Small and large unilamellar vesicle membranes as model system for bile acid diffusion in hepatocytes

Uptake of bile acids into the liver cell occurs via active transport or pas sive diffusion, In a model system, passive diffusion was studied in liposom es using pyranine fluorescence. Rate constants for the diffusion of diverse more polar or more apolar bile acids were examined. Hydrophobic lithocholi c acid (LCA) revealed a maximal rate constant of 0.057 s(-1); with the pola r ursodeoxycholic acid (UDCA), the value was 0.019 s(-1). UDCA (3 mol%) eff ectively decreased the rate constant of 0.1 mM chenodeoxycholic acid (CDCA) , whereas cholesterol reached a similar decrease only between 5 and 10 mol% . At higher concentrations of CDCA (above 1 mM) or LCA (0.3-0.4 mM), breaki ng up of liposomal structure was confirmed by light-scattering decrease and increase of carboxyfluorescein fluorescence. Changes in lipid composition of phosphatidylcholine (PC)- small unilamellar vesicles (SUVs) or large uni lamellar vesicles (LUVs) also caused decreasing rate constants. For a cardi olipin (CL):PC ratio of 1:20 the CDCA (0.1 mM) rate constant was 71% lower (0.015 s(-1)) and for a sphingomyelin (SM):PC ratio of 2:1 the rate constan t was 50% lower (0.026 s(-1)). Changes in membrane fluidity were detected u sing membrane anisotropy measurements with the 1,6-diphenyl-1,3,5-hexatrien e (DPH) method. Membrane fluidity was reduced with cholesterol- but not wit h CL- or SM-containing SUVs (ratio: cholesterol, CL, SM:PC of 1:5), This mo del system is currently used for the analysis of more complex lipid vesicle s resembling the plasma/hepatocyte membrane, which is either stabilized or destabilized by appropriate conditions. The results should become clinicall y relevant. (C) 1999 Academic Press.