Quasi-elastic light scattering studies of native hepatic bile from the dog: comparison with aggregative behavior of model biliary lipid systems

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Abstract
Using quasi-elastic light scattering (QLS), the macromolecular components in hepatic bile obtained from the dog were characterized, and results were compared with data from model bile solutions containing the bile salt (BS) sodium taurocholate (TC), egg lecithin (L) and cholesterol (Ch). Native bile samples were obtained by direct catheterization of the common bile duct in a previously cholecystectomized dog fitted with a Thomas duodenal cannula. Hepatic bile was sampled during 3 secretory states: unstimulated fasting bile (A); stimulated secretion during an i.v. TC infusion (B) and secretin-stimulated secretion (C). All 3 samples had comparable molar ratios of L/BS (0.21 .+-. 0.03) and Ch/L (0.027 .+-. 0.006) but differed in the total lipid concentration (BS + L + Ch): 13.1 .+-. 0.8, 6.7 .+-. 0.8, and 3.0 .+-. 0.4 g/dl, respectively. From the QLS autocorrelation functions measured on samples B and C, 3 macromolecular components (denoted 1.alpha., 1.beta. and 2) were resolved. Component 1.alpha. (hydrodynamic radius R1.alpha. = 10 .+-. 2 .ANG.) is comparable in size to the micellar aggregates of model systems. Component 1.beta. (R1.beta. = 67 .+-. 7 .ANG.) appears to reflect an average of biliary proteins. Component 2 (R2 = 650 .+-. 15 .ANG.) is a trace component whose size and sedimentation behavior are compatible with those of the canalicular membrane vesicles postulated to be present in bile. Serial dilution of the B and C bile samples with Tris buffer (0.15 M NaCl, pH 8.0) showed a remarkable similarity in the behavior of the 1.alpha. component as compared to the mean hydrodynamic radius .hivin.Rh of similarly diluted model bile solutions. When a critical dilution factor, d.gamma., is reached, .hivin.Rh increases abruptly from .apprx. 30-400 .ANG.. Above a 2nd dilution factor, d.alpha., it then decreases to a value of .apprx. 150 .ANG.. Similar results were obtained on sample A but were shifted to higher dilutions. Such behavior is consistent with the presence of mixed disk micelles in native bile which undergo a micelle-to-vesicle transition upon dilution. From the d.gamma. and d.alpha. values, estimates of the intermicellar bile salt concentrations were made for all 3 samples (range 1.4-6.2 mM) which agree well with previous experimental results on model and native bile. These studies offer compelling evidence for the existence of micellar aggregates in native bile whose size, structure and equilibria are similar to those found in model bile solutions.