Effects of taurocholate on the size of mixed lipid micelles and their associations with pigment and proteins in rat bile

Effect of beta-cyclodextrin dietary supplementation on biliary proteins and their resulting cholesterol nucleating activity in pigs

We explored the possibility that the biliary protein fraction may support part of the variation in the nucleating activity previously measured in gallbladder biles of pigs. Eighteen gallbladder aspirates freshly obtained from three dietary groups (0, 5, or 10% beta-cyclodextrin) of six pigs were chromatographed to purify their total protein fraction. Proteins were quantified, and analysed through electrophoresis and immunoblotting or enzyme-linked immunosorbent assay for albumin, and five putative effectors of cholesterol crystallisation, mucins, immunoglobulin A, 130 kDa, apolipoprotein A-I, and anionic polypeptide fraction. Each total protein fraction was also assayed for its ability to influence cholesterol precipitation, when added to supersaturated model bile. The current data provided evidence that the cholesterol crystallisation-promoting activity of biliary proteins in model biles increased with the beta-cyclodextrin dietary content. This occurred without any significant change in the total biliary protein content, but was associated with a significant decrease in the concentration of albumin and apolipoprotein A-I, resulting in changes in the overall balance of proteins in bile. Comparison of these results with the crystallisation figures previously obtained from the corresponding native biles led us to conclude that biliary proteins might influence the outcome of the crystallisation process, namely the final crystal concentration at equilibrium, but would not systematically represent a major driving force for determining the velocity of crystal formation in native bile of pigs.

Inducing cholesterol precipitation from pig bile with beta-cyclodextrin and cholesterol dietary supplementation

BACKGROUND/METHODS

In this study, pigs fed for 3 weeks a well-balanced semi-purified diet enriched with 0.3% cholesterol and 0, 5 or 10% beta-cyclodextrin were proposed as new animal donors of gallbladder bile exhibiting different rates of cholesterol crystallization, in order to gain insight into the early mechanisms underlying cholesterol precipitation in vivo. The appearance and growth of cholesterol crystals were monitored in the incubated freshly collected gallbladder biles through light microscopy and concomitant time-sequential determination of crystallized cholesterol concentration, and interpreted in terms of the composition of the bile.

RESULTS

Although the concentration of total lipids and proteins and the relative proportions of bile acids, phospholipids, and cholesterol remained unchanged under beta-cyclodextrin, the cholesterol crystallization increased in the following order: 0<<10<5% beta-cyclodextrin. Concomitantly, the proportion of chenodeoxycholic acid in bile, and the hydrophobicity index of the biliary bile acid mixture increased in the following order: 0<5<10% beta-cyclodextrin (the same as reported elsewhere for the decrease in the antinucleating ApoA1), while sn-2 arachidonoyl biliary lecithins were specifically increased with 5% beta-cyclodextrin in the diet.

CONCLUSIONS

We hypothesized that lecithin molecular species may be the determinant factor in modulating high cholesterol crystallization rates in biles otherwise enriched with hydrophobic bile acids.

An improved ultracentrifugation method for the separation of cholesterol carriers in bile

Vesicles and micelles, the major carriers of cholesterol in bile, play a role in the formation of cholesterol gallstones. A simple and rapid ultracentrifugation method was developed to isolate these biliary cholesterol carriers when only microliter amounts of bile were available. The proposed method employs a 46 to 0% sucrose density gradient, a NVT90 near-vertical rotor, and a centrifugation time of one hour. As little as 25 microL of bile can be used with no disruption of the carriers. The method was validated by comparison with gel filtration column chromatography using 6 mM taurocholate in the elution buffer. The sucrose linear density gradient ultracentrifugation procedure described here is simple, fast, and compares favorably with the gel filtration chromatography method for the separation of cholesterol carriers from bile.

Tubulovesicular transcytotic pathway in isolated rat hepatocyte couplets in culture. Effect of colchicine and taurocholate

Isolated rat hepatocyte couplets in short-term culture (6 h) were labeled for 3 min with horseradish peroxidase (HRP) to characterize the transcytotic vesicle transport pathway in this cell culture system that retained an "apical" canalicular membrane polarity. Microtubules were identified with monoclonal antibodies to beta-tubulin and fluorescein iso-thiocyanate-labeled goat-antimouse antibody and were concentrated in the apical domain, a structural polarity that was eliminated by pretreatment with colchicine. In control cells, HRP immediately labeled vesicles and tubules in the submembrane regions of the periphery of the cell. Within 10 min tubules and vesicles were prominently labeled in pericanalicular regions, a process blocked by colchicine but not by lumicolchine or taurocholate administration. A quantitative morphometric analysis utilizing a Zeiss Videoplan-2 image analyzer established that (a) HRP-containing structures increased in density, area, length, and diameter in the pericanalicular region by 10 min; (b) colchicine, but not lumicolchicine, pretreatment diminished their density, area, and length; and (c) taurocholate (50 microM), a choleretic and biliary lipid-stimulating bile acid, had no effect on HRP density or percentage of area in the pericanalicular region, but decreased the diameter of the pericanalicular HRP-containing structures and increased the percentage of tubules containing HRP from 29% to 40%. Tubules were particularly prominent in thick sections (400 nm) in both peripheral and pericanalicular regions and were viewed as continuous anastomosing linear arrays in stereo-paired micrographs. These studies established that isolated rat hepatocyte couplets maintain a highly polarized tubulovesicular transcytotic pathway in short-term culture that is micro-tubule-dependent. Taurocholate stimulates the transformation of tubules from vesicles in this isolated rat hepatocyte couplet system.

Identification of a phosphatidylcholine active phospholipase C in human gallbladder bile

This study describes the identification of a phospholipase C activity against phosphatidylcholine in delipidated human gallbladder bile. All biles were obtained from cholesterol gallstone patients and were negative on bacterial culture. The biliary enzyme was inhibited by EDTA and had a pH optimum of between 7-8. All of the 15 gallbladders examined contained significant phospholipase C activity (32.85 +/- 8.37 nmol/h/mg delipidated protein). The finding of a phospholipase C in gallbladder bile of patients with cholesterol gallstones may be one of the factors responsible for or related to the rapid in vitro nucleation seen in these biles.

Bilirubin excretion into bile after partial hepatectomy in rats

Biliary excretion of bilirubin was studied in Wistar rats at different intervals following two-thirds hepatectomy. Endogenous bilirubin excretion was not significantly modified during liver regeneration. The maximal biliary excretion rate (Tm) of bilirubin, expressed per 100 g of body weight, significantly decreased immediately after hepatectomy with a recovery to presurgery values by 16 days. Values expressed per g of liver were significantly lowered only by 12- and 24h posthepatectomy. In these early periods there was a significant increase in the liver concentration of unconjugated bilirubin and a decrease in bile flow following bile pigment infusion. Maximal excretion of bilirubin in the rat seems to change parallel to the regeneration of liver mass except during the first 24h in which the inhibitory effects exerted by the high load of exogenous bilirubin would cause a decline in its own biliary output with an added cholestatic effect.

Biliary proteins and the nucleation defect in cholesterol cholelithiasis

A study was performed to determine whether differences in gallbladder proteins might be present in patients with rapidly nucleating bile. Gallbladder and hepatic bile protein concentrations were measured using a fluorometric assay. The method was validated by an independent technique, i.e., hydrolysis and amino acid analysis. Persons with cholesterol gallstones had significantly higher gallbladder bile protein concentrations than patients without gallbladder disease or patients with pigment stones. The protein concentration correlated with the in vitro nucleation time in the cholesterol stone group. Gallbladder bile proteins were also purified by chromatography and gradient ultracentrifugation. Proteins from patients with cholesterol gallstones accelerated the nucleation time of control bile, whereas protein from controls had little effect. Hepatic bile protein concentrations were similar in persons with and without cholesterol gallstones. The gallbladder-to-hepatic bile ratios of a variety of solutes were examined. The ratio for protein in the cholesterol gallstone group can be explained straightforwardly by water reabsorption in the gallbladder, whereas the very low ratio in patients without cholesterol gallstones suggests that their gallbladders reduce protein mass by a process such as protein absorption or degradation during water absorption in the gallbladder.

Ultracentrifugal isolation of vesicular carriers of biliary cholesterol in native human and rat bile

We have utilized ultracentrifugation of native bile-Metrizamide density gradients to isolate a vesicular transport system of biliary lipids in both man and rat. We identified vesicular structures by electron microscopy. Fresh bile specimens were obtained from bile fistula rats (unsaturated bile) and from patients 1 week after bile duct surgery (supersaturated bile). Metrizamide was dissolved in bile (33% w/v), and continuous density gradients were performed with undiluted bile (density limits = 1.020 to 1.300 gm per ml). The relative distribution of biliary cholesterol, phospholipid and bile salt was studied as a function of the density of the fractions. Approximately 50% of total rat biliary cholesterol and between 61 and 90% of human biliary cholesterol was concentrated in the lightest fractions of the gradients (density less than 1.060 gm per ml). In contrast, less than 20% of bile salts was present in fractions with densities lower than 1.060 gm per ml. The highest amounts of bile salts and phospholipids of the bile-Metrizamide density gradients were found in the density range of 1.075 to 1.100 gm per ml in both human and rat bile. More than 80% of biliary proteins was found in fractions with densities greater than 1.075 gm per ml, and only 2% was found in the cholesterol-rich fraction with density less than 1.060 gm per ml in both species. When bile salt concentration was raised in rat bile from 38 to 97 mM by adding taurocholate, the low density cholesterol-rich fraction almost disappeared. Electron microscopy of negatively stained preparations of the fractions with density less than 1.060 gm per ml showed 40 to 120 nm vesicles, which were not apparent in the other fractions. Similar vesicles were demonstrated also in fresh rat bile and within the canaliculi after acute depletion of the bile salt pool (biliary bile salt concentration of 3.45 mM; total biliary lipid concentration of 0.25 gm%). The structure of these vesicles was shown in thin sections of liver specimens. They appeared as internal cavities surrounded by a single, continuous 6-nm-thick bilayer. These studies demonstrate that a high proportion of biliary cholesterol is transported in vesicles in human supersaturated native bile and that vesicular carriers are also responsible for the transport of a significant amount of biliary cholesterol in unsaturated rat bile. The presence of vesicles in unsaturated hepatic bile strongly supports the thesis that biliary lipids may be secreted as vesicles from the hepatocyte into the canaliculi.

Intrahepatic sources of biliary-like micelles

To study the hepatocellular origins of biliary lipids, bile acid-lipid micelles were isolated from rat liver subfractions in vitro by Sephadex G-100 column chromatography, using 10 mM sodium taurocholate in the column eluant. Micelles of similar size to biliary micelles were obtained from microsome suspensions and from Golgi content that had been isolated in 5 mM sodium taurocholate-containing subcellular fractionation media. The phospholipid composition of these "biliary-like' micelles was similar but not identical to that of biliary micelles; cholesterol was detected in the micelles from Golgi content but not microsomes. 'Biliary-like' micelles could not be isolated from plasma membranes (including highly purified canalicular membranes), Golgi membranes or liver cytosol. These data suggest that biliary lipid micelles (or micelle precursors) may originate from the endoplasmic reticulum and Golgi apparatus.

Phospholipid degradation in, and protein content of, rat fistula bile. Contamination of bile with pancreatic juice

The extent to which pancreatic juice can contaminate bile collected from a rat with a biliary fistula has been investigated by cannulating the bile duct proximal to either the duodenum or the liver, and by stimulating pancreatic flow with secretin. Bile collected via a fistula proximal to the duodenum showed marked pancreatic contamination. Thus, bile collected via a fistula proximal to the duodenum has a higher flow rate, a greater total protein and amylase content and a different polypeptide profile than bile collected via a fistula proximal to the liver. The phospholipid content also differed in that phosphatidylcholine was converted enzymically to lysophosphatidylcholine. Secretin increased bile flow and the biliary output of total protein and amylase when the fistula was proximal to the duodenum, but had no effect upon these parameters when the fistula was proximal to the liver, or in the isolated perfused rat liver.

Hydrolysis of intralipid by pancreatic lipase and phospholipase A2-gel filtration studies

Intralipid was incubated with pancreatic lipase (EC 3.1.1.3) and/or phospholipase A2 (EC 3.1.1.4) at two bile salts/phosphatidylcholine molar ratios and at two different triglyceride hydrolysis rates using various amounts of lipase. Incubations were studied by gel filtration. Results show: During lipase action, three phases of lipids coexist: an emulsified phase, a micellar phase and an intermediate heavy phase sized between the two others. The equilibrium between each phase is dependent upon the bile salts concentration. Under these conditions, pancreatic lipase was at 60% bound to the emulsified phase, whereas pancreatic phospholipase A2 was bound at 94% to the micellar phase.

Biliary proteins

The study of biliary proteins has grown enormously in the last 10 years. Although much has been recently learned about the nature, origin and hepatobiliary transport of these proteins, little is known of their function in bile or their effect on its physical state. This review will focus on description of the proteins and mechanisms by which they are secreted into bile.