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

Assessing causal associations of bile acids with obesity indicators: A Mendelian randomization study

Maintaining a balanced bile acids (BAs) metabolism is essential for lipid and cholesterol metabolism, as well as fat intake and absorption. The development of obesity may be intricately linked to BAs and their conjugated compounds. Our study aims to assess how BAs influence the obesity indicators by Mendelian randomization (MR) analysis. Instrumental variables of 5 BAs were obtained from public genome-wide association study databases, and 8 genome-wide association studies related to obesity indicators were used as outcomes. Causal inference analysis utilized inverse-variance weighted (IVW), weighted median, and MR-Egger methods. Sensitivity analysis involved MR-PRESSO and leave-one-out techniques to detect pleiotropy and outliers. Horizontal pleiotropy and heterogeneity were assessed using the MR-Egger intercept and Cochran Q statistic, respectively. The IVW analysis revealed an odds ratio of 0.94 (95% confidence interval: 0.88, 1.00; P = .05) for the association between glycolithocholate (GLCA) and obesity, indicating a marginal negative causal association. Consistent direction of the estimates obtained from the weighted median and MR-Egger methods was observed in the analysis of the association between GLCA and obesity. Furthermore, the IVW analysis demonstrated a suggestive association between GLCA and trunk fat percentage, with a beta value of -0.014 (95% confidence interval: -0.027, -0.0004; P = .04). Our findings suggest a potential negative causal relationship between GLCA and both obesity and trunk fat percentage, although no association survived corrections for multiple comparisons. These results indicate a trend towards a possible association between BAs and obesity, emphasizing the need for future studies.

Choleresis and inhibition of biliary lipid secretion induced by piperacillin in the rat

1. The effects of the administration piperacillin on bile flow and biliary lipid secretion were studied in male Wistar rats. 2. Intravenous injection of piperacillin at doses ranging from 0.3 to 3.0 mmol/kg bodyweight led to an increase in its biliary concentration and excretion rate. Maximal biliary excretion was reached at a dose of 2.0 mmol/kg piperacillin. 3. Excretion of the antibiotic into bile was associated with a marked choleresis. A linear relationship was observed between bile flow and piperacillin excretion, 5.7 micro L bile being produced per micro mol piperacillin excreted into the bile. 4. Continuous i.v. infusion of piperacillin at 2.0 mmol/100 g per min did not result in significant changes in bile acid or cholesterol secretion, but biliary phospholipid secretion was markedly reduced. The inhibitory effect on phospholipid secretion was also present when biliary lipid output had been previously increased by an infusion of taurocholate (200 nmol/100 g per min). Addition of taurocholate did not reverse the impairment of phospholipid secretion induced by piperacillin. 5. These results indicate that acute administration of piperacillin in the rat induces a marked choleresis by stimulating bile acid-independent bile flow. The significant impairment in phospholipid secretion suggests a specific effect on intracellular supply and/or translocation across the canalicular membrane.

Transport of the sulfated, amidated bile acid, sulfolithocholyltaurine, into rat hepatocytes is mediated by Oatp1 and Oatp2

The uptake of the sulfated bile acid sulfolithocholyltaurine (SLCT) was investigated in isolated rat hepatocytes and in HeLa cells transfected with complementary DNAs (cDNAs) of organic anion transporting polypeptides (Oatps) 1 and 2 cloned from rat liver. In hepatocytes, transport of SLCT was greatly reduced by bromosulfophthalein (BSP), estrone sulfate, the precursor bile acids cholyltaurine and lithocholyltaurine, and 4,4'-diisothiocyanostilbene-2-2'-disulfonic acid (DIDS). However, SLCT transport was insensitive to 4-methylumbelliferyl sulfate, harmol sulfate, digoxin, fexofenadine, and lack of sodium ion. Because the estimation of kinetic constants was enhanced with use of inhibitors, BSP (1-50 micromol/L) was added to isolated rat hepatocytes to assess the various transport components for SLCT uptake. The resulting data showed a nonsaturable pathway and at least 2 pathways of different Michaelis-Menten constants (K(m)) (70 and 6 micromol/L) and similar maximum velocities (V(max)) (1.73 and 1.2 nmol/min/mg protein) and inhibition constants of 0.63 and 10.3 micromol/L for BSP. In expression systems, SLCT was taken up by Oatp1 and Oatp2 expressed in HeLa cells with similar K(m) values (12.6 +/- 6.2 and 14.6 +/- 1.9 micromol/L). These K(m) values were comparable to that observed for the high-affinity pathway in rat hepatocytes. In conclusion, the results suggest that transport of SLCT into rat liver is mediated in part by Oatp1 and Oatp2, high-affinity pathways, a lower-affinity pathway of unknown origin, and a nonsaturable pathway that is compatible with a transport system of high K(m) and/or passive diffusion.

Interactions between organic anions, micelles and vesicles in model bile systems

Biliary lipid secretion probably involves both 'micellization' and 'vesiculization' of bile-canalicular membrane lipids. Several hydrophilic organic anions inhibit the secretion of lipids into the bile without altering bile salt secretion [Verkade, Vonk and Kuipers (1995) Hepatology 21, 1174-1189]. Hydrophobic organic anions do not interfere with biliary lipid secretion. We investigated whether the organic-anion-induced inhibition of biliary lipid secretion in vivo could be attributed to inhibition of micellization, by the application of in vitro models of micellization. Carboxyfluorescein was entrapped in a self-quenching concentration in small unilamellar vesicles (SUV) composed of cholesterol/egg phosphatidylcholine (molar ratios 0, 0.2 and 0.5). Certain organic anions clearly affected the bile-salt-induced release of fluorescence from these SUV, reflecting interference with micellization. However, the effects of hydrophilic and hydrophobic organic anions did not correspond with their effects on biliary lipid secretion in vivo, irrespective of the bile salt species used (taurocholate, taurodeoxycholate or tauroursodeoxycholate) and of the lipid composition of the SUV. Ultracentrifugation and dynamic light-scattering studies indicated that organic anions do interact with bile salt/ phosphatidylcholine/cholesterol mixed micelles, but that they do not inhibit micellization, for example by competing with phosphatidylcholine and/or cholesterol for incorporation into mixed micelles. In conclusion, the present in vitro data indicate that the in vivo mechanism of organic-anion-induced inhibition of biliary lipid secretion is not mediated by inhibition of micellization.

Relationship between biliary lipid and protoporphyrin secretion; potential role of mdr2 P-glycoprotein in hepatobiliary organic anion transport

BACKGROUND/AIMS

Erythropoietic protoporphyria, caused by ferrochelatase deficiency, leads to protoporphyrin accumulation in the liver. Therapeutic attempts to increase the secretion of this hydrophobic organic anion into bile are hampered by a lack of understanding of the secretory mechanism(s) involved. We have investigated biliary secretion of protoporphyrin in rats and mice, primarily targeted on the role of biliary lipids in this process.

METHODS

Gel permeation chromatography was applied to investigate the association of porphyrins with lipid fractions in bile. Secretion of endogenous porphyrins was studied in (GY mutant) rats and mdr2 P-glycoprotein deficient mice, under conditions of widely varying biliary lipid secretion rates.

RESULTS

Gel permeation chromatography revealed that, in native human and rat bile, protoporphyrin associated with cholesterol/phospholipid vesicles upon elution with bile salt-free buffer. In contrast, the more hydrophilic coproporphyrin isomers I and III were found only in bile salt/organic anion hybrid particles and smaller aggregates. Interruption of the enterohepatic circulation in normal Wistar rat resulted in parallel decrease of endogenous protoporphyrin-, lipid-, and bile salt secretion, but did not alter the secretion of coproporphyrin I and III. Uncoupling of lipid- from bile salt secretion by sulfated taurolithocholate resulted in impaired secretion into bile of protoporphyrin only. Conversely, secretion of coproporphyrin I and III, but not that of protoporphyrin, was impaired in mutant Groningen Yellow rats with defective ATP-dependent hepatobiliary organic anion transport. In mice homozygous for a disruption of the mdr2 P-glycoprotein gene, resulting in complete absence of phospholipids in bile and strongly reduced cholesterol output, secretion of protoporphyrin was reduced by 90%, whereas that of coproporphyrin I and III was affected to a much lesser extent.

CONCLUSIONS

Our data demonstrate a close association between protoporphyrin and lipid secretion into bile, indicating that these processes are, at least functioning coupled. This finding implicates a role of mdr2 P-glycoprotein activity in hepatobiliary removal of the hydrophobic organic anion protoporphyrin. Hence, it may be speculated that protoporphyrin secretion can be influenced by drugs, diet or other means that affect biliary lipid secretion.

Effects of organic anions and bile acids on biliary lipid excretion in hyperbilirubinemic mutant Sprague-Dawley rats

The effects of organic anions and bile acids on biliary lipid excretion were studied in EHBR, a hyperbilirubinemic mutant Sprague-Dawley rat. A marked delay in the biliary excretion of BSP, cefpiramide, rose bengal and ursodeoxycholate-disulfate was observed in these animals. The marked decrease in the biliary excretion of phospholipids and cholesterol and the uncoupling of biliary bile acids and lipids that occurred after the administration of BSP, cefpiramide and ursodeoxycholate-disulfate in control Sprague-Dawley rats was absent in EHBR. Rose bengal did not change biliary lipid excretion in either the control Sprague-Dawley rats or the EHBR. Although taurocholate markedly increased bile flow and biliary bile acid excretion in both types of rats, the increase in biliary lipid excretion observed in the control Sprague-Dawley rats was absent in EHBR. These findings indicate that EHBR have an impairment of hepatic lipid transfer that is enhanced by bile acids, possibly at the level of intracellular vesicular lipid transport.

Papaverine inhibits transcytotic vesicle transport and lipid excretion into bile in isolated perfused rat liver

Papaverine is a nonspecific smooth muscle relaxant and a phosphodiesterase inhibitor. Its effects on biliary excretion of lipids and horseradish peroxidase were investigated in a single-pass isolated perfused rat liver model. A constant infusion of papaverine (1.6 mumol/min; 40 mumol/L) significantly increased bile flow (microliters per minute per gram of liver) before (2.03 +/- 0.09 vs. 1.0 +/- 0.06) and after sodium taurocholate infusion (2.77 +/- 0.10 vs. 1.88 +/- 0.11). However, papaverine significantly and reversibly reduced biliary excretion of phospholipids and cholesterol (nanomoles per minute per gram of liver) after a 1.0 mumol/min sodium taurocholate infusion, from 7.45 +/- 0.83 and 1.42 +/- 0.15 to 1.75 +/- 0.18 and 0.39 +/- 0.06, respectively (p less than 0.01), whereas secretion of bile acids was unaffected. When a 1-min pulse of horseradish peroxidase (25 mg) was infused in isolated perfused rat liver after a continuous infusion of N6,O-2'-dibutyryladenosine 3',5'-cyclic monophosphate (0.25 mumol/min; 6.25 mumol/L), horseradish peroxidase appeared in bile in an early (4 to 6 min) and late (20 to 25 min) peak. Papaverine significantly reduced the late peak, from 1.211 +/- 0.264 to 0.498 +/- 0.107 (p less than 0.01). Papaverine had no significant effects on either cyclic AMP or cyclic GMP in the liver and bile, although it has been reported that papaverine is a phosphodiesterase inhibitor. These findings indicate that papaverine inhibits biliary excretion of lipids but not bile acids, and they suggest that papaverine has an inhibitory effect on transcytotic vesicle transport independent of an increase of cyclic nucleotides in hepatocytes.

Taurolithocholate-induced inhibition of biliary lipid and protein excretion in the rat

Taurolithocholate (TLC), a natural bile salt, induces selective impairment on canalicular membrane of the hepatocyte, which seems to be a major determinant of its cholestatic effect in experimental animals. In order to extend existing studies about the effects of TLC on bile secretion, we examined in TLC-treated rats the biliary excretion of compounds that are transported to canalicular membrane via vesicles, such as lipids and proteins. The single intravenous injection of TLC (3 mumol/100 g body wt.) inhibited transiently the biliary bile salt excretion, while the biliary excretion of lipids (i.e., cholesterol and phospholipids) and proteins remained inhibited even though the biliary excretion and composition of bile salts were normalized. Under such a condition, TLC also inhibited the transcellular vesicular pathway to the exogenous protein horseradish peroxidase entry into bile, without altering the paracellular biliary access of the protein. The hepatic uptake of horseradish peroxidase was unaffected by TLC-treatment. The results indicate that TLC can inhibit the biliary excretion of compounds that reach the canaliculus via a vesicular pathway, such as lipids and proteins, by a mechanism not related to a defective bile salt excretion. Possible explanations for these findings are discussed.

The uncoupling of biliary lipid from bile acid secretion by organic anions in the rat

A number of organic anions has been shown to inhibit biliary phospholipid and cholesterol secretion without affecting bile acid secretion. However, the mechanism of this uncoupling phenomenon is still unclear. This study shows a comparison of the effects of ampicillin (18 mumol/100g body wt), sulfated taurolithocholic acid (0.2 and 1.0 mumol/100 g body wt), and indocyanine green (0.6 mumol/100 g body wt) in control and Groningen Yellow-Wistar rats with chronic (8 days) biliary drainage. Groningen Yellow rats have a hereditary defect in hepatobiliary transport of various organic anions. Bile secretion, but not hepatic uptake, of the three organic anions was strongly impaired in Groningen Yellow rats compared with controls. Ampicillin and sulfated taurolithocholic acid caused a strong uncoupling in control rats but had no effect or a much smaller effect on lipid secretion in Groningen Yellow rats. Indocyanine green did not affect lipid secretion, in either control or in Groningen Yellow rats. Gel-filtration chromatography of bile showed a specific coelution of ampicillin and sulfated taurolithocholic acid with the bile acid fraction, whereas indocyanine green coeluted with the phospholipid/cholesterol fraction. This study concludes that the uncoupling by ampicillin and sulfated taurolithocholic acid occurs after their secretion into bile and is caused by interaction of these compounds with bile acids. It is hypothesized that this interaction inhibits the capacity of bile acids to induce secretion of phospholipids and cholesterol into the bile.

Concentrative biliary secretion of ceftriaxone. Inhibition of lipid secretion and precipitation of calcium ceftriaxone in bile

The hepatic transport of ceftriaxone, a third-generation cephalosporin, was characterized in the rat and hamster; its effect on bile flow and bile acid-induced biliary lipid secretion was also measured. In anesthetized rats with biliary fistulae, the Tmax was about 5 mumol.min-1.kg-1, and in the hamster the Tmax was about 1 mumol.min-1.kg-1. The compound was not biotransformed. At high secretion rates, the concentration of cephalosporin in bile increased to 27 mmol/L, a concentration far exceeding the solubility product of its calcium salt [2 x 10(-6) (mol/L)2], which precipitated from bile. In the rat, ceftriaxone induced choleresis (22 microL/mumol ceftriaxone, the expected value for a dianionic compound). In the isolated perfused rat liver, ceftriaxone had a fractional hepatic extraction rate averaging 3%; the compound was concentratively secreted into bile, the bile-perfusate ratio ranging from 35-250. Ceftriaxone inhibited phospholipid and cholesterol secretion induced by endogenous or exogenous bile acids; the rate of inhibition was linearly proportional to the canalicular secretion rate of ceftriaxone. Hepatic transport of ceftriaxone had no influence on hepatic secretion of ursodeoxycholyltaurine. In contrast, the net hepatic transport of ursodeoxycholic acid, ursodeoxycholyltaurine, or cholyltaurine inhibited ceftriaxone transport in a dose-dependent manner. It is concluded that ceftriaxone and bile acids share a common mechanism for hepatic transport in the rat and also interact in the processes involved in biliary lipid secretion. Biliary secretion of unbiotransformed ceftriaxone occurs at high concentrations; secondary Ca2+ entry results in the formation of supersaturated canalicular bile and subsequent precipitation as a calcium salt in the biliary tract. These data explain the formation of biliary sludge that occurs in patients undergoing high-dose ceftriaxone therapy.

The role of calcium precipitation in the sulfoglycolithocholate-induced cholestasis of the bile fistula hamster

Sulfate glycolithocholic acid (SGLC) has been shown to be highly cholestatic in the rat. This study was performed in order to gain understanding of the mechanisms of SGLC-induced cholestasis and the aim of the investigation was to explore the hypothesis that SGLC could cause a precipitation of calcium in bile. We studied the effects of intravenously administrated SGLC on bile flow, biliary lipids secretion and calcium excretion in the female bile fistula hamster. We also performed in-vitro studies with a Ca2(+)-selective electrode in order to measure the calcium binding capacity of SGLC. The results showed that after 1 h of infusion of 8 mumol/100 g body weight [14C]SGLC bile flow dropped to zero. During the infusion period a fine white sludge was visible in the test tube used for bile collection. TLC and HPLC analysis of both the supernatant and the precipitate showed that unchanged SGLC was excreted into bile. Up to 20% of biliary SGLC and more than 50% of the total Ca2+ present in bile was precipitated. The SGLC/Ca2+ molar ratio in the precipitate was 1.12 +/- 0.3 (mean +/- S.D. of four experiments). Light and electron microscopy of the liver did not show any specific abnormalities. The Ca2+ binding activity of SGLC in vitro, was highest among the bile acids tested at a concentration of 0.1 mM, when almost 100% of bile acids are in the monomeric (non-micellar) form. This suggests that among the bile acids, SGLC exerts the strongest binding activity on free calcium ions.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory action of cyclobutyrol on the secretion of biliary cholesterol and phospholipids

A number of organic anions are known to decrease biliary secretion of cholesterol and phospholipid without affecting bile acid secretion. Cyclobutyrol (CB) is a choleretic agent which also inhibits biliary lipid secretion. Using isolated perfused rat liver we have studied this inhibition in relation to possible mechanisms suggested for other anions. Shortly after its administration to the isolated perfused liver, CB decreases biliary outputs of cholesterol and phospholipid, without changes in bile acid secretion, at low (450 nmol/min), high (1350 nmol/min) and nil taurocholate infusion rates. The absolute inhibition does not appear to be decreased by elevated bile acid secretion. There is a differential effect on secretion of cholesterol and phospholipid, more marked at low bile acid secretion rates. Biliary outputs of the canalicular membrane enzymes 5'-nucleotidase and alkaline phosphodiesterase I are also depressed by CB administration, but the anion does not affect the biliary output of bovine serum albumin or the output of rat serum albumin into the perfusion fluid. Since CB does not inhibit intracellular vesicular transport or apparently inhibit intracanalicular events, its effect is different from the effect of several other anions. From these studies it appears that the most likely effect of CB is exerted at the level of the canalicular membrane.

Inhibition of biliary cholesterol and phospholipid secretion during cyclobutyrol-induced hydrocholeresis

The effects of sodium cyclobutyrate, a synthetic hydrocholeretic drug, on biliary lipid secretion and on the biliary outputs of several plasma-membrane enzymes were investigated in anaesthetized rats. Administration of a single oral dose of cyclobutyrol (0.72 mmol/kg body wt.) reduced biliary concentration and output of cholesterol and phospholipid. However, bile acid secretion was not significantly modified. This uncoupling effect of lipid secretion remained even when the choleretic response to the drug had ceased. It additionally led to a statistically significant decrease in the cholesterol/bile acid and phospholipid/bile acid molar ratios and in the lithogenic index of the bile. The biliary outputs of the plasma-membrane enzymes alkaline phosphatase and gamma-glutamyltransferase were markedly reduced by the drug. When cyclobutyrol was administered to rats which had been previously fed with a high-cholesterol diet, the effects of cyclobutyrol persisted, but were less marked. Our results demonstrate that the bile acid-independent choleresis induced by cyclobutyrol (related to its pharmacokinetic effect) is accompanied by a pharmacodynamic action that selectively reduces the secretion of biliary lipids. This is due to an uncoupling of the secretion of cholesterol and phospholipids from that of bile acids. Possible explanations for the biliary response to cyclobutyrol are discussed.