Relationship between lipid fluidity and water permeability of bovine tracheal epithelial cell apical membranes

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Abstract
Apical membrane vesicles were prepared from bovine tracheal epithelial cells. These membranes were enriched in alkaline phosphatase specific activity 35-fold compared to cellular homogenates. Steady-state fluorescence polarization studies of these membranes, using three fluorophores, demonstrated that they possessed a relatively low fluidity. Studies using the probe 1,6-diphenyl-1,3,5-hexatriene detected thermotropic transitions at 25.7 .+-. 0.4 and 26.8 .+-. 0.6.degree. C in these membranes and their liposomes, respectively. Analysis of the composition of these membranes revealed a fatty acyl saturation index of 0.59 .+-. 0.02, a protein/lipid ratio (w/w) of 0.60 .+-. 0.06, a cholesterol/phospholipid ratio (mol/mol) of 0.83 .+-. 0.11, and a sphingomyelin/lecithin ratio (mol/mol) of 0.64 .+-. 0.10. Membrane vesicles were osmotically active when studied by a stopped-flow nephelometric technique. Arrhenius plots of rates of osmotic water efflux demonstrated break points at approximately 28 and 18.degree. C, with activation energies of 16.7 .+-. 0.2 kcal mol-1 from 35 to 28.degree. C, 8.3 .+-. 0.5 kcal mol-1 from 28 to 18.degree. C, and approximately 3.0 kcal mol-1 below 18.degree. C. Treatment of membrane vesicles with benzyl alcohol, a known fluidizer, decreased lipid order (increased fluidity) and increased the rate of osmotic water efflux. The present results suggest that water crosses tracheal epithelial cell apical membranes by solubility-diffusion across the lipid domain and that increases in fluidity correlate with increases in the water permeability of these membranes.