AJP - Gastrointestinal and Liver Physiology, Vol 260, Issue 1 70-G79, Copyright © 1991 by American Physiological Society
Interconversions of lipid aggregates in rat and model bile
T. E. Little, S. P. Lee, H. Madani, E. W. Kaler and K. Chinn
Department of Medicine, University of Washington, Seattle 98195.
The dynamic interchange of cholesterol and the phase transition between
nonmicellar and micellar aggregates in rat and model bile were
characterized with gel-permeation chromatography, quasi-elastic light
scattering, turbidity measurements, and by radiolabeling lipid aggregates
in bile. Cholesterol partitioned into either the micellar or nonmicellar
phases independent of the lipid aggregate structure. In model bile,
increasing bile salt concentrations led to a decrease in the relative
proportion of nonmicellar aggregates beginning at 5 mM taurocholate (TC),
while the relative cholesterol content of the nonmicellar fraction
increased from 1.0 to 2.7 +/- 2.0 (means +/- SD). In rats, creation of a
biliary fistula resulted in a decrease of bile salts from 41 to 4 mM. Mixed
micelles increased from 25 to 120 A in radius, while nonmicellar aggregates
increased from 180 to 800 A in radius. Addition of TC to model bile
(cholesterol:lecithin = 1:1) vesicles with total lipid concentrations less
than 7 mM yielded a progressive shift of vesicles (450 A) to mixed micelles
(30 A). For mixtures with higher total lipid concentrations, addition of TC
promoted substantial vesicle aggregation and resulted in formation of a
third phase containing lipid aggregates larger in size than the initial
vesicles. These results suggest that rapid exchange of cholesterol occurs
in bile and that significant remodeling of vesicles can occur. These
alterations in vesicles include both enrichment in cholesterol content and
formation of larger aggregates during increases in bile salt concentration.
