EFFECTS OF SUBMICELLAR BILE-SALT CONCENTRATIONS ON BIOLOGICAL MEMBRANE-PERMEABILITY TO LOW-MOLECULAR-WEIGHT NONIONIC SOLUTES

Bile salts have been hypothesized to mediate cytotoxicity by increasin g membrane permeability to aqueous solutes. We examined whether submic ellar bile salt concentrations affect model and native membrane permea bility to small uncharged molecules such as water, urea, and ammonia. Osmotic water permeability (P-f) and urea permeability were measured i n large unilamellar vesicles composed with egg yolk phosphatidylcholin e (EYPC) +/- cholesterol (Ch) or rat liver microsomal membranes by mon itoring self-quenching of entrapped carboxyfluorescein (CF). Ammonia p ermeability was determined utilizing the pH dependence of CF fluoresce nce. Submicellar bile salt concentrations did not significantly alter P-f of EYPC +/- Ch or rat liver microsomal membranes. At taurodeoxycho late (TDC) or tauroursodeoxycholate concentrations approaching those t hat solubilized membrane lipids, CF leakage occurred from vesicles, bu t P-f remained unchanged. Higher bile salt concentrations (0.5-2 mM TD C) did not alter P-f of equimolar EYPC/Ch membranes. The activation en ergy for transmembrane water flux was unchanged (12.1 +/- 1.2 kcal/mol for EYPC) despite the presence of bile salts in one or both membrane hemileaflets, suggesting strongly that bile salts do not form transmem brane pores that facilitate water flux. Furthermore, submicellar bile salt concentrations did not increase membrane permeability to urea or ammonia. We conclude that at submicellar concentrations, bile salts do not form nonselective convective channels that facilitate transmembra ne transport of small uncharged molecules. These results suggest that bile salt-mediated transport of specific substrates, rather than nonse lective enhancement of membrane permeability, underlies bile salt cyto toxicity for enterocytes and hepatocytes.