Standing-gradient osmotic flow. Examination of its validity using an analytical method.

The solutions to the non-linear differential equations governing solute-solvent coupling in the intercellular spaces of epithelial layers have been obtained by using an analytical method, rather than the usual numerical ones. When the present series solution includes second-order correction terms, the concentration and velocity profiles obtained by the analytical method agree very well with those coming from numerical solutions. This method has further allowed us to examine the standing-gradient hypothesis when applied to the backwards fluid transport system of the corneal endothelium. With the information presently available for the relevant parameters (osmotic permeability, rate of transport, radius and length of the spaces, and location of the pumping sites), near-isotonicity of the transported fluid would not be explained by the standing-gradient model.

Monomeric human red cell glucose transporter (Glut1) in non-ionic detergent solution and a semi-elliptical torus model for detergent binding to membrane proteins.

The self-association state of the human red cell glucose transporter (Glut1) in octaethylene glycol n-dodecyl ether (C12E8) and n-octyl beta-D-glucopyranoside (OG) solution was analyzed in the presence of reductant by gel filtration with light-scattering, refractivity and absorbance detection, and by ultracentrifugation. The C12E8-Glut1 complex was essentially monomeric, whereas OG-Glut1 also formed dimers and larger oligomers. C12E8-Glut1 retained substantial glucose transport activity even after depletion of endogenous lipids by gel filtration, as shown by reconstitution and transport measurements. Removal of endogenous lipids from OG-Glut1 abolished the activity unless phosphatidylcholine was included in the eluent. The binding of C12E8 and OG to Glut1 was determined by gel filtration with refractivity and absorbance detection or with radioactive tracer to be 1.86 +/- 0.07 and 1.84 +/- 0.09 g/g polypeptide, respectively. A structural model was proposed in which non-ionic detergent forms a semi-elliptical torus (SET) surrounding the transmembrane protein. The torus thickness was assumed to be equal to the radius (short half-axis) of a spherical (oblate ellipsoidal) free detergent micelle and the polar head groups of the detergent molecules were predicted to be situated just outside the hydrophobic surface of the protein. The experimental detergent binding values and those obtained from the SET model together confirmed that Glut1 was monomeric in C12E8 solution and provided constraints on the shape and size of the hydrophobic transmembrane region of Glut1 in alpha-helical and beta-barrel topology models.

The effect of vitreous humor on fluid transport by rabbit corneal endothelium.

The effects of vitreous humor and some of its components on the function of rabbit corneal endothelium were investigated and compared to those of artificial and natural aqueous humor. The rate of fluid pumping across the endothelium was measured while that layer was in contact with: (1) complete medium, an artificial aqueous solution; (2) pooled rabbit aqueous humor; (3) filtered and (4) unfiltered pooled rabbit vitreous humor; and (5) solid residue from vitreous humor. The relative magnitudes of the rates were: complete medium greater than aqueous larger than or equal to filtered vitreous greater than vitreous larger than or equal to solid residue. A definite adverse effect of unfiltered vitreous and of its solid residue on endothelial pumping was observed. It is suggested that the solid (collagenous) elements in vitreous humor may be responsible for these effects and perhaps for those generically referred to as the "vitreous touch syndrome" in clinical studies.