The lipid fluidity of rat colonic brush-border membrane vesicles modulates Na+-H+ exchange and osmotic water permeability.

Brush-border membrane vesicles were prepared from rat colonic epithelial cells. Steady-state fluorescence polarization techniques, using the fluorophores 1,6-diphenyl-1,3,5-hexatriene and DL-12-(9-anthroyl)stearic acid (12-AS), revealed that benzyl alcohol (25-75 mM) but not methyl alcohol (50-125 mM) significantly increased the fluidity of these vesicles. Benzyl alcohol (50 and 75 mM) but not methyl alcohol also increased amiloride-sensitive sodium-stimulated proton efflux from these vesicles at all concentrations of sodium tested (2.5-50.0 mM), as assessed by changes in the fluorescence of acridine orange. Benzyl alcohol, at 50 and 75 mM concentrations, increased the maximal velocity (Vmax) of this exchange process by approximately 58 and 75%, respectively. Neither concentration, however, altered the Km for sodium. Osmotic water flow, measured as rate constants of osmotic shrinkage of these vesicles using a stopped-flow nephelometric technique, was also increased by 75 mM benzyl alcohol but not by a similar concentration of methyl alcohol. The present data, therefore, demonstrate that the fluidity of rat colonic brush-border membranes can influence Na+-H+ exchange and osmotic water flow across these vesicles.