Biliary Phospholipid Research Articles

Pharmacologic perturbation of rat liver lysosomes: Effects on release of lysosomal enzymes and of lipids into bile

Hepatocyte lysosomes disassemble materials derived from intracellular sources, including lipid-containing membranes, by a process called autophagy. In addition, hepatocyte lysosomes can release their contents into bile by exocytosis. Therefore, using both in vivo and in vitro models, we tested the hypothesis that acute pharmacologic induction of autophagy would modify the biliary excretion of lysosomal protein and of lipids. We treated rats with a single dose of chloroquine (10 mg/kg), glucagon (1 mg/kg), or control solutions and collected bile via bile fistulas. Both chloroquine and glucagon immediately caused a marked and parallel decrease in biliary excretion of three lysosomal enzymes, N-acetyl-β-glucosaminidase, β-glucuronidase, and β-galactosidase, to 25%–30% of baseline values (p < 0.01). This decrease was sustained for 2 h after glucagon and 4 h after chloroquine administration. In contrast, biliary lipid changes were minor: a slight lowering of biliary cholesterol secretion after chloroquine (p < 0.05), but no change in biliary bile acids, cholesterol, and phospholipid secretion after glucagon. Changes in biliary excretion of lysosomal enzymes accompanying chloroquine and glucagon administration were associated with morphologic evidence of autophagy as assessed by electron microscopy and by increased fragility of hepatic lysosomes as assessed by latency of N-acetyl-β-glucosaminidase. These in vivo changes in biliary lysosomal enzyme excretion induced by chloroquine and glucagon were confirmed in vitro using the isolated perfused rat liver. Thus, acute induction of autophagy results in conservation of hepatic lysosomal protein and has virtually no effect on biliary lipid excretion.

Spontaneous cholelithiasis in sheep: Prevalence survey and analysis of gallstones and bile

In an abattoir survey of a consecutive series of 206 sheep at a local slaughterhouse, gallstones (concretions with a diameter equal to or greater than 1 mm) were found in the gall bladder of 24 animals. There were stones in 19 out of 146 adult sheep and in 5 out of 60 lambs, with the higher frequency in the male sex in both groups. Qualitative analysis of the stones showed that all of them were pigment (bilirubin) stones. There was no statistically significant difference (paired t test, P>0·05) between the amounts of biliary cholesterol, phospholipids, bile acids and total and indirect bilirubin in 12 sheep with gallstones and 12 control sheep without gallstones, paired for age, sex and day of slaughter.

Triton WR-1339-induced changes in serum lipids and biliary lipid secretion.

Biliary lipid secretion rates were measured in fed rats after an intravenous injection of Triton WR-1339 (TWR, 60 mg/100 g body wt), an agent that inhibits lipoprotein removal from the circulation. Serum triglyceride, phospholipid (PL), and cholesterol (CH) concentrations rose within 3 h of TWR to 45, 6.6, and 10 times control values, respectively, at 24-36 h. Serum lipids fell rapidly at 48 h and were normal by 72-96 h after TWR. TWR did not alter bile flow, hepatic bile acid transport, or biliary bile acid output. Within 0.5 h of TWR, biliary PL and CH outputs fell greater than 70%, and taurocholate-stimulated biliary PL secretion was markedly reduced. Biliary PL and CH secretion rates were approximately 30 and approximately 40% suppressed, respectively, 24 h after TWR, 160 and 330% elevated, respectively, at 48 h, and normally by 72 h, despite normal taurocholate-stimulated biliary PL secretion. Biliary beta-glucuronidase secretion (a lysosomal enzyme) was unchanged for 3 h after TWR but was increased at 24, 48, and 72 h, independent of biliary lipid secretion. Thus TWR acutely dissociates bile acid from lipid secretion without impairing bile acid transport or biliary lysosomal discharge. Late changes in biliary lipid secretion relate closely to TWR-induced change in serum lipid metabolism but not to stimulation of biliary lysosomal discharge.

Regulation of bile acid synthesis. I. Effects of conjugated ursodeoxycholate and cholate on bile acid synthesis in chronic bile fistula rat.

Bile acid synthesis is thought to be regulated by a negative feedback mechanism which is presumably dependent upon the flux of bile acids in the enterohepatic circulation. To characterize further the role of bile acids in regulation of bile acid synthesis, we have administered pure taurine or glycine conjugates of ursodeoxycholic acid or cholic acid to chronic bile fistula rats by continuous intraduodenal infusion, thus simulating restoration of the enterohepatic circulation. The effects of these bile salt infusions on bile acid synthesis, biliary cholesterol and phospholipid secretion and on the activities of the hepatic microsomal enzymes cholesterol 7 alpha-hydroxylase and HMG-CoA reductase were evaluated. Because the rate of biliary bile salt secretion in rats with intact exteriorized enterohepatic circulation averaged 27.1 +/- 1.4 mumoles per 100 gm rat per hr, infusion rates for bile fistula studies were chosen to match (24 to 36 mumoles per 100 gm rat per hr) or exceed (48 mumoles per 100 gm rat per hr) this physiological flux. Infusion of tauroursodeoxycholic acid for 48 hr at 24 and 48 mumoles per 100 gm rat per hr failed to suppress cholic acid synthesis. Bile flow and biliary cholesterol and phospholipid secretion exhibited small, dose-dependent increases with tauroursodeoxycholic acid infusions. No suppression of cholesterol 7 alpha-hydroxylase or HMG-CoA reductase activity was observed. By contrast, taurocholic acid inhibited synthesis of chenodeoxycholate and its metabolites alpha- and beta-muricholate by 10% (NS), 66% (p less than 0.05) and 75% (p less than 0.05) at infusion rates of 24, 36 and 48 mumoles per 100 gm rat per hr, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the hepatocyte microtubular system in the excretion of bile salts and biliary lipid: implications for intracellular vesicular transport.

The role of the hepatocyte microtubular system in the transport and excretion of bile salts and biliary lipid has not been defined. In this study the effects of microtubule inhibition on biliary excretion of micelle- and non-micelle-forming bile salts and associated lipid were examined in rats. Low-dose colchicine pretreatment had no effect on the baseline excretion of biliary bile salts and phospholipid in animals studied 1 hr after surgery (basal animals), but slightly retarded the excretion of tracer [14C]taurocholate relative to that of lumicolchicine-pretreated (control) rats. However, colchicine pretreatment resulted in a marked reduction in the excretion of 2 mumol/100 g doses of a series of four micelle-forming bile salts of differing hydrophilicity, but had no significant effect on the excretion of the non-micelle-forming bile salt, taurodehydrocholate. Continuous infusion of 0.2 mumol of taurocholate/(100 g.min) following 24 hr of biliary drainage (depleted/reinfused animals) resulted in physiologic bile flow with biliary excretion rates of bile salts, phospholipid, and cholesterol that were markedly inhibited (mean 33, 39, and 42%, respectively) by colchicine or vinblastine pretreatment. Excretion of tracer [14C]taurocholate also was markedly delayed by colchicine in these bile salt-depleted/reinfused animals. In contrast, colchicine did not inhibit bile salt excretion in response to reinfusion of taurodehydrocholate. Thus, under basal conditions, the microtubular system appears to play a minor role in hepatic transport and excretion of bile salts and biliary lipid. However, biliary excretion of micelle-forming bile salts and associated phospholipid and cholesterol becomes increasingly dependent on microtubular integrity as the transcellular flux and biliary excretion of bile salts increases, in both bile salt-depleted and basal animals. We postulate that cotransport of micelle-forming bile salts and lipids destined for biliary excretion, via an intracellular vesicular pathway, forms the basis for this microtubule dependence.

Studies on choleretic constituents in Artemisia capillaris THUNB.

Twelve constiuents (1, capillarin; 2, scoparone; 3, scopolein; 4, isoscpoletin; 5, copillatremisin-7-methyl ether; 6, cirsimaritin; 7, capillarisin; 8, artepillin A; 9, capillartemisin B1; 10, artepillin C; 11, capillin; 12, capillene) were isolated after tesing the choleretic activity of various fractions derived from the water extract of Artemisia capillaris. In tests of the choleretic activity in Wistar rats, four constituents (2, 8, 9, 10) were found to make an overwhelming contribution to the activity. Five constituents (3, 4, 8, 10, 12) were found for the first time in this plant. All of the compounds caused bile secretion to increase without increasing biliary bile acid, cholesterol or phospholipid excretion.

Rôle d'une eau sulfatée calcique et magnésienne sur l'élimination du cholestérol chez le rat

In the rat, the activity of the thermal water of Capvern and of its ionic components on the modification of the flow rate and composition of hepatic bile has been investigated. Two groups of rats were selected: the first group received per os an overload of the thermal water during six weeks. At that time on empty stomach a great increase of biliary phospholipids was observed. An intraduodenal injection of the thermal water produced a significant and rapid increase of biliary flow rate and biliary cholesterol. the animals of the second group received through intraduodenal injections the thermal water or an anionic solution composed of calcium, of magnesium sulphate or of both salts at their concentration in the thermal water, respectively. We observed an increase in biliary flow rate and biliary calcium concentration in the rats receiving the solution of calcium sulphate only and an increase of choleresis in the rats receiving the thermal water only. These data suggest that the thermal water has a specific action on the metabolism of cholesterol in increasing its biliary elimination particularly through the calcium ion content of the water.

Biliary lipid secretion in the rat during infusion of increasing doses of unconjugated bile acids

The aim of the present study was to examine the secretion of biliary components in rats during infusion of increasing doses of either deoxycholic acid, chenodeoxycholic acid or cholic acid and to test the hypothesis that biliary phospholipids may regulate the hepatic bile acid secretory capacity. Analysis of bile samples, collected every 10 min throughout the infusion period showed that there was an elevation of bile acid, phospholipid, cholesterol and alkaline-phosphodiesterase secretion, with all the bile acids, peaking and then gradually declining. Their secretory rates maximum differed and were inversely related to their detergent strength. However, the secretory rates maximum and total output of phospholipids and cholesterol were similar for all bile acids infused. The per cent contribution of phosphatidylcholine to total bile acid-dependent phospholipid secretion was reduced from 84% (in the pre-infusion period) to 59, 46 and 13% at the end of the cholic acid, chenodeoxycholic acid and deoxycholic acid infusions, respectively. This decrease in the per cent contribution of phosphatidylcholine was associated with an increase in the contribution of both spinghomyelin and phosphatidylethanolamine. The biliary phospholipid fatty acid pattern corroborated these changes in the phospholipid classes. Since sphingomyelin and phosphatidylethanolamine are major phospholipids in bile canalicular and other hepatocellular membranes, the marked increase in their secretion in bile during the infusion of high doses of bile acids may indicate solubilization of membrane phospholipids, resulting in membrane structural changes responsible for the reduced excretory function of the liver.

Biliary lipid secretion in the rat. The uncoupling of biliary cholesterol and phospholipid secretion from bile acid secretion by sulfated glycolithocholic acid

Glycolithocholic acid and its sulfated derivative are major metabolites of the secondary bile acid lithocholic acid in man. Both compounds are known to induce cholestasis in experimental animals. We compared the effects of these endogenous hepatotoxins on bile production and biliary lipid composition in rats with chronic biliary drainage. The compounds were administered enterally at relatively low rates (5–50% of the rats' endogenous bile acid secretion in these experiments) to simulate enterohepatic circulation. Both compounds were substantially secreted into bile (more than 90% of dose); sulfated glycolithocholic acid unchanged and glycolithocholic acid after hepatic hydroxylation predominantly in the form of glyco-β-muricholic acid (cf. Kuipers et al. (1986) Am. J. Physiol. 251, G189–G194). Neither glycolithocholic acid nor its sulfated derivative affected the biliary excretion of endogenous bile acids or bile flow in these experiments. In spite of this, phospholipid and cholesterol secretion were significantly reduced by sulfated glycolithocholic acid but were not altered by glycolithocholic acid. Phospholipid and cholesterol secretion rapidly decreased to 25 and 50% of their initial values, respectively, at biliary output rates of sulfated glycolithocholic acid up to 2 μmol/h, and did not further decrease when this output was increased to 6 μmol/h. Small unilamellar liposomes consisting of cholesterol, [ Me- 14C]choline-labeled phosphatidylcholine, phosphatidylserine and [ 3H]cholesteryl oleate in a 5:4: 1:0.1 molar ratio were employed to label intrahepatic lipid pools. Administration of sulfated glycolithocholic acid slightly reduced bile acid synthesis from [ 3H]cholesteryl oleate, but significantly reduced the biliary secretion of [ 14C]phospholipid. Glycolithocholic acid did not affect the hepatic processing of liposomal lipids. It is concluded that sulfated glycolithocholic acid at low doses causes the uncoupling of biliary lipid secretion from that of bile acids, which might represent an initiating event in sulfated glycolithocholic acid hepatotoxicity.

Gallstone disease without gallstones: A partial answer

Although bile acid and bile lipid metabolism have been studied in established cholelithiasis, little is known about them in patients destined to develop gall stones, but in whom the stones have not yet appeared (prestone gall stone disease). After confirmed complete gall stone dissolution and withdrawal of treatment, gall stones recur frequently. Before the stones reappear, these patients have ‘poststone gall stone disease.’ In 13 such patients we confirmed complete gall stone dissolution with two normal cholecystograms and in 11 of the 13 by normal ultrasonography, measured bile acid and bile lipid composition in fasting duodenal bile, bile acid synthesis from marker corrected three day faecal bile acid excretion, bile acid pool size using an abbreviated isotope dilution technique, ‘steady-state’ bile lipid secretion using a duodenal amino acid perfusion system and then calculated the enterohepatic cycling frequency of the bile acid pool and the relationship between pool size and body weight. The results confirm that after withdrawal of treatment the biliary cholesterol saturation index reverts to levels (1.6 ± SEM 0.4) comparable with those before dissolution therapy first began (1.6 ± 0.2; NS). The mean bile acid pool size in the 13 patients of 4.4±0.5 mmol was comparable with that in untreated gall stone patients. Pool size was significantly smaller in the nine non-obese patients (3.5 ± 0.3), than in the four obese (6.0 ± 0.8; p < 0.05). It also correlated significantly with body weight (r = 0.72) and with %IBW (r = 0.79). The coefficients of variation for biliary bile acid, phospholipid and cholesterol secretion were high, but the mean hourly secretion rates were of the same order as those seen in untreated gall stone patients studied with the amino acid duodenal perfusion stimulus. These results provide a baseline for assessing the response to postdissolution treatment and may indicate metabolic events leading to gall stone formation.

Effects of triton WR 1339 and orotic acid on lipid metabolism in rats

In order to investigate the effect of hepatic cholesterol flux on biliary bile acids, Triton WR 1339 and orotic acid were administered to rats, and the biliary cholesterol, phospholipids and bile acids were analyzed together with serum lipoproteins and hepatic lipids. Triton, which raised serum very low density lipoprotein and lipid levels and decreased serum high density lipoprotein liver lipid levels, increase the biliary cholic acid group/chenodeoxycholic acid group ratio (CA/CDCA) in the bile without affecting the total amount of bile acids and the other biliary lipids. Orotic acid, which decreased serum lipid and lipoprotein concentrations and increased liver lipid levels, increased the biliary excretion of cholesterol and phospholipids, but produced no significant change in the total amount of bile acids and in the CA/CDCA ratio in bile.

Inhibition of biliary phospholipid and cholesterol secretion by cefoperazone.

The effect of cefoperazone, a third-generation cephalosporin, on biliary lipid secretion in rats was examined. Rats were anesthetized with ether and the mid-lumbar vein and common bile duct cannulated. Bile acid secretion was maintained by intravenous taurocholic acid infusion (28 mumol/hr). A 1-hr control period was followed by intravenous cefoperazone infusion at either submaximal (20 mumol/hr), or supramaximal (60 mumol/hr) concentrations. At the cefoperazone infusion rate of 20 mumol/hr (biliary secretion of 7.1 +/- 1.6 mumol/hr) phospholipid secretion fell 19% and cholesterol secretion fell 31%; at a cefoperazone infusion rate of 60 mumol/hr (biliary secretion rate of 27.1 +/- 5.1 mumol/hr) phospholipid and cholesterol secretion were further reduced 40% and 56%, respectively, of controls. All changes were significant (P less than 0.01). Inhibition of both cholesterol and phospholipid secretion paralleled each other, was dose-dependent, and reversible. Cefoperazone's inhibitory action was abolished at a bile acid infusion rate of 108 mumol/hr. Cefoperazone was not found to be associated with bile acid micelles or mixed micelles as determined by ultracentrifugation and gel filtration. Thus, the effect of cefoperazone on biliary lipid secretion is not due to the impairment of mixed micelle formation in the canalicular lumen but rather its inhibitory effect appears to be due to a presecretory event.

Effect of complete sulfation of bile acids on bile formation in rats.

The effect of sulfation of common bile acids on the formation of bile was investigated in male rats by infusing them with the sulfate esters of cholic, chenodeoxycholic, deoxycholic, lithocholic or dehydrocholic acid in four step-wise, increasing doses. Each dose was infused for 30 min and bile collected every 10 min. Control studies were performed by using either albumin solution (the bile acid carrier) or corresponding nonsulfated bile acids at concentrations similar to those of the sulfated products. The secretion of sulfated bile acids was slower and less than that of nonsulfated bile acids, demonstrating transport maximum kinetics rather than the secretory rate maximum characteristic of nonsulfated bile acids. Sulfation significantly increased bile salt-independent bile flow and the choleretic potency of the bile acids tested. With the exception of deoxycholic acid, which had a slight stimulatory effect, bile acid sulfation generally prevented a rise in bile acid-dependent phospholipid and cholesterol secretion. In fact, it reduced biliary phospholipid and cholesterol secretion associated with the secretion of endogenous bile acids. These data are in agreement with the physicochemical properties of sulfated bile acids. They indicate that sulfation prevents the cholestatic action of nonsulfated bile acids, perhaps by increasing bile flow via a high choleretic potential and/or by stimulating bile acid-independent bile flow. The effect of sulfated bile acids on the secretion of biliary phospholipids may protect the canalicular membrane from the detergent properties of bile acids and may thus block the cholestasis which results from high, nonsulfated bile acid concentrations.

Biliary Excretion of Apolipoprotein B by the Isolated Perfused Rat Liver: Relationship to Receptor-Mediated Uptake of Human Low-Density Lipoprotein and Biliary Lipid Secretion

Since we previously reported that apolipoproteins, the lipid transport proteins of lipoproteins, are present in human bile, we tested the hypothesis that apolipoproteins in bile are derived from circulating lipoproteins and that they are involved in the biliary excretion of lipids. We perfused isolated rat livers with lipoprotein-free solutions and collected bile before and after addition of human low and high density lipoproteins. When human low density lipoprotein was added to solutions perfusing livers isolated from rats previously treated with ethinyl estradiol, a hormone that increases receptor-mediated uptake of low density lipoproteins by hepatocytes, the concentrations in bile of cholesterol and phospholipid, but not bile acids, increased by approximately 25% together with the appearance of small amounts (approximately 1% of the amount perfused) of a metabolized form of human apolipoprotein B in bile. Acetylation of low density lipoprotein, a procedure that prevents this lipoprotein from binding to its receptor on hepatocytes, both abolished the appearance of immunoreactive apolipoprotein B in bile and blocked the increase in biliary cholesterol and phospholipid concentrations. Whereas the addition of a high density lipoprotein to solutions perfusing isolated livers of normal rats was associated with the appearance of small amounts (approximately 1% of the amount perfused) of apolipoproteins A-I and A-II in bile, there were no accompanying changes in the concentrations of biliary lipids. These results suggest that apolipoproteins or their fragments in bile are derived from circulating lipoproteins; in the case of apolipoprotein B, biliary excretion is dependent upon receptor-mediated uptake of low density lipoprotein by the hepatocyte. Our data are also consistent with the hypothesis that certain apolipoproteins may be involved in biliary lipid secretion, although in an as yet unclear manner.

Effect of a Low Protein Diet on Bile Flow and Composition in Rats

The effect of feeding a low protein (LP) diet on bile flow and biliary lipid and protein secretion was studied with young female rats. Animals fed a 8% protein diet (LP) for 4, 8 and 12 wk had a significantly lower bile flow and a lower biliary bile acid and protein secretion rate compared with rats fed a 26% protein (normal) diet (NP). The bile acid-independent fraction of bile flow was significantly increased. The slope of the regression line to zero bile acid was markedly smaller in the LP than in the NP group, indicating lower bile acid-dependent bile flow. Biliary phospholipid and cholesterol secretion rates were significantly higher in the LP than in the NP group, demonstrating an uncoupling between bile acid and lipid secretion at the low rates of bile acid secretion. The percentage increase for these parameters was of similar magnitude (30%). Absolute concentrations of bile acid, phospholipid and cholesterol were significantly higher in LP-fed rats than in NP rats, while relative concentration of bile acid was lower and those of cholesterol and phospholipid were higher. Analysis of biliary bile acids showed that the percent contribution of chenodeoxycholic acid and of deoxycholic acid increased significantly in the LP-fed rats, while that of cholic acid decreased. These data indicate that the lower bile flow in the rats fed LP can be attributed to lower bile salt-dependent flow associated with significantly lower choleretic potency of bile acids secreted.

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.

Liver cell membrane solubilization may control maximum secretory rate of cholic acid in the rat.

The factors modulating the maximum secretory rate of cholic acid were investigated. Rats were infused intravenously with cholic acid in measured stepwise increasing doses (1, 2, 3, and 4 mumol X min-1 X 100 g body wt-1). Each dose was infused for 30 min and bile samples were collected every 10 min. Bile flow, bile acid, cholesterol, individual biliary phospholipids, and the fatty acid profiles of the biliary phospholipids were determined. Microsomal and bile canalicular membrane-enriched fractions were isolated from cholic acid-treated rats at the end of the experiment. Membranes were analyzed for cholesterol, phospholipid, and phospholipid fatty acid composition. During cholic acid infusion, the secretion rates of bile acid, cholesterol, phospholipid, and bile flow initially increased and then declined. No evidence of liver cell damage was observed by light or electron microscopy. Maximum phospholipid secretion rate (13.5 nmol X min-1 X g-1) occurred before peak bile flow and bile acid secretory rate maximum (4.72 microliter X min-1 X g-1 and 375 nmol X min-1 X g-1). When phospholipid output declined, the proportion of sphingomyelins and phosphatidylethanolamine relative to phosphatidylcholine increased. This was also reflected in the fatty acid composition. Cholic acid infusion caused a decline in microsomal and bile canalicular membrane phospholipid content without affecting their phospholipid composition. Depletion of membrane phospholipid resulted in an increase in the cholesterol:phospholipid ratio, which is suggested to be the underlying mechanism for modulating cholic acid secretion.

Effect of bilirubin and bile acids on biliary phospholipid secretion during bromobenzene-induced hepatic injury in the rabbit

The interrelationship between the biliary secretion of phospholipids, bilirubin and bile acids was studied in New Zealand rabbits after acinar zone 3 hepatocyte damage induced by bromobenzene (4 mmol/kg body wt; i.p.). Treatment with the toxin did not significantly modify biliary phospholipid secretion, suggesting that the transport of phospholipids is mainly a zone 1 function. Bilirubin infusion at 1 mumol/min kg body wt induced a significant inhibition in biliary phospholipid secretion both in control and treated animals. This effect was overcome by the additional infusion of sodium glycodeoxycholate at 1.6 mumol/min kg body wt, although higher increases in phospholipid output were found in the control than in the treated rabbits. This would be related to the lowered recruitment of zone 3 cells for secretion in bromobenzene-damaged livers.

Inhibition of biliary phospholipid secretion by bilirubin in partially hepatectomized rats

The effect of i.v. administration of bilirubin on biliary phospholipid secretion was studied in sham operated and two-thirds hepatectomized Wistar rats. Following bilirubin infusion at 260 nmol/100 g body wt/min, a maximal biliary bilirubin excretion was reached in the sham operated rats. No modifications of bile flow or bile acid secretion appeared, but biliary phospholipid secretion was significantly inhibited. At 12 and 24 h after hepatectomy, the maximal bilirubin excretion was markedly lowered but biliary phospholipid secretion still remained significantly inhibited. The infusion of taurocholate at 200 nmol/100 g body wt/min counteracted the impairment of phospholipid secretion, both in sham operated and hepatectomized animals. Bile acid secretions more than two-fold that of the controls were needed to reach half maximal biliary phospholipid secretion in bilirubin-infused animals, with or without the addition of taurocholate.

Disproportional changes in the rates of biliary bile acid, phospholipid and cholesterol secretion in the SHR/N-corpulent rat

Lean and obese male rats of the SHR/N- cp strain were fed a plain chow diet ad libitum and studied at 11–13 weeks of age. Compared to their lean littermates the obese animals manifested 2–4-fold higher plasma levels of cholesterol, triacylglycerol and glucose. In the obese rats, the mass of the liver was increased more than 70% and there was a significant increase in the total hepatic content of free cholesterol (42%), cholesteryl ester (2.9-fold) and triacylglycerol (7.4-fold). Despite the difference in liver mass, the rate of bile secretion was the same in the lean and obese groups. Bile acid pool size was 2.5-fold greater in the obese rats and this was reflected in an enhanced rate of biliary bile acid secretion. In contrast, there was little or no increase in biliary cholesterol and phospholipid secretion, so the relative content (molar percentage) of these lipids decreased significantly in the obese animals. In both the lean and obese groups, biliary cholesterol and phospholipid output could be enhanced by the intravenous administration of taurocholate, but the proportion of both lipids in the bile of the obese rats remained significantly lower than in their lean controls. Thus the difference in biliary lipid composition between the two groups is not due to a limited availability of cholesterol and phospholipid for transport into bile, but more likely reflects a difference in the reaction whereby the secretion of cholesterol and phospholipid is coupled to the transport of bile acids across the canalicular membrane.