Partial characterization of regulation of biliary lecithin hydrophobicity: association with organic anion-induced solute cholestasis in rats

Modifying hepatic phospholipid synthesis associates with biliary phospholipid secretion rate in a transporter-independent manner in rats: relation to canalicular membrane fluidity

Biliary phospholipid secretion is mediated by a multidrug resistance gene product, and its molecular subselection occurs at the site of secretion to modulates bile metastability. The aim of this study was to determine the effect of modifying hepatic phospholipid synthesis on canalicular phospholipid transporter expression and membrane fluidity. Bile-duct cannulation was performed in male Sprague-Dawley rats pretreated with or without intravenous infusion of dimethylethanolamine, an intermediate phospholipid metabolite along the pathway of phosphatidylcholine synthesis of phosphatidylethanolamine N-methylation (0.01 mg/min/100 g body wt) for 15 hr, followed by sodium taurocholate infusion (50 nmol/min/100 g body wt) with or without sulfobromophthalein (50 nmol/min/100 g body wt). Dimethylethanolamine enhanced biliary phospholipid secretion in association with a decrease in biliary phospholipid hydrophobicity. Dimethylethanolamine also increased canalicular membrane fluidity defined by 1,6-diphenyl-1,3,5-hexatriene fluorescence depolarization, whereas the expression of multidrug resistance gene product and multidrug resistance associated protein was unchanged. In contrast, a disproportionate reduction of biliary phospholipid secretion caused by sulfobromophthalein (uncoupling) was enhanced by under the treatment with dimethylethanolamine. In conclusion, the increase in biliary phospholipid secretion and canalicular membrane fluidity without a drastic change of its canalicular transporter by dimethylethanolamine suggests that such a canalicular membrane fluidity facilitates the transporter activity and/or phospholipid molecular movement from the canalicular outer membrane into the bile. A more drastic reduction in phospholipid secretion under sulfobromophthalein-caused uncoupling indicates the possibility of a preferential distribution of relatively hydrophilic phosphatidylcholine molecules to bile salt micelles since sulfobromophthalein is known to reduce the micellar capacity to extract membrane lipids for biliary secretion.

Dose-dependent conjugation of sulfobromophthalein and hepatic transit time in bile fistula rats: role of the microtubule-dependent vesicle pathway

Sulfobromophthalein (BSP) is selectively taken up by the liver and secreted into the bile as unconjugated and conjugated forms. Our previous study demonstrated that unconjugated BSP, but not conjugated BSP, caused the dissociation of biliary lipid secretion from that of bile acids, suggesting that the hepatic BSP conjugation rate partly regulated biliary lipid secretion. To evaluate the mechanisms through which biliary lipid secretion is regulated by exogenous organic anions, we intravenously administered BSP to male Sprague-Dawley rats at various doses either continuously or as a bolus. Then the relationship of the dose of BSP to its conjugation rate, hepatic transit time, and biliary lipid secretion was determined. BSP decreased biliary secretion of cholesterol and phospholipids in a dose-dependent manner without affecting bile acid secretion. In contrast, the proportion of conjugated BSP in bile was associated with the dose. Although the serum clearance of BSP after bolus infusion was constant regardless of the dose administered (50 or 200 nmol/100 g), BSP secretion was delayed with increasing doses: unconjugated BSP was secreted predominantly in the early phase (0-15 min after bolus injection), and conjugated BSP was the predominant form in the late phase (15-30 min). Pretreatment with colchicine reduced the conjugation rate and hepatic transit time of BSP, suggesting that the microtubule-dependent vesicle pathway plays a role in biliary excretion and conjugation of BSP. We conclude that biliary lipid secretion is influenced by organic anions with an affinity for bile acids such as BSP and that this effect is dependent upon the hepatic metabolic rate, i.e., conjugation rate. The hepatic transit time also plays a key role in this process by influencing metabolism.

Cyclosporin A reduces canalicular membrane fluidity and regulates transporter function in rats

Changes of the biliary canalicular membrane lipid content can affect membrane fluidity and biliary lipid secretion in rats. The immunosuppressant cyclosporin A is known to cause intrahepatic cholestasis. This study investigated whether cyclosporin A influenced canalicular membrane fluidity by altering membrane phospholipids or transporter expression. In male Sprague-Dawley rats, a bile-duct cannula was inserted to collect bile, and sodium taurocholate was infused (100 nmol/min per 100 g) for 60 min. During steady-state taurocholate infusion, cyclosporin A (20 mg/kg) or vehicle was injected intravenously and then bile was collected for 80 min. After killing the rats, canalicular membrane vesicles were prepared. Expression of canalicular membrane transporters was assessed by Western blotting and canalicular membrane vesicle fluidity was estimated by fluorescence polarization. Cyclosporin A reduced biliary lipid secretion along with a disproportionate reduction of lipids relative to bile acids. Cyclosporin A significantly decreased canalicular membrane fluidity along with an increase of the cholesterol/phospholipid molar ratio. Only expression of the transporter P-glycoprotein was increased by cyclosporin A. Because canalicular membrane transporter expression was largely unchanged by cyclosporin A despite a marked decrease of biliary lipid secretion, transporter activity may partly depend upon canalicular membrane fluidity.

Transcytotic vesicle fusion is reduced in cholestatic rats: redistribution of phospholipids in the canalicular membrane

Cholestasis, which affects phospholipid trafficking, therefore would be expected to alter canalicular membrane phospholipid composition and fluidity, as well as fatty acid composition of membrane phospholipid. These alterations may affect transcytotic vesicle fusion and would be expected to cause variety of cholestatic phenomena. The aim of this study was to determine the effect of cholestasis on transcytotic vesicle fusion. Sprague-Dawley rats with extrahepatic and intrahepatic cholestasis were prepared by bile duct ligation (6 hr or three days) and phalloidin infusion (0.4 mg/kg body weight), respectively. Liposomes of phosphatidylserine/phosphatidylcholine were labeled with octadecyl rhodamine B chloride. Fusion of liposomes to canalicular membrane vesicle preparations from cholestatic and control rats was induced by the addition of calcium. The degree of fusion was evaluated by measuring the increase in rhodamine fluorescence. Membrane phospholipid content also was analyzed. Rates of liposomal fusion to membranes from cholestatic rats were decreased compared to controls. The saturated/unsaturated and saturated/polyunsaturated fatty acid ratios were increased in membrane phosphatidylcholine and decreased in membrane sphingomyelin from cholestatic rats. Cholesterol/phospholipid ratios were increased. Thus, in the presence of cholestasis, a redistribution of phospholipid species within canalicular membranes is associated with decreased transcytotic vesicle fusion. Cholestasis likely decreases membrane fluidity. The regulation of phospholipid species within hepatocellular membranes may play an important role in intrahepatic lipid transport.

Effects of bilirubin ditaurate on biliary secretion of proteins and lipids: influence on the hepatic vesicle transport system

BACKGROUND

Several organic anions cause dissociation of biliary lipid secretion from bile acid secretion (uncoupling). As bile lipids originate from liver microsomes and are transported by carrier proteins and/or transcytotic vesicles, such a reduction of biliary lipid secretion may lead to cytosolic accumulation of vesicles. This study investigated whether bilirubin conjugate, a physiologically important organic anion, caused uncoupling and whether hepatic retention of compounds carried by transcytotic vesicles occurred subsequently, using bilirubin ditaurate, a synthetic commercially available compound.

METHODS

Cannulation of the bile duct and femoral vein was done in male Sprague-Dawley rats. Sodium taurocholate was infused intravenously at a constant rate of 100 nmol/min per 100 g bodyweight. Bilirubin ditaurate (50 nmol/min per 100 g bodyweight) was infused concomitantly, followed by periodical bile collection for analysis of lipids, total protein and immunoglobulin A.

RESULTS

Biliary bile acid secretion was not changed significantly by infusion of bilirubin ditaurate. In contrast, the secretion of cholesterol, phospholipids and immunoglobulin A was decreased by 57.3, 48.7 and 44.8%, respectively. The biliary cholesterol:phospholipid ratio was increased by 19%. Uncoupling was caused by bilirubin ditaurate and biliary immunoglobulin A secretion was decreased.

CONCLUSIONS

As immunoglobulin A is a major protein carried by intrahepatic transcytotic vesicles, uncoupling may involve impairment of intrahepatic vesicular transport. Also, a reduction of immunoglobulin A secretion into bile by organic anion-induced uncoupling may weaken biliary immunity.

Partial characterization of mechanisms of cytoprotective action of hydrophilic bile salts against hydrophobic bile salts in rats: relation to canalicular membrane fluidity and packing density

Bile salts regulate the subselection of phosphatidylcholine species secreted into bile and thereby modulate bile metastability. The aim of this study was to determine whether bile salts alter phosphatidylcholine species of the canalicular membrane, and if they do, to clarify whether the cytoprotective action of hydrophilic bile salts is associated with modulation of phosphatidylcholine composition in cell membrane bilayers. Bile salt-pool-depleted rats were infused intravenously with sodium taurocholate at a constant rate (200 nmol/min/100 g body wt) for 2 hr, followed by infusion of either sodium tauroursodeoxycholate, sodium tauroalphamuricholate, or sodium taurobetamuricholate (200 nmol/min/100 g) for 2 hr. Biliary outputs of cholesterol and phosphatidylcholine and phosphatidylcholine hydrophobicity in bile and subcellular fractions were determined. The cytoprotective action of hydrophilic bile salts was determined by the release of canalicular membrane-localizing enzymes (alkaline phosphatase, leucine aminopeptidase) into bile. Tauroursodeoxycholate, taurobetamuricholate, and tauroalphamuricholate decreased the release of these enzymes when compared to values under taurocholate infusion. Bile phosphatidylcholine hydrophobicity was also decreased by the bile salts, whereas the cholesterol/phosphatidylcholine ratio was increased. In contrast, phosphatidylcholine hydrophobicity in the canalicular membrane was increased by these three bile salts. In conclusion, hydrophilic bile salts promote biliary secretion of relatively hydrophilic phosphatidylcholine secretion into bile, and consequently phosphatidylcholine hydrophobicity in canalicular membranes increased. Such an alteration in phosphatidylcholine species within canalicular membrane enhances its lateral packing density with less fluidity, and this may account, in part, for the cytoprotective action of hydrophilic bile salts against hydrophobic bile salts.

Chronokinetics of active biliary ampicillin secretion in rats

The existence of temporal variation in biliary excretion has been demonstrated for dibromosulfophthalein and ampicillin (AMP). This study was performed to investigate if the 24 h rhythm of active AMP biliary secretion could be attributed to circadian rhythms in the capacity and/or binding affinity of the active secretion mechanism. In this study, 12 Sprague-Dawley rats, housed under a 12 h light/12 h dark environment, were used. Each rat received four 1 h infusions of incremental doses of AMP during either the active (24:00 group) or rest phase (12:00 group) under pentobarbital anesthesia. High doses of AMP were administered to saturate the biliary secretion of AMP via the anion carrier system. Bile and plasma were collected at steady state for each infusion and analyzed by a microbiological assay. The systemic clearance of AMP was increased approximately twofold during the active phase (24:00 group) compared to the resting phase (12:00 group). Plots of bile excretion rate versus plasma concentration indicated saturation of the anion carrier system. Analysis of the data using the Michaelis-Menten model revealed no significant difference in the binding affinity (1/Km) of the biliary anion carrier system between the 12:00 and 24:00 groups. However, the maximum AMP excretion rate attained in the bile (maximum transport or Vmax) showed a 50% increase during the active phase, thus implicating a day-night variation in transport capacity of the anionic pathway. Therefore, temporal variation in the capacity of the secretory mechanisms is a determinant contributor to the proposed circadian rhythm observed in the biliary elimination of AMP.

Bile salt hydrophobicity modulates subselection of biliary lecithin species in rats depleted of bile salt pool

Although bile salts play an important role in the secretion of biliary lipid, little is known about the relationship between bile salt hydrophobicity and the selection of lecithin species to be secreted into bile. We therefore investigated whether bile salts modulate the selection of biliary lecithin subspecies. Rats that were depleted of the bile salt pool were infused with taurocholate (50, 100, 200, and 400 nmol/min/100 g body weight), taurochenodeoxycholate (25, 50, 100, and 200 nmol/min/100 g body weight), tauroursodeoxycholate (100, 200, 400, and 800 nmol/min/100 g body weight), or taurobetamuricholate (100, 200, 400, and 800 nmol/min/100 g body weight). Bile was collected to analyze bile flow, bile acid output, cholesterol levels, and lecithin levels. The hydrophobic-hydrophilic balance of the bile salts and biliary lecithin species was assessed by determining the retention times during reverse-phase high-performance liquid chromatography. Biliary lecithin secretion rates correlated with the hydrophobicity index of the biliary bile salts administered. Thus, biliary lecithin hydrophobicity increased with increasing bile salt hydrophobicity, whereas the molar cholesterol-lecithin ratio in the bile decreased. In conclusion, bile salt hydrophobicity regulates the selection of biliary lecithin subspecies during biliary secretion and thereby modulates, at least in part, bile cholesterol metastability. Thus, bile salt hydrophobicity accounts for the physicochemical conditions determining bile lipid metastability.

Extracellular and intracellular regulation of biliary lecithin hydrophobicity

Bromosulfophthalein and papaverine have been demonstrated to inhibit biliary lipid secretion without affecting secretion of bile salts in normal rats, so-called uncoupling. Bromosulfophthalein inhibits the capacity of intracanalicular bile salt micelles to induce biliary lipid secretion, and papaverine inhibits vesicular transport within the hepatocyte. We compared the effects of bromosulfophthalein and papaverine on biliary lipid secretion in normal Sprague-Dawley rats and Eizai hyperbilirubinuria rats. The fatty acyl chain saturation in biliary lecithin increased during bromosulfophthalein infusion and decreased during papaverine infusion in Sprague-Dawley rats. Bromosulfophthalein had no effect on biliary lipid secretion in Eizai rats, while papaverine induced uncoupling. The degree of fatty acyl chain saturation in biliary lecithin was unchanged during bromosulfophthalein infusion, but decreased with papaverine in Eizai rats. We deduce that selection of biliary lecithin species occurs at various points in the lipid transport pathway at intracellular and intracanalicular sites.

Effect of cholestasis induced by organic anion on the lipid composition of hepatic membrane subfractions and bile in rats

Several organic anions inhibit the secretion of cholesterol and phospholipid into bile without affecting total bile acid secretion (uncoupling). The uncoupling induced by sulphobromophthalein (BSP) alters the fatty acid composition of biliary lecithin. The purpose of this study was to investigate the relationship between the lipid composition of bile and of liver subcellular membrane fractions during BSP-induced uncoupling. After depletion of the bile salt pool, rats fitted with a bile duct cannulus were infused with sodium taurocholate given either alone or with BSP. Bile was collected and liver microsomes and canalicular membranes were isolated for analysis of lipid composition. In bile, uncoupling increased the cholesterol/phospholipid ratio (C/P ratio) and the saturated/unsaturated fatty acid ratio (S/U ratio) in phosphatidylcholine. The C/P ratio was increased in the canalicular membrane, but the membrane phosphatidylcholine S/U ratio was decreased during uncoupling. In microsomes, the S/U ratio of membrane phosphatidylcholine was slightly increased, but the C/P ratio was unaffected during uncoupling. These results support the hypothesis that an increased secretion of hydrophobic phosphatidylcholine species from the canalicular membrane into bile reduces the proportion of hydrophobic phosphatidylcholine species in the canalicular membrane during uncoupling. The decreased contribution of hydrophobic phosphatidylcholine species may ameliorate the decrease in membrane fluidity resulting from the accumulation of cholesterol in the canalicular membrane and stimulate the synthesis of hydrophobic phosphatidylcholine species in the microsomes.

Transcytotic vesicle fusion with canalicular membranes is modulated by phospholipid species: implications for biliary lipid secretion

Phospholipid species modulate bile metastability and the subselection of such species for biliary secretion occurs at the canalicular membrane. In this study, the role of phospholipid head groups and hydrophobic indices in transcytotic vesicle fusion with the canalicular membrane inner leaflet was investigated using rat canalicular membrane vesicles (CMV) and liposomes. The CMV were purified from Sprague-Dawley rat liver, and small unilamellar vesicles (SUV) of phosphatidylserine (PS), phosphatidylcholine (PC) and mixtures of PS/PC (1:1, 2:1 and 4:1) were labelled with 8 mol% of octadecyl rhodamine B chloride (R18). The PC species used in this study were egg yolk PC (EYPC), soybean PC (SBPC), dipalmitoyl PC (DPPC) and dilinoleoyl PC (DLPC). Fusion of SUV with CMV was initiated by the addition of a millimolar concentration of Ca2+ and the degree of fusion was estimated by the increase of R18 fluorescence. Ca(2+)-dependent fusion of SUV consisting of PS, and PS/PC (4:1) with CMV was observed (PS > PS/PC; 4:1), whereas no detectable fusion was evident between CMV and SUV of PC alone or PS/PC (1:1 or 2:1). The rank order of fusibility between CMV and SUV of PS/PC (4:1) containing various PC species was PS/DLPC > PS/SBPC > PS/EYPC > PS/DPPC. The hydrophobic index of PC as determined by high performance liquid chromatography (HPLC) was related closely to liposome fusibility (r = -0.88). These results suggest that transcytotic vesicle fusion with the canalicular membrane inner leaflet is regulated by the phospholipid hydrophobicity of the vesicles.