Osmotically-induced and photo-induced deformations of disk membranes.

Abstract

Abstract Effects of the osmotic pressure of suspending medium on the size and shape of isolated bovine rod outer segment disk membranes in the unbleached and bleached states were studied by elastic and quasielastic light scattering in dilute solutions of aqueous sucrose (0-0.1 wt.%). Data for the translational diffusion coefficients, D , and the elastically scattered intensities were analyzed with use of the oblate ellipsoidal shell model, and the axial ratio, ρ, and the major semiaxis, b , of the ellipsoid were deduced as a function of the concentration, c , of sucrose for each photochemical state (unbleached and bleached) of the membranes. The unbleached and bleached membranes suspended in water ( c = 0 ) were found to be spherical vesicles, i.e., ρ = 1 , of 0.49 μm radius. As c was increased, however, ρ for the bleached membranes rather steeply increased approaching 3 at c = 0.1 wt.% , whereas that for the unbleached membranes gradually increased (up to 2 at c = 0.1% ) after c exceeded 0.02%; at a fixed c , ρ increased upon bleaching. In all the cases, the deformation took place while b remained unchanged. The resulting contraction of the intravesicular volume induced either by the photo-bleaching or the osmotic pressure was accompanied by a significant decrease in the surface area of the membranes. The photo-induced contractions in the volume and the surface area amounted to 30–45% and 10–20%, respectively, in the concentration range from 0.02 to 0.1%. The dependence of D on c for each photochemical state was quantitatively explained by a simple model based on the Ca+ efflux from the intravesicular space to the bathing medium upon bleaching.