Quasielastic light scattering evidence for vesicular secretion of biliary lipids
Cohen, D.E.; Angelico, M.; Carey, M.C.
American Journal of Physiology 257(1 Pt 1): G1-G8
1989
ISSN/ISBN: 0002-9513 PMID: 2750900 Document Number: 337296
We employed quasielastic light scattering, negaive-stain, and freeze-fracture electron microscopy to study the time-dependent physicochemical behavior of biliary lipids in fresh rat bile. Three to five minutes after bile collection, the earliest light scattering measurements and electron microscopy revealed unilamellar vesicles (mean hydrodynamic radius, .hivin.Rh = 430-740 .ANG.) coexisting with mixed micelles (.hivin.Rh = 20-120 .ANG.) in all biles. Both percent biliary vesicels (1 to > 70%) and micellar sizes varied inversely with bile salt concentration (range 1.6-72 mM) both during endogenous pool drainage and sodium taurocholate infusion. With bile salt concentrations in the vicinity of or below the estimated critical micellar concentration, biliary vesicle concentrations remained constant or increased slightly with passage of time. However, with micellar bile salt concentrations, complete conversion of vesicles to micelles occurred at rates that were directly proportional to bile salt concentration. Back-extrapolation of weighted .hivin.Rh averages of micelles plus vesicles as functions of time gave sizes of .apprx. 470 .ANG. at 1 min, suggesting the predominance of homogeneously sized unilamellar vesicles at the earliest stages of bile formation. After micellization of lipids, mixed protein aggregates of vesicle size were demonstrated in all biles. These experiments elucidate the dynamic coexistence of lipid vesicles and mixed micelles in cholesterol unsaturated biles and demonstrate that vesicle-micelle interconversions of biliary lipid aggregates are normal physiological phenomena within the biliary tree. Our results are consistent with the hypothesis that, during bile formation, biliary lecithin and cholesterol are cosecreted into bile canaliculi as unilamellar vesicles and that mixed micelle formation is achieved not only by micellar concentrations of bile salts but also with passage of time during bile flow.