface area, negligible resistance to surface shear deformation, and a small but finite resistance to bending. The presence of cholesterol in the membrane is known to increase its resistance to area dilation. In this report, a new method for measuring bilayer membrane bending stiffness has been used to investigate the effect of cholesterol on the bending rigidity of SOPC (1,stearoyl-2,oleoyl-phosphatidylcholine) membranes. The curvature elasticity (kc) for membranes saturated with cholesterol was measured to be 3.3 x 10(-19) J, approximately 3-fold larger than that the modulus for cholesterol-free SOPC membrane. These findings are consistent with previous measurements of bending stiffness based on thermal fluctuations, which showed a similar approximately 3-fold increase in the modulus with cholesterol addition (Evans and Rawicz, 1990, Phys. Rev. Lett. 64:2094) and provide further substantiation of the important contribution that cholesterol makes to membrane cohesion and stability.

The insulin-sensitizing compound troglitazone has evolved into a promising therapeutic agent for type II diabetes. It improves insulin sensitivity and lipoprotein metabolic profiles and lowers blood pressure in humans and rodents. Because troglitazone has insulin-like effects on a number of tissues, we hypothesized that it may reduce vascular tone through stimulation of endothelial-derived nitric oxide (NO) production or by diminution of vascular smooth muscle cell (VSMC) intracellular calcium ([Ca2+]i). Our results show that troglitazone decreases norepinephrine-induced contractile responses in the rat tail artery, an effect not reversed by the NO inhibitor L-nitroarginine methyl ester (L-NAME). In contrast, troglitazone significantly inhibited L-type Ca2+ currents in freshly dissociated rat tail artery and aortic VSMCs and in cultured VSMCs. The data suggest that troglitazone attenuates vascular contractility via a mechanism involving VSMC [Ca2+]i but independent from endothelial generation of NO. Because insulin has been shown to affect vascular tone by both of these mechanisms, troglitazone only partially mimics insulin action in this tissue.

Recently, a new approach to measure the bending stiffness (curvature elastic modulus) of lipid bilayer membrane was developed (Biophys. J., Vol. 55; pp. 509-517, 1989). The method involves the formation of cylindrical membrane strands (tethers) from bilayer vesicles. The bending stiffness (B) can be calculated from measurements of the tether radius (Rt) as a function of the axial force (f) on the tether: B = f.Rt/2 pi. In the present report, we apply this method to determine the bending stiffness of bilayer membranes composed of mixtures of SOPC (1-stearoyl-2-oleoyl phosphatidyl choline) and POPS (1-palmitoyl-2-oleoyl phosphatidyl serine). Three different mixtures were tested: pure SOPC, SOPC plus 2 percent (mol/mol) POPS, and SOPC plus 16 percent POPS. The bending stiffness determined for these three different lipid mixtures were not significantly different (1.6-1.8 x 10(-12) ergs). Because POPS carries a net negative charge, these results indicate that changes in the density of the membrane surface charge have no effect on the intrinsic rigidity of the membrane. The values we obtain are consistent with published values for the bending stiffness of other membranes determined by different methods. Measurements of the aspiration pressure, tether radius and the tether force were used to verify a theoretical relationship among these quantities at equilibrium. The ratio of the theoretical force to the measured force was 1.12 +/- 0.17.

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