The role of Ca2+ signalling and InsP3R in the pathogenesis of intrahepatic cholestasis of pregnancy

BACKGROUND

In order to contribute new insights for future prevention and treatment of intrahepatic cholestasis of pregnancy (ICP), and to promote positive pregnancy outcomes, we evaluated serum Ca2+ levels and inositol 1,4,5-trisphosphate receptor (InsP3R) expression in the liver tissue of a rat ICP model.

METHODS

After establishing the model by injection of oestradiol benzoate and progesterone into pregnant rats, animals were divided into normal control (n = 5) and ICP model groups (n = 5). The expression of InsP3R protein in the liver, and serum levels of Ca2+, glycocholic acid and bile acid were detected.

RESULTS

InsP3R mRNA and protein were significantly lower in the ICP model group compared to the normal group, as determined by qPCR and immunohistochemistry, respectively. Serum enzyme-linked immunosorbent assay results revealed significantly higher levels of glycocholic acid and bile acid in the ICP model group compared to the normal group, while Ca2+ levels were significantly lower. The levers of Ca2+ were significantly and negatively correlated with the levels of glycocholic acid. The observed decrease in Ca2+ was associated with an increase in total bile acids, but there was no significant correlation.

CONCLUSIONS

Our results revealed that the expression of InsP3R and serum Ca2+ levels was significantly decreased in the liver tissue of ICP model rats. Additionally, Ca2+ levels were found to be negatively correlated with the level of glycocholic acid.

A novel analytical approach towards in-vitro bile acid binding studies to Colesevelam Hydrochloride tablets: An ultra-high performance liquid chromatography tandem mass spectrometric method

Colesevelam hydrochloride is a bile acid sequestrant used as a low density lipoprotein (LDL) reducing agent in hyperlipidemia with an additional advantage to improve glycemic control in type 2 diabetes patients. The objective of the study was to develop and validate a liquid chromatography tandem mass spectroscopic method for the simultaneous in-vitro estimation of bile acid salts of Glycocholic acid (GC), Glycochenodeoxycholic acid (GCDC) and Taurodeoxycholic acid (TDC) and its application in performing in-vitro binding study with Colesevelam Hydrochloride tablets. The method was developed using C-18 (50 x 4.6 mm, 3 μm) column with detection on negative ion mode and acquisition time of 3.5 min. The calibration range was linear from 0.0002 mM to 0.0065 mM for GC, 0.0002 mM to 0.0065 mM for GCDC and 0.0001 mM to 0.0021 mM for TDC. The precision was less than 3.0% and accuracy was found well within the range of 85 to 115%. The validated method was further applied to conduct in-vitro equilibrium binding study. The data was subjected to Langmuir isotherm and affinity constant (k₁) and capacity constant (k₂) were calculated.

Introduction of aromatic ring-containing substituents in cyclic nucleotides is associated with inhibition of toxin uptake by the hepatocyte transporters OATP 1B1 and 1B3

Analogs of the cyclic nucleotides cAMP and cGMP have been extensively used to mimic or modulate cellular events mediated by protein kinase A (PKA), Exchange protein directly activated by cAMP (Epac), or protein kinase G (PKG). We report here that some of the most commonly used cyclic nucleotide analogs inhibit transmembrane transport mediated by the liver specific organic anion transporter peptides OATP1B1 and OATP1B3, unrelated to actions on Epac, PKA or PKG. Several cAMP analogs, particularly with 8-pCPT-substitution, inhibited nodularin (Nod) induced primary rat hepatocyte apoptosis. Inhibition was not mediated by PKA or Epac, since increased endogenous cAMP, and some strong PKA- or Epac-activating analogs failed to protect cells against Nod induced apoptosis. The cAMP analogs inhibiting Nod induced hepatocyte apoptosis also reduced accumulation of radiolabeled Nod or cholic acid in primary rat hepatocytes. They also inhibited Nod induced apoptosis in HEK293 cells with enforced expression of OATP1B1 or 1B3, responsible for Nod transport into cells. Similar results were found with adenosine analogs, disconnecting the inhibitory effect of certain cAMP analogs from PKA or Epac. The most potent inhibitors were 8-pCPT-6-Phe-cAMP and 8-pCPT-2′-O-Me-cAMP, whereas analogs like 6-MB-cAMP or 8-Br-cAMP did not inhibit Nod uptake. This suggests that the addition of aromatic ring-containing substituents like the chloro-phenyl-thio group to the purines of cyclic nucleotides increases their ability to inhibit the OATP-mediated transport. Taken together, our data show that aromatic ring substituents can add unwanted effects to cyclic nucleotides, and that such nucleotide analogs must be used with care, particularly when working with cells expressing OATP1B1/1B3, like hepatocytes, or intact animals where hepatic metabolism can be an issue, as well as certain cancer cells. On the other hand, cAMP analogs with substituents like bromo, monobutyryl were non-inhibitory, and could be considered an alternative when working with cells expressing OATP1 family members.

[The influence of amylin on the bile acid spectrum in rats]

In acute experiments on the rats with cannulated common biliary duct, the influence ofamylin on the level of bile secretion and bile acids spectrum was investigated. It was shown that subcutaneous administration of amylin at the dose 1 mg/kg body weight doesn't affect the volume of secreted bile. Under these conditions, the concentration of taurocholic acid was increased and the concentration of tauroconjugates of chenodeoxycholic and deoxycholic acids was decreased in the bile. At the same time, the concentration of glycocholates remained constant and of unconjugated bile acids was decreased. This redistribution of bile acids spectrum leads to an increase in the coefficient of conjugation. Amylin changes the ratio of trygydroxy- and dygydroxycholates in secreted bile leading to an increase in the coefficient of hydroxylation. These results suggest that amylin enhances the processes of conjugation and hydroxylation of bile acids in hepatocytes that results in improvement of detergent properties of the bile, particularly, the ability of the bile to maintain the cholesterol in dissolved state. At the lowest effective dose, amylin does not alter the concentration of glucose in the blood.

Bile acid salt binding with colesevelam HCl is not affected by suspension in common beverages

It has been previously reported that anions in common beverages may bind to bile acid sequestrants (BAS), reducing their capacity for binding bile acid salts. This study examined the ability of the novel BAS colesevelam hydrochloride (HCl), in vitro, to bind bile acid sodium salts following suspension in common beverages. Equilibrium binding was evaluated under conditions of constant time and varying concentrations of bile acid salts in simulated intestinal fluid (SIF). A stock solution of sodium salts of glycochenodeoxycholic acid (GCDC), taurodeoxycholic acid (TDC), and glycocholic acid (GC), was added to each prepared sample of colesevelam HCl. Bile acid salt binding was calculated by high-performance liquid chromatography (HPLC) analysis. Kinetics experiments were conducted using constant initial bile acid salt concentrations and varying binding times. The affinity, capacity, and kinetics of colesevelam HCl binding for GCDC, TDC, and GC were not significantly altered after suspension in water, carbonated water, Coca-Cola, Sprite, grape juice, orange juice, tomato juice, or Gatorade. The amount of bile acid sodium salt bound as a function of time was unchanged by pretreatment with any beverage tested. The in vitro binding characteristics of colesevelam HCl are unchanged by suspension in common beverages.

A bile salt hydrolase of Brucella abortus contributes to the establishment of a successful infection through the oral route in mice

Choloylglycine hydrolase (CGH), a bile salt hydrolase, has been annotated in all the available genomes of Brucella species. We obtained the Brucella CGH in recombinant form and demonstrated in vitro its capacity to cleave glycocholate into glycine and cholate. Brucella abortus 2308 (wild type) and its isogenic Deltacgh deletion mutant exhibited similar growth rates in tryptic soy broth in the absence of bile. In contrast, the growth of the Deltacgh mutant was notably impaired by both 5% and 10% bile. The bile resistance of the complemented mutant was similar to that of the wild-type strain. In mice infected through the intragastric or the intraperitoneal route, splenic infection was significantly lower at 10 and 20 days postinfection in animals infected with the Deltacgh mutant than in those infected with the wild-type strain. For both routes, no differences in spleen CFU were found between animals infected with the wild-type strain and those infected with the complemented mutant. Mice immunized intragastrically with recombinant CGH mixed with cholera toxin (CGH+CT) developed a specific mucosal humoral (immunoglobulin G [IgG] and IgA) and cellular (interleukin-2) immune responses. Fifteen days after challenge by the same route with live B. abortus 2308 cells, splenic CFU counts were 10-fold lower in mice immunized with CGH+CT than in mice immunized with CT or phosphate-buffered saline. This study shows that CGH confers on Brucella the ability to resist the antimicrobial action of bile salts. The results also suggest that CGH may contribute to the ability of Brucella to infect the host through the oral route.

Poly(N,N,N-trimethylammoniumalkyl acrylamide chloride) Based Hydrogels for Serum Cholesterol Reduction

Hydrogels present an attractive alternative to nanoscale block copolymer aggregates and microscale resin beads as potential asystemic serum cholesterol reduction materials. Not only would the oral delivery of these materials be more pleasant than the sand-like bile salt anion sequestrant beads but also the hydrogel preparation is much simpler than the copolymer aggregate analogues [Cameron, N. S.; Eisenberg, A.; Brown, G. R. Biomacromolecules 2002, 3, 116-123. Cameron, N. S.; Eisenberg, A.; Brown, G. R. Biomacromolecules 2002, 3, 124-132]. Our goal was to explore these materials building on our experience with bulk resins and self-assembled copolymers. In this paper, following a brief introduction to hydrogels and their application to hypercholesterolemia, the synthesis, characterization, and preliminary glycocholate binding properties of poly(N,N,N-trimethylammoniumalkyl acrylamide chloride)gel are presented [Cameron, N. S.; Eisenberg, A.; Brown, G. R. Polym. Preprints 2002, 43, 771-772].

Effect of ursodeoxycholic acid on the impairment induced by maternal cholestasis in the rat placenta-maternal liver tandem excretory pathway

We investigated the effects of ursodeoxycholic acid (UDCA; 60 microg/day/100 g b.wt.) on the impairment induced by maternal obstructive cholestasis during pregnancy (OCP) in the rat placenta-maternal liver tandem excretory pathway. A blunted catheter was implanted in the common bile duct on day 14 of pregnancy, and the tip was cut on day 21. [(14)C]Glycocholate (GC) was then administered through the umbilical artery of "in situ" perfused placenta (placental transfer test) or through the maternal jugular vein (biliary secretion test), and GC bile output was measured. OCP impaired both GC placental transfer and maternal biliary secretion. UDCA moderately improved the latter but had a more marked beneficial effect on GC placental transfer. Histological examination revealed trophoblast atrophy and structural alterations, e.g., loss of apical membrane microvilli in OCP placentas. Gene expression level was investigated by real-time quantitative reverse transcription-polymerase chain reaction and Western blot analysis. OCP reduced both placental lactogen II (a trophoblast-specific gene) mRNA and the functional amount of epithelial tissue, determined by transplacental diffusion of antipyrin. Using a rapid filtration technique, impairment in the ATP-dependent GC transport across trophoblast apical plasma membranes obtained from OCP placentas was found. UDCA partially prevented all these changes. The expression level of organic anion transporters Oatp1, Oatp2, and Oatp4, and multidrug resistance-associated proteins Mrp1, Mrp2, and Mrp3 in whole placenta were not affected or were moderately affected by OCP but greatly enhanced by UDCA. In summary, UDCA partially prevents deleterious effects of OCP on the rat placenta-maternal liver tandem excretory pathway, mainly by preserving trophoblast structure and function.

Sensitivity of bile acid transport by organic anion-transporting polypeptides to intracellular pH

We investigated the influence of intracellular pH (pHi) on [14C]-glycocholate (GC) uptake by human hepatoblastoma HepG2 cells that express sodium-independent (mainly OATP-A and OATP-8), but not sodium-dependent, GC transporters. Replacement of extracellular sodium by choline (Chol) stimulated GC uptake but did not affect GC efflux from loaded cells. Amiloride or NaCl replacement by tetraethylammonium chloride (TeACl) or sucrose also increased GC uptake. All stimulating circumstances decreased pHi. By contrast, adding to the medium ammonium or imidazole, which increased pHi, had no effect on GC uptake. In Chinese hamster ovary (CHO) cells expressing rat Oatp1, acidification of pHi had the opposite effect on GC uptake, that is, this was reduced. Changes in extracellular pH (pHo) between 7.40 and 7.00 had no effect on GC uptake at pHi 7.30 or 7.45 when pHo<pHi. However, GC uptake was inhibited at pHo 7.40 and 7.80 when pHo>pHi. Inhibition was not proportional to the pHo-pHi difference. Intracellular acidification decreased V(max), but had no effect on K(m). In sum, sodium-independent GC transport can be affected by intracellular acidification, possibly due both to modifications in the driving forces and to the particular response to protonation of carrier proteins involved in this process.

Differential effects of metformin on bile salt absorption from the jejunum and ileum

AIMS

The antidiabetic drug metformin is often associated with a small reduction in total circulating cholesterol, but the mechanism responsible is unknown. As bile salts contribute significantly to cholesterol homeostasis, this study has investigated the effect of metformin on the absorption of bile salts by the jejunum and ileum, and their transfer into bile.

METHODS

Sodium-[1-14C]-glycocholate was administered into the jejunum or ileum of anaesthetized rats with and without metformin (250 mg/kg). Appearance of 14C-glycocholate in plasma and bile was followed for 150 min.

RESULTS

Absorption of 14C-glycocholate from the ileum, which is a high-capacity active process, was 10-fold greater than absorption from the jejunum, which is mainly a passive process. Metformin increased threefold the absorption of 14C-glycocholate from the jejunum. Metformin similarly increased the appearance of jejunal 14C-glycocholate in plasma and bile. In contrast to the jejunum, absorption of 14C-glycocholate from the ileum was suppressed by more than half with metformin. This was associated with corresponding reductions of 14C-glycocholate in plasma and bile.

DISCUSSION

Thus, metformin induced a large suppression of active bile salt absorption from the ileum compared with a small increase in passive absorption from the jejunum. This suggests that the ileal effect of metformin to reduce overall bile salt absorption could contribute to the modest cholesterol-lowering effect of this drug.

Maldigestion and malabsorption of dietary lipid during severe childhood malnutrition

BACKGROUND

Diets rich in lipid are used to provide energy density in treating children with severe malnutrition, but the extent to which their digestion and absorption can cope with the load effectively is uncertain.

AIM

To determine the extent of impaired digestion or absorption, in three groups of eight malnourished children (aged 5-23 months) using isotopic probes of the predominant fatty acids in coconut and corn oil used to fortify the diet.

METHODS

Each child received oral doses of one of three (13)C labelled triglycerides (trilaurin, triolein, or trilinolein). The recovery of (13)C label in stool either as triglyceride (TAG) or fatty acid (FA), was used to assess digestion and absorption. In a separate test, the recovery of label in stool following an oral dose of [(13)C]-glycocholate was measured to assess bile salt malabsorption.

RESULTS

The median recovery of label in stool was 9% (range 1-29%) of administered dose. Following treatment there was a reduction in stool (13)C excretion for the labelled TAG (<1%). In half the subjects, label was recovered as TAG in stool (median 0.6%, range 0-44%). Most label in stool was recovered as FA (median 30%, range 0-100%). Following [(13)C]-glycocholate, label was recovered in excess in about one third of studies.

CONCLUSION

Abnormalities in the gastrointestinal handling of lipid were observed in over 50% of children with severe malnutrition, reflecting problems in absorption, although impaired solubilisation or hydrolysis could also be contributory factors. The underlying lesion improves as treatment progresses, leading to concomitant improvement in function.

Characterization of cholylglycine hydrolase from a bile-adapted strain of Xanthomonas maltophilia and its application for quantitative hydrolysis of conjugated bile salts

Purified bile salt hydrolase from bile-adapted Xanthomonas maltophilia displays Michaelis-Menten kinetics on cholylglycine and cholyltaurine and hydrolyzes bile salts also in crude bovine bile. The protein is a dimer and is resistant to proteinases and to heating at 55 to 60 degrees C for up to 60 min, in agreement with calorimetric data.

Absorption of water-soluble compounds with different molecular weights and

The absorption of water soluble compounds with different molecular weights, such as phenol red (MW 354), trypan blue (MW 960), fluorescein isothiocyanate dextrans, (MW 4400 and 9100) was studied in the lung, nasal cavity, buccal cavity, small and large intestine of rats. For all the compounds, maximal absorption was observed when administered to the lung. The rank order of absorption of each compound from various administration sites was lung>small intestine> or =nasal cavity> or =large intestine> or =buccal cavity. In addition, the relationship between logarithm absorption % of the compounds from various administration sites and logarithm molecular weights of these compounds was examined. The absorption of compounds gradually decreased with increasing molecular weight for each site of administration. Moreover, the absorption of [Asu1.7]-eel calcitonin (ECT) from these sites and the effect of 10 mM sodium glycocholate, an absorption enhancer, on its absorption were also investigated in rats. When ECT alone was administered into these sites, the lung had the best absorption site of ECT, followed by the nasal cavity, the large intestine, the small intestine and the buccal cavity. Therefore, the absorption of ECT was also dependent on the administration site, although the rank order of absorption % of ECT was different from the other compounds. Sodium glycocholate (NaGC) remarkably increased ECT absorption from the small intestine, while we found marginal increase in its absorption from the lung even in the presence of NaGC. These findings provided useful fundamental information that might aid in the selection of administration routes for drugs of differing molecular weights including peptide drugs as far as the degree of drug absorption is concerned.

Gastrointestinal handling and metabolic disposal of 13C-labelled tripalmitin during rehabilitation from childhood malnutrition

We investigated the gastrointestinal handling and post-absorptive metabolic handling of [1,1,1-13C]tripalmitin and [1-13C]glycocholate during recovery from severe childhood malnutrition. Eight children were studied on three occasions: at admission (phase 1), during rapid catch-up growth (phase 2) and when weight-for-height had reached 90 % of the reference (phase 3). Breath samples were obtained over a 24 h period and stools were collected over 3 d following the administration of each tracer. At admission, the lipid content of stool expressed as a percentage of ingested lipid was 6 (range 0.7-28.9) but less variation was shown between children at phase 2 (3.3 (range 0.9-4.1)) and phase 3 (1.4 (range 0.4-2.5)). The excretion of 13C in stool varied markedly between children at admission (11.1 (sd 5.4) % administered dose) and during rehabilitation (phase 2, 15.4 (sd 16.5) % administered dose; phase 3, 6.2 (sd 10.2) % administered dose). About 5 % of the absorbed label was recovered on breath at each stage (% absorbed dose; phase 1, 5.1 (sd 6.0); phase 2, 5.2 (sd 3.1); phase 3, 6.4 (sd 6.6)). None of the children exhibited significant bile salt malabsorption as a consequence of small intestinal overgrowth. Of the 13C measured in stool, more label was recovered in fatty acids than triacylglycerols during each of the three phases and this was interpreted to reflect a failure to absorb the products of digestion. The results show that not all the children had problems associated with the digestion and absorption of 13C-labelled tripalmitin in severe malnutrition and during recovery, which was not reflected in gross lipid balance across the gastrointestinal tract. Absorbed lipid was more likely to be deposited as adipose tissue than to satisfy the immediate needs for energy.

Expression of members of the multidrug resistance protein family in human term placenta

The placenta serves, in part, as a barrier to exclude noxious substances from the fetus. In humans, a single-layered syncytium of polarized trophoblast cells and the fetal capillary endothelium separate the maternal and fetal circulations. P-glycoprotein is present in the syncytiotrophoblast throughout gestation, consistent with a protective role that limits exposure of the fetus to hydrophobic and cationic xenobiotics. We have examined whether members of the multidrug resistance protein (MRP) family are expressed in term placenta. After screening a placenta cDNA library, partial clones of MRP1, MRP2, and MRP3 were identified. Immunofluorescence and immunoblotting studies demonstrated that MRP2 was localized to the apical syncytiotrophoblast membrane. MRP1 and MRP3 were predominantly expressed in blood vessel endothelia with some evidence for expression in the apical syncytiotrophoblast. ATP-dependent transport of the anionic substrates dinitrophenyl-glutathione and estradiol-17-beta-glucuronide was also demonstrated in apical syncytiotrophoblast membranes. Given the cellular distribution of these transporters, we hypothesize that MRP isoforms serve to protect fetal blood from entry of organic anions and to promote the excretion of glutathione/glucuronide metabolites in the maternal circulation.

Solution structure of ileal lipid binding protein in complex with glycocholate

Ileal lipid binding protein (ILBP) is a cytosolic lipid-binding protein that binds both bile acids and fatty acids. We have determined the solution structure of porcine ILBP in complex with glycocholate by homonuclear and heteronuclear two-dimensional NMR spectroscopy. The conformation of the protein-ligand complex was determined by restrained energy minimization and simulated annealing calculations after docking the glycocholate ligand into the protein structure. The overall tertiary structure of ILBP is highly analogous to the three-dimensional structures of several other intracellular lipid binding proteins (LBPs). Like the apo-structure, the bile-acid complex of ILBP is composed of 10 anti-parallel beta-strands that form a water-filled clam-shell structure, and two short alpha-helices. Chemical shift data indicated that the bile acid ligand is bound inside the protein cavity. Furthermore, 13C-edited heteronuclear single-quantum correlation-NOESY experiments showed NOE contacts between several aromatic residues located in the proposed bile acid portal region and the 13C-labeled ligand. A single bile acid molecule is bound inside the protein, with the steroid moiety penetrating deep into the water-accessible internal cavity, such that ring A is located right above the plane of the Trp49 indole ring. The carboxylate tail of the ligand is protruding from the proposed bile acid portal into the surrounding aqueous solution. The body of the steroid moiety is oriented with the nonpolar face in contact with the mostly hydrophobic residues of beta-strands C, D and E, while the polar face shows contacts with the side-chains of Tyr97, His99, Glu110 and Arg121 in beta-strands H, I and J. Thus, the conformational arrangement of the ligand complex suggests that the binding affinity of ILBP for bile acid molecules is based mainly on strong hydrophobic interactions inside the protein cavity. Furthermore, this binding mode explains how ILBP can transport unconjugated and conjugated bile acids.

Effect of maternal cholestasis on bile acid transfer across the rat placenta-maternal liver tandem

Cholestasis of pregnancy induces alterations in bile acid transport by human trophoblast plasma membrane (TPM) vesicles. We investigated whether maternal cholestasis affects the overall ability of the rat placenta to carry out vectorial bile acid transfer from the fetus to the mother. Complete obstructive cholestasis (OCP) was maintained during the last week of pregnancy and released at term (day 21), before experiments were performed. In situ single-pass perfusion of one placenta per rat with 250 nmol [(14)C]glycocholic acid (GC) revealed an impaired uptake in OCP rats (2.28 vs. 5.53 nmol in control rats). Approximately 100% of GC taken up by control placentas was secreted in maternal bile over 120 minutes (5.38 nmol), whereas this was only 61% (1.40 nmol) of the GC taken up by OCP placentas. When 5 nmol GC was administered through the jugular vein no significant difference between both groups in total GC bile output was found. The efficiency (V(max)/K(M)) of adenosine triphosphate (ATP)-dependent GC transport by vesicles from the maternal side of TPM was decreased (-41%) in OCP. Moreover, histological examination of the placentas suggested a reduction in the amount of functional trophoblast in the OCP group. This was consistent with a lower antipyrine diffusion across the placenta in these animals. In sum, our results indicate that maternal cholestasis affects the ability of the placenta to efficiently carry out bile acid transfer from fetal to maternal blood. Changes in both the structure and the functionality of the chorionic tissue may account for this impairment.

Dissolution of hydrocortisone in human and simulated intestinal fluids

PURPOSE

To compare solubility and dissolution rate of hydrocortisone in aspirated human intestinal fluids (HIFs) with simulated intestinal fluids (SIFs) and buffer.

METHODS

Solubility and flux from a rotating disk of hydrocortisone were measured. The bile salt content, pH and osmotic pressure were determined in HIFs.

RESULTS

In fasted state the solubility of hydrocortisone was higher in HIFs than in the buffer and SIFs. The flux of hydrocortisone in HIFs was similar to the flux in the buffer but lower than the flux in SIFs at fasted state. Addition of intestinal surfactants in SIFs increased solubility and flux at both fasted and fed state. The increase in solubility was caused by micelle formation in SIFs. The increase in flux may partly be explained by increased solubility. The bile salt content of the HIFs did not correlate with the solubility or the flux but pH in the HIFs seems to have some effect on the components of the HIFs resulting in increased solubility.

CONCLUSIONS

It is possible to perform comparable dissolution tests in HIFs and SIFs. The lack of correlation between the results in HIFs and the bile salt content may be explained by the relatively low lipophilicity of the model drug.

Bile salt hydrolase activity of three strains of Lactobacillus acidophilus

Three strains of Lactobacillus acidophilus, two from human intestinal origin (016 and L1) and one from porcine intestinal origin (ATCC 43121), were tested for their bile salt deconjugation activity. The L. acidophilus ATCC 43121 had more deconjugating activity of both sodium glycocholate and sodium taurocholate at pH 6.5 than did either L. acidophilus 016 or L1. The activity of intracellular bile salt hydrolase found in strain ATCC 43121 was 14-fold higher than that in either of the other two strains. The optimum pH for deconjugation of sodium glycocholate was between 4 and 5.5 for all three strains. For deconjugation of sodium taurocholate, the optimum pH was between 3.5 and 4.5 for strains L1 and ATCC 43121 and was between pH 5 and 6 for strain O16. The molecular mass of the enzyme in all three strains of L. acidophilus was estimated to be 126 kDa by Sephadex G-200 gel filtration. All three strains exhibited more bile salt hydrolase activity towards sodium glycocholate than towards sodium taurocholate.

Measurement of bile salt hydrolase activity from Lactobacillus acidophilus based on disappearance of conjugated bile salts

Bile salt hydrolase activity of Lactobacillus acidophilus was measured based on the disappearance of sodium glycocholate and sodium taurocholate from the reaction mixture using HPLC. The amount of sodium glycocholate and sodium taurocholate that disappeared was proportional to the amount of sodium cholate that appeared in the mixture as detected by HPLC. Sodium glycocholate did not precipitate at the enzyme reaction conditions (37 degrees C and pH 5.4) for determining bile salt hydrolase activity. The bile salt hydrolase assay was insensitive to low oxidation-reduction potential when measuring bile salt hydrolase from L. acidophilus, an intestinal microorganism. However, EDTA and freezing temperatures were necessary to maintain stability of the partially purified enzyme during storage.

Dietary inulin lowers plasma cholesterol and triacylglycerol and alters biliary bile acid profile in hamsters

The mechanisms by which inulin may elicit its lipid-lowering effect are not well elucidated. To examine the lipid-lowering potential of inulin and especially its effect on bile acid metabolism, male golden Syrian hamsters were fed semipurified diets containing 20 g/100 g fat, 0.12 g/100 g cholesterol and 0 (control), 8, 12 or 16% inulin for 5 wk. Plasma total cholesterol concentrations were significantly lowered by 18, 15 and 29% in hamsters fed 8, 12 and 16% inulin, respectively. Dietary inulin specifically decreased VLDL cholesterol, which was significantly lower in hamsters fed 16% inulin compared with controls (1.1 +/- 0.3 vs. 2.9 +/- 0.6 mmol/L). LDL and HDL cholesterol were not significantly affected by dietary inulin. Plasma triacylglycerol was significantly reduced by 40 and 63% in hamsters fed 12 and 16% inulin, respectively. Hepatic total cholesterol and particularly esterified cholesterol accumulation were significantly lower in hamsters fed 8% inulin compared with controls. All three levels of dietary inulin caused distinct alterations in the bile acid profile of gallbladder bile. Taurochenodeoxycholic acid was significantly lower, whereas glycocholic and glycodeoxycholic acid were greater in hamsters fed inulin. Daily fecal bile acid excretion (micromol/d) tended to be greater (P = 0.056) in inulin-fed hamsters compared with controls, whereas daily neutral sterol excretion was not affected. These data demonstrate that the lipid-lowering action of inulin is possibly due to several mechanisms, including altered hepatic triacylglycerol synthesis and VLDL secretion and impaired reabsorption of circulating bile acids.

Transport and biotransformation of the new cytostatic complex cis-diammineplatinum(II)-chlorocholylglycinate (Bamet-R2) by the rat liver

Rat liver uptake and bile output of the cytostatic complex cis-diammineplatinum(II)-chlorocholylglycinate (Bamet-R2) were studied. Up to 100 microM, Bamet-R2 uptake by rat hepatocytes in primary culture followed saturation kinetics (Vmax = 0.65 +/- 0.12 nmol/5 min per mg protein; K(M) = 45.2 +/- 10.7 microM). Bamet-R2 uptake was lower than that of cholylglycinate (CG) but higher than that of cisplatin. Replacement of 116 mM NaCl by 116 mM choline chloride did not significantly reduce Bamet-R2 uptake. Addition of 500 microM CG, cholic acid, estrone sulfate, or ouabain to 50 microM Bamet-R2-containing incubation media inhibited Bamet-R2 uptake. No liver biotransformation of Bamet-R2 occurred, as indicated by HPLC analysis of bile collected from anesthetized rats after intravenous administration of the drug. Bamet-R2 uptake and secretion into bile by isolated rat livers exceeded those of cisplatin but were lower than those of CG. Differences between Bamet-R2 and CG were more marked for bile output than for liver uptake. Thus, higher Bamet-R2 than CG or cisplatin liver content was found. Co-administration of Bamet-R2 and CG revealed that CG induced a slight reduction in Bamet-R2 uptake and a marked inhibition in Bamet-R2 bile output. By contrast, Bamet-R2 had no effect on CG on either liver uptake or bile output. In sum, the present data indicate that Bamet-R2 is efficiently taken up and secreted into bile by the rat liver by mechanisms shared in part by natural bile acids.

Bile salt deconjugation and cholesterol removal from media by Lactobacillus casei

Lactobacillus casei N19 and E5 and Lactobacillus acidophilus L1 and ATCC 43121 were compared for their ability to deconjugate bile salts and remove cholesterol from MRS broth during growth at pH 6.0 and during growth without pH control. Samples grown without pH control dropped to pH 4.2 to 4.5 during 20 h of incubation, depending on the culture. The plate counts indicated that populations in all cultures were near their maximum numbers after 16 h of growth. The amount of cholesterol removed from the broth was similar for both strains of L. acidophilus grown with and without pH control. However, the strains of L. casei differed significantly in the amount of cholesterol removed during growth with or without pH control. Both cultures of L. casei that were grown at pH 6.0 removed very little cholesterol from the broth, but cells grown without pH control removed up to 60 micrograms of cholesterol/ml. All cultures of both species deconjugated 60 to 90% of the bile salts. Lactobacillus acidophilus L1 was the only culture to demonstrate differences between the two pH treatments in the amount of bile salts deconjugated; however, there was no difference in the amounts of cholesterol removed. These results indicate that most of the cholesterol removal from broth by L. acidophilus was due to assimilation, perhaps by the incorporation of cholesterol into the cellular membrane. Lactobacillus casei most likely removes cholesterol from broth by means of the destabilization of cholesterol micelles and the coprecipitation of the cholesterol with the deconjugated bile salts at pH less than 6.0.

Beneficial effect of ursodeoxycholic acid on alterations induced by cholestasis of pregnancy in bile acid transport across the human placenta

BACKGROUND/AIMS

The existence of impairment in bile acid transport across the placenta during intrahepatic cholestasis of pregnancy and the effect of ursodeoxycholic acid treatment (1 g/day) were investigated.

METHODS

Kinetic parameters were calculated from experiments carried out on membrane vesicles obtained from basal (TPMb, fetal-facing) and apical (TPMa, maternal-facing) trophoblast plasma membranes. Bile acid uptake was measured using varying concentrations of [14C]-glycocholate and a rapid filtration technique.

RESULTS

The maximal velocity of transport (Vmax), the apparent affinity constant (Kt) and the efficiency (Ef) of transport (Vmax/Kt) of the anion:bile acid exchanger located at the TPMb were reduced in intrahepatic cholestasis of pregnancy. Ursodeoxycholic acid induced a reversal of Vmax, Kt and Ef to normal values. Owing to the 3-fold increase in Vmax, with no change in Kt, intrahepatic cholestasis of pregnancy induced an enhancement in Ef of ATP-independent bile acid transport across TPMa. Both Vmax and Ef were restored to normal values by ursodeoxycholic acid. Finally, in ATP-dependent bile acid transport across TPMa, a reduction in the Ef due to an increase in Vmax together with a more pronounced increase in Kt was found. This impairment was also reversed by ursodeoxycholic acid.

CONCLUSIONS

These results suggest that placenta bile acid transport systems are impaired in intrahepatic cholestasis of pregnancy. Moreover, together with the confirmed beneficial effect for intrahepatic cholestasis of pregnancy patients, such as the relief of pruritus and the improvement in biochemical markers of cholestasis, ursodeoxycholic acid treatment restores the ability of the placenta to carry out vectorial bile acid transfer.

Interaction of iron(II) with bile salts

Iron(II) ions react with small aggregates of cholate, glycocholate, chenodeoxycholate, and deoxycholate to form soluble and colloidal compounds. Taurocholate under conditions used does not react with the Fe2+ ion. Small aggregates of dihydroxy bile salts (predominating in the premicellar region, at concentrations of the bile salt above 1 mmol dm-3) have a larger affinity for Fe2+ compared to those formed from cholate anions. In their interactions with small aggregates of cholate anions, the Fe2+ ion shows an affinity comparable to that of Cu2+ and Cd2+ and somewhat larger than that of Zn2+. Small aggregates of cholate show a higher ability to mask Fe2+ than those of taurocholate and glycocholate. Interaction of glycocholic acid anions with Fe2+ ions is sufficient to prevent iron(II) precipitation.

Giardia lamblia: evidence for carrier-mediated uptake and release of conjugated bile acids

Giardia lamblia trophozoites colonize the human small intestine, where they are exposed to high concentrations of conjugated bile acids. Previous work has shown that bile acids enhance trophozoite survival, multiplication, and differentiation into the cyst stage. Therefore, experiments were performed to test whether carrier-mediated uptake of conjugated bile acids is present in this primitive parasite. Uptake of both cholyltaurine (C-tau) and cholylglycine (C-gly) was increased manyfold after culturing trophozoites in medium lacking bile acids. Absence of uptake at 4 degrees C and inhibition by other conjugated bile acids provided additional evidence for carrier-mediated uptake. Uptake of C-tau was greater than that of C-gly under all experimental conditions and appeared to be mediated by a different carrier. The major evidence for different carriers is that C-tau uptake was Na(+)-dependent, while C-gly uptake was not. In addition, C-tau uptake was more strongly inhibited by DTNB and several organic anions than C-gly uptake. Radiolabeled C-tau and C-gly were each released rapidly from trophozoites at 37 degrees C but not at 4 degrees C, suggesting that release of conjugated bile acids was also carrier-mediated. These findings are consistent with the notion that multiple transporters for conjugated bile acids are present in a lower eukaryote. We speculate that intracellular bile acids may facilitate lipid trafficking and membrane biosynthesis.

Effect of maternal cholestasis on biliary lipid and bile acid secretion in the infant rat

Partial and reversible impairment of bile formation has been reported to occur in the offspring of rats undergoing common bile duct ligation during the last third of pregnancy. This situation was defined as latent cholestasis of the neonate and was suggested to be related to the multilamellar bodies partially occupying the canalicular lumen. The current study was undertaken to investigate the presence of alterations in the secretion of biliary lipids in these infant rats. Using both high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) analyses, no changes caused by maternal cholestasis were found in either the conjugation pattern, or in the ratio of primary to secondary major bile acids in bile samples collected from 4-week-old and 8-week-old rats. However, a decrease in the proportion of cholate together with an increase in the amount of alpha- and omega-muricholate were found at 4 weeks of age. These changes were different from those observed in the pattern of maternal plasma bile acids, in which beta-, but not alpha-muricholate, concentrations were increased. Moreover, studies performed by labeling the bile acid pool of the cholestatic mother-fetus tandem with [14C]glycocholic acid (GC) at day 16 of pregnancy indicated that only a minor proportion (approximately 10%) of bile acids found in 4-week-old pups was of maternal origin. Changes in the bile acid pool composition were fully reversed by 8 weeks of age. Bile lecithin and cholesterol output were determined by enzymatic techniques, both under basal conditions and during stepwise taurocholate (TC) infusion. At the time when multilamellar bodies were found, i.e., 4 weeks after birth, no change in either nonstimulated or TC-induced cholesterol output was observed. By contrast, both spontaneous and TC-induced lecithin secretion were markedly higher (+200%) in pups of cholestatic mothers as compared with control rats. These differences were abolished at 8 weeks of age. At this time, cholesterol output was significantly lower than that found in younger animals. This reduction was more pronounced in the control than in the cholestatic group. Histological examination of liver samples collected from the cholestatic group at 4 weeks of age revealed the presence of multilamellar bodies not only in the canalicular lumen but also within vesicular structures located in the pericanalicular area or near the Golgi apparatus. Both intracellular and intracanalicular bodies were present before and after TC infusion for 2 hours. These results indicate that maternal cholestasis in rats induces profound alterations in biliary lipids and bile acid secretion in their pups. Because bile acids are important activators of different steps responsible for biliary lipid secretion (intracellular trafficking, releasing into bile, and solubilization), alterations in maternal bile acid pool size and composition may affect the fetal development of biliary lipid secretion mechanisms, which may result in the appearance of multilamellar bodies within bile canaliculi, which in turn may be involved in the reversible latent cholestasis observed in these infants rats.

Evidence that interference with binding to hepatic cytosol binders can inhibit bile acid excretion in rats

We previously identified that Y' bile acid binders (3alpha-hydroxysteroid dehydrogenases) interact with bile acids in intact rat hepatocytes using [3beta-3H, C24-14C]bile acids and that indomethacin, a competitive inhibitor of 3alpha-hydroxysteroid dehydrogenase, inhibits 3H-loss from the C3-position of bile acids as well as inhibits hepatic bile acid removal and excretion. To study the kinetics of these inhibitory effects, glycocholate transport was studied in the absence and presence of indomethacin in the single-pass perfused rat liver. Indomethacin decreased net hepatic glycocholate uptake in the perfused liver, which was confirmed in isolated hepatocytes and basolateral liver plasma membrane vesicles. However, indomethacin markedly increased the sinusoidal efflux and decreased the biliary excretion of glycocholate in the perfused liver. These observations indicate that the effect of indomethacin to delay biliary glycocholate excretion is related to either intracellular or canalicular glycocholate transport. The latter possibility seemed unlikely because indomethacin did not inhibit electrogenic or adenosine triphosphate (ATP)-dependent glycocholate uptake by canalicular liver plasma membrane vesicles. Thus, the current data support an important role for binding of bile acids to cytosolic proteins in overall hepatic transport and suggest that specific interference with cytosolic binding can interfere with the excretion of bile acids.

Bumetanide is not transported by the Ntcp or by the oatp: evidence for a third organic anion transporter in rat liver cells

The loop diuretic bumetanide which inhibits hepatic bile acid uptake competitively according to its transport kinetics has been proposed to serve as a substrate of a multispecific bile acid transport system in liver parenchymal cells. However, when the in vitro transcripts of two cloned hepatic bile acid uptake carriers, the Ntcp (Na+/taurocholate cotransporting polypeptide) and the oatp (organic anion transporting polypeptide), was expressed for three days in Xenopus laevis oocytes [3H]bumetanide uptake was not increased although bile acid uptake was stimulated. The data presented show that bumetanide is taken up by a third organic anion transport system which is different from the cloned bile acid transporters.

Hepatobiliary transport of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors conjugated with bile acids

To obtain prodrugs with affinity to liver parenchymal cells, the hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors HR 780 and lovastatin (syn. mevinolin) were conjugated with the bile acids cholic acid, taurocholic acid, and glycocholic acid. Hepatic uptake and biliary excretion of the coupled drugs were investigated and compared with the noncoupled drugs. Studies were performed with livers of normal Wistar rats, and TR-/GT- Wistar rats with deficient drug excretion. The experiments showed that the parent drug HR 780 was slowly excreted into bile. In contrast, the excretion of the bile acid-conjugated HR 780 derivatives S 3554 (conjugated with cholate), S 3898 (conjugated with glycocholate), and S 4193 (conjugated with taurocholate) was rapid and very efficient in both groups of rat strains. The bile acid-conjugated HMG-CoA reductase inhibitors showed a 10 to 20 times higher affinity for the uptake systems of bile acids than the noncoupled parent drug compounds, and even higher affinities than the bile acids themselves. The cholate conjugate of HR 780 (compound S 3554) was shown to be a noncompetitive inhibitor of taurocholate uptake and a competitive inhibitor of sodium-independent cholate uptake (Ki = 1 mumol/L). Uptake of radiolabeled S 3554 into isolated rat hepatocytes was observed to be rapid, cell specific, saturable, energy dependent, and carrier mediated. However, the carrier for S 3554 uptake was found not to be the cloned Na(+)-dependent taurocholate cotransporting polypeptide Ntcp. Expression of this carrier cRNA in Xenopus laevis oocytes did not stimulate S 3554 uptake.

Molecular and functional characterization of an organic anion transporting polypeptide cloned from human liver

BACKGROUND & AIMS

Based on a recently cloned rat liver organic anion transporter, we attempted to clone the corresponding human liver organic anion transporting polypeptide.

METHODS

A human liver complementary DNA library was screened with a specific rat liver complementary DNA probe. The human liver transporter was cloned by homology with the rat protein and functionally characterized in Xenopus laevis oocytes.

RESULTS

The cloned human liver organic anion transporting polypeptide consists of 670 amino acids and shows a 67% amino acid identity with the corresponding rat liver protein. Injection of in vitro transcribed complementary RNA into frog oocytes resulted in the expression of sodium-independent uptake of [35S]bromosulfophthalein (Michaelis constant [Km], approximately 20 mumol/L), [3H]cholate (Km, approximately 93 mumol/L), [3H]taurocholate (Km, approximately 60 mumol/L), [14C]glycocholate, [3H]taurochenodeoxycholate, and [3H]tauroursodeoxycholate (Km, approximately 19 mumol/L). Northern blot analysis showed cross-reactivity with messenger RNA species from human liver, brain, lung, kidney, and testes. Polymerase chain reaction analysis of genomic DNA from a panel of human-rodent somatic cell hybrids mapped the cloned human organic anion transporter to chromosome 12.

CONCLUSIONS

These studies show that the cloned human liver organic anion transporter is closely related to, but probably not identical to, the previously cloned rat liver transporter. Furthermore, its additional localization in a variety of extrahepatic tissues suggests that it plays a fundamental role in overall transepithelial organic anion transport of the human body.

Characterization of bile salt uptake by Giardia lamblia

We have shown previously that Giardia lamblia takes up conjugated bile salt in vitro, and have now investigated the mechanism by which this occurs. Uptake of sodium taurocholate (TC) and glycocholate (GC) with respect to time had an initial exponential component followed by a linear component, consistent with a combination of both active and passive transport processes. The presence of an active transport process was further supported by experiments which showed that bile salt uptake: (i) was concentration dependent (apparent Km's for TC and GC were 0.21 and 0.63 mM, respectively); (ii) was competitively inhibitable; (iii) was reduced by the metabolic inhibitor sodium fluoride (50 mM) and low temperature (4 degrees C). Bile salt was not taken up by glutaraldehyde-fixed parasites, indicating that bile salt was not merely being adsorbed on to the parasite surface. Differential centrifugation of lyzed parasites following exposure to radiolabelled GC, showed that the majority of bile salt was located in the cytosol fraction (76%) with a relatively minor component associated with cell membrane, indicating that bile salt had been internalized. Bile salt analysis of extracts of parasites and culture medium indicated that GC had not been metabolized by Giardia. Thus, like the mammalian ileum, Giardia appears to take up conjugated bile salts by active and passive transport processes. Conjugated bile salts are known to promote encystation and thus these uptake mechanisms may constitute an important survival mechanism for the parasite enabling it to complete its life cycle.

Enhanced secretion of glycocholic acid in a specially adapted cell line is associated with overexpression of apparently novel ATP-binding cassette proteins

Secretion of anionic endo- and xenobiotics is essential for the survival of animal and plant cells; however, the underlying molecular mechanisms remain uncertain. To better understand one such model system--i.e., secretion of bile acids by the liver--we utilized a strategy analogous to that employed to identify the multidrug resistance (mdr) genes. We synthesized the methyl ester of glycocholic acid (GCE), which readily enters cells, where it is hydrolyzed to yield glycocholic acid, a naturally occurring bile acid. The rat hepatoma-derived HTC cell line gradually acquired resistance to GCE concentrations 20-fold higher than those which inhibited growth of naive cells, yet intracellular accumulation of radiolabel in resistant cells exposed to [14C]GCE averaged approximately 25% of that in nonresistant cells. As compared with nonresistant cells, resistant cells also exhibited (i) cross-resistance to colchicine, a known mdr substrate, but not to other noxious substances transported by hepatocytes; (ii) increased abundance on Northern blot of mRNA species up to 7-10 kb recognized by a probe for highly conserved nucleotide-binding domain (NBD) sequences of ATP-binding cassette (ABC) proteins; (iii) increased abundance, as measured by RNase protection assay, of mRNA fragments homologous to a NBD cRNA probe; and (iv) dramatic overexpression, as measured by Western blotting and immunofluorescence, of a group of 150- to 200-kDa plasma membrane proteins recognized by a monoclonal antibody against a region flanking the highly conserved NBD of mdr/P-glycoproteins. Finally, Xenopus laevis oocytes injected with mRNA from resistant cells and incubated with [14C]GCE secreted radiolabel more rapidly than did control oocytes. Enhanced secretion of glycocholic acid in this cell line is associated with overexpression of ABC/mdr-related proteins, some of which are apparently novel and are likely to include a bile acid transport protein.

Change of zonal bile acid processing after partial hepatectomy in the rat

The aim of this study was to analyze whether partial hepatectomy alters functional liver heterogeneity with respect to bile acid processing. One, 5 and 21 days after liver resection (approximately 80% of liver mass) in male Sprague-Dawley rats (300-400 g), isolated livers were perfused in either the antegrade or the retrograde direction, respectively, with 32 nmol cholate/min per g liver. Uptake, metabolism and biliary secretion kinetics were determined by bolus injection of 14C-cholate. Uptake and biliary recovery (within 30 min) of cholate were > 90% in all groups. One day postresection, liver mass had already doubled and it regenerated to over 80% 5 days after resection. Serum bile acid concentration increased rapidly, peaking 6 h after resection (176.7 +/- 28.5 mumol/l) (mean +/- SEM). Twenty-one days after resection it fell to control values (23.2 +/- 3.8 mumol/l). T25 (T50), the time (min) necessary to excrete 25% (50) of the bile acid load into bile, was strikingly different between periportal and pericentral cells of controls (1.8 vs 5.7 and 3.4 vs 8.1). Five days after resection this difference became smaller (1.4 vs 2.9 and 2.8 vs 5.5) due to accelerated biliary cholate secretion in pericentral cells. Pericentral cells of controls metabolized cholate more extensively to taurocholate (approximately 83%) and glycocholate (approximately 13%) than periportal cells of controls (65%, 10%), leading to a 5-fold higher proportion of unmetabolized cholate in periportal than pericentral cells (25% vs 5%). Five days after resection the percentage of taurocholate decreased significantly at the expense of an increased formation of glycocholate.(ABSTRACT TRUNCATED AT 250 WORDS)

ATP-dependent bile acid transport across microvillous membrane of human term trophoblast

The main fate for fetal bile acids is to be transferred to the mother by the trophoblast. In this study, ATP-dependent bile acid transport across the maternal- and the fetal-facing plasma membranes (mTPM and fTPM, respectively) of the human trophoblast was investigated. With the use of [14C]glycocholate (GC) and a rapid-filtration technique, GC transport by mTPM and fTPM was measured in the absence or the presence of 3 mM ATP plus an ATP-regenerating system. GC efflux from preloaded mTPM or fTPM vesicles was found to be insensitive to ATP. By contrast, GC uptake by mTPM, but not by fTPM, was significantly increased (approximately threefold) by ATP. This was temperature sensitive and occurred into an osmotically reactive space. Kinetic analysis revealed that GC uptake by mTPM was saturable and fit the Michaelis-Menten equation both in the absence and in the presence of ATP. ATP-dependent transport was not abolished by a protonophore (carbonyl cyanide p-trifluormethoxyphenyl hydrazone) together with 100 mM K+ (in = out) plus a K+ ionophore (valinomycin). It specifically required hydrolyzable ATP, although CTP had a slight stimulatory effect. Neither Na+ nor Cl- (100 mM, in = out) was mandatory. Moreover, 100 mM gradients of either Na+ (in << out) or Cl- (in >> out) had no effect on ATP-dependent GC uptake. This was inhibited by vanadate and bile acid analogues but not by several cholephilic organic anions and a variety of adenosine triphosphatase inhibitors. These results provide strong evidence for the existence of an ATP-dependent transport system for bile acids across the apical membrane of human trophoblast, which may play an important role in the control of the overall fetal-maternal bile acid traffic.

[A study on the mechanism of the choleretic effect of proglumide]

It has been found in this laboratory that proglumide (PGM) can increase hepatic bile flow in humans and in several species of animals, and lower gallstone formation in experimental animals. In order to further investigate the mechanisms of this choleretic effect of PGM, studies with isolated rat liver cells and plasma membranes were performed. The results indicated that PGM could increase the activity of membrane Na+, K(+)-ATPase significantly. On the other hand, PGM decreased the net uptake of 14C-glycocholic acid by rat liver cells. These data suggest that the choleretic effect of PGM is likely to be mediated through the enhancement of membrane Na+, K(+)-ATPase activity (which would in turn increase water and electrolyte output), rather than by affecting bile acid uptake by liver cells. It was also observed that PGM could reverse the inhibitory effect of somatostatin on the activity of membrane Na+, K(+)-ATPase. These results provide some clues for the elucidation of the mechanisms of the inhibitory effect of PGM on gallstone formation in experimental animals.

Direct energization of bile acid transport into plant vacuoles

Bile acids were shown to be transported into barley mesophyll vacuoles. Uptake of the cholate conjugates taurocholate and glycocholate is strictly ATP-dependent. Uptake of taurocholate is a saturable process (Km = 40 microM) and is inhibited by vanadate but not by bafilomycin, a specific inhibitor of the vacuolar H(+)-ATPase. Together with the observation that the non-hydrolyzable ATP analog AMPPNP (5'-adenylyl beta,gamma-imidodiphosphate) does not stimulate, but rather inhibits, the ATP-dependent uptake of taurocholate, and that a 3-fold accumulation of the bile acid is observed in the presence of bafilomycin, these results suggest that taurocholate is transported into the vacuole by a primary active process as is the case for its canalicular secretion in rat liver (Nishida, T., Gatmaitan, Z., Che, M., and Arias, I. M. (1991) Proc. Natl. Acad. Sci. U. S. A. 88, 6590-6594). Taurocholate uptake is inhibited by other bile acids and is slightly stimulated by glutathione S-conjugates. The different responses of the glutathione S-conjugate (Martinoia, E., Grill, E., Tommasini, R., Kreuz, K., and Amrhein, N. (1993) Nature 364, 247-249) and the taurocholate transporters, respectively, to substrates, oligomycin, GTP, and UTP suggest the presence of at least two ATPases specifically involved in the transport of conjugates across the tonoplast. As cholate and its conjugates have so far not been reported to occur in plants, the physiological function of the novel transport ATPase described here is presently unknown.

The effect of non-starch polysaccharide supplementation on circulating bile acids, hormone and metabolite levels following a fat meal in human subjects

The effects of guar gum, sugar-beet fibre (SBF) and wheat bran supplementation of a high-fat test meal were compared with an NSP-free control meal and a meal containing an equivalent amount of the ion-exchange resin cholestyramine in healthy non-obese human volunteers. Their effects on gastric emptying, postprandial circulating bile acids, triacylglycerols and gastrointestinal hormone levels were studied. The in vitro binding of NSP and cholestyramine to [1-14C]glycocholic acid was measured and compared with their in vivo effect. Guar gum and cholestyramine supplementation significantly lowered circulating postprandial bile acid, triacylglycerol and gastric inhibitory polypeptide concentrations, but sugar-beet fibre and wheat bran were without effect. Liquid gastric emptying, as assessed by circulating paracetamol levels, was slightly accelerated in the guar gum-supplemented meal. Glycocholic acid bound strongly to the insoluble fraction of cholestyramine and the soluble fraction of guar gum. The insoluble fractions of SBF and wheat bran bound only small quantities of glycocholate; no bile acid binding was detected in the soluble fractions of these NSP. The study demonstrates that measurement of postprandial bile acids enables an indirect measurement to be made of bile acid binding to NSP in vivo. The results support the hypothesis that the hypocholesterolaemic action of guar gum is largely mediated via interruption of the enterohepatic bile acid circulation, but indicate that the hypocholesterolaemic action of SBF is mediated by another mechanism.

Interaction between cholephilic anions and bile acid transport across basal membrane of human trophoblast

The sensitivity of radiolabeled bile acid (BA) binding and transport by basal plasma membrane (BPM) vesicles of human trophoblast to cholephilic organic anions (COAs) was studied by a rapid filtration technique. Glycocholate (GC) efflux from preloaded (15 microM GC) vesicles was investigated in the presence of 300 microM COAs at the trans-side of the membrane. Bilirubin (BR) diglucuronide and rose bengal induced a very strong transstimulating effect, whereas phalloidin and phenol red showed a negligible effect. This effect was from strong to moderate for indocyanine green > bromosulfophthalein (BSP) > or = fusidic acid > or = phenolphthalein > or = BR ditaurate > or = rifamycin SV > or = rifampicin. BSP-induced transstimulation was not additive to the "velocity effect" previously reported for bicarbonate. At the cis-side, BSP reduced the saturable component of taurocholate (TC) binding to BPM vesicles. BSP also induced a partial and mixed type of inhibition both in TC uptake [inhibitor constant (Ki) 227 microM] and efflux (Ki 209 microM). Two binding sites with overlapping specificity for BAs and other COAs are proposed in this carrier, the site for non-BA COA presumably corresponding to that for bicarbonate. In summary, the results indicate that several COAs can act as potential substrates for the BA carrier located at the BPM of human trophoblast. This stresses the "biliary-like" role of the placenta and suggests the possibility of developing new functional tests for this organ on the basis of fetal-maternal transfer of nontoxic cholephilic dyes.

Selective induction by phenobarbital of the electrogenic transport of glutathione and organic anions in rat liver canalicular membrane vesicles

Glutathione is excreted into bile via a low affinity, electrogenic, ATP-independent transport system which is cis-inhibited and trans-stimulated by certain organic anions (Fernández-Checa, J. C., Takikawa, H., Horie, T., Ookhtens, M., and Kaplowitz N. (1992) J. Biol. Chem. 267, 1667-1673). This transport system differs from the sinusoidal carrier in several respects, such as affinity for transport and inhibitor specificity. Another differential aspect is the selective increase by phenobarbital pretreatment of GSH excretion into bile without changing the sinusoidal release into blood. To determine if phenobarbital induces the GSH transporter in the canalicular membrane and if this is reflected in the induction of organic anion transport, we have used rat liver canalicular (cLPM) and sinusoidal (bLPM) enriched membrane vesicles from liver of control (saline) and phenobarbital-treated rats. cLPM vesicles prepared from phenobarbital-pretreated rats exhibited a significant, 46% increase in Vmax for transport (9.02 +/- 0.3 versus 6.17 +/- 0.5 nmol/mg/15 s) without a change in the Km for GSH transport (14.0 +/- 1.1 versus 16.7 +/- 2.7 mM, respectively). Kinetic parameters for GSH transport in bLPM vesicles remained unchanged after phenobarbital treatment versus control (Vmax, 4.67 +/- 0.2 versus 4.77 +/- 0.2 nmol/mg/15 s; Km, 7.79 +/- 0.8 versus 6.95 +/- 0.8 mM, respectively). Phenobarbital treatment increased the electrogenic transport of [35S]sulfobromophthalein (BSP) (5 and 50 microM) but not the electrogenic uptake of [14C] glycocholic acid (10 and 200 microM). In addition, the ATP-dependent transport of [35S]BSP, [3H]leukotriene C4, and [14C]glycocholic acid into cLPM vesicles was not altered by phenobarbital treatment. The ATP-independent transport of [35S]BSP in cLPM was cis-inhibited and trans-stimulated by GSH, supporting the view that BSP and GSH share a common multispecific transporter. Thus, among the various canalicular transport systems, the multispecific electrogenic organic anion and GSH transport system is selectively induced by phenobarbital treatment.

Interaction of bile salts with calcium hydroxyapatite: inhibitors of apatite formation exhibit high-affinity premicellar binding

Of the major human bile salts, only the glycine-conjugated dihydroxy species prevent the transformation of amorphous calcium phosphate to calcium hydroxyapatite, a component of gallstones; we have proposed that this inhibition occurs by competition between the bile salt and HPO4(2-) anions for binding site on the apatite crystal embryo. Now we show that the binding affinity of bile salts to fully mature hydroxyapatite has the following order: glycine-conjugated dihydroxy salts > taurine-conjugated dihydroxy salts > glycocholate approximately taurocholate. Glycine-conjugated dihydroxy bile salts bound with high affinity as "premicellar" aggregates, but the remaining species appeared to bind as a wider range of aggregate sizes. Glycochenodeoxycholate binding was decreased as the pH increased from 6.6 to 9.8 and the apatite surface charge reversed from net positive to net negative. Binding was competitively inhibited by HPO4(2-), but not by H2PO4-. Ca2+ promoted the binding of glycochenodeoxycholate, taurochenodeoxycholate and glycocholate, and for the latter two bile salts the increase was associated with enhanced "premicellar" binding. The binding of taurocholate was not influenced by Ca2+. When either glycocholate or taurocholate was mixed with glycochenodeoxycholate, mixed aggregates were formed that had a lower affinity for apatite than had pure glycochenodeoxycholate aggregates. Because only glycine-conjugated dihydroxy bile salts inhibit apatite formation, these results suggest that inhibition depends on high-affinity "premicellar" bile salt-apatite binding.

Substrate-specific differences in the rate of bile acid carrier reorientation: studies on human placental basal vesicles

The initial rate of transport of the bile acid glycocholic acid (GCA) has been measured in influx and efflux across placental basal membrane vesicles, and the mechanism of inhibition of its transport by the analogue taurochenodeoxycholic acid (TCDCA) analysed kinetically. This analogue, although trans-stimulating GCA efflux, inhibits influx in a way which does not depend upon substrate concentration; moreover, its potency as an inhibitor is markedly influenced by whether it is placed on one or on both sides of the vesicles membrane. These findings can be accounted for by postulating that both GCA and TCDCA are translocated through the carrier, but that the rate of loaded carrier reorientation is higher than that of the free carrier only when loaded with TCDCA and not with GCA.

Characterization of an extracellular factor that stimulates bile salt hydrolase activity in Lactobacillus sp. strain 100-100

Bile salt hydrolase activity in Lactobacillus sp. strain 100-100 is strictly intracellular. The strain produces an extracellular factor that stimulates the intracellular hydrolase activity. The factor is inducible by conjugated bile salts, has an apparent molecular mass over 12 kDa but less than 25 kDa, is stable in air, and resistant to pronase and heat. It is partially extractable into organic solvents and inactivated by a sulphydryl group inhibitor. We postulate that the factor functions by a novel mechanism to facilitate entry of conjugated bile salts into the bacterial cells.

Bile secretion of sulfated glycolithocholic acid is required for its cholestatic action in rats

To test our hypothesis that the cholestatic action of sulfated glycolithocholic acid (SGLC) in the rat is related to its interaction with calcium in the biliary tree [R. van der Meer, R. J. Vonk, and F. Kuipers. Am. J. Physiol. 254 (Gastrointest. Liver Physiol. 17): G644-G649, 1988], we have now compared its effects on bile formation in control Wistar rats and mutant Groningen Yellow (GY) Wistar rats. Intravenous injection of 0.6 mumol/100 g body wt of [14C]SGLC in unanesthetized rats with permanent biliary drainage did not induce cholestasis in either of the strains; however, its biliary secretion was strongly impaired in GY rats (12% dose at 1 h after injection vs. 95% dose in controls). Injection of 6.0 and 12.0 mumol/100 g body wt of [14C]SGLC caused an almost complete cessation of bile flow in control rats within 3 and 1 h, respectively. In contrast, administration of the same doses did not cause cholestasis in GY rats. Cholestasis in control rats was preceded by coprecipitation of [14C]SGLC and calcium in bile and incomplete biliary recovery of radioactivity. The hepatic content 15 min after injection of [14C]SGLC (6.0 mumol/100 g body wt) was similar in control and GY rats, 51 and 49% of the dose, respectively. Administration of glycolithocholic acid, the unsulfated parent compound of SGLC (6.0 mumol/100 g body wt), induced a rapid but reversible cessation of bile flow in both controls and GY rats; in this case no precipitation was observed in bile. This study shows that rapid bile secretion of SGLC is required for the induction of cholestasis.(ABSTRACT TRUNCATED AT 250 WORDS)

Zonal distribution of cysteine uptake in the perfused rat liver

When in situ perfused rat livers were administered tracer or physiologic concentrations of [35S]cysteine, a zone III (perivenous) predominance of uptake was observed in either antegrade or retrograde single-pass perfusion, as determined by quantitative densitometry of autoradiographs of liver section. This pattern remained unchanged from 30 s to 5 min observed. At higher supraphysiologic doses a more uniform acinar distribution of cysteine uptake was observed. Uptake rates of cysteine in antegrade perfusion indicated an apparent saturable component at low but physiologic cysteine concentrations. That uptake rather than metabolic trapping accounts for this perivenular pattern was supported by finding identical zonal distribution under conditions in which GSH and protein synthesis were markedly inhibited. Furthermore, increasing or decreasing hepatic cysteine pool sizes did not affect the extraction or zonation. These results suggest that a low Km transport system for cysteine is localized in zone III of the hepatic acinus.

Cholyl-lysylfluorescein: synthesis, biliary excretion in vivo and during single-pass perfusion of isolated perfused rat liver

A fluorescent bile salt, cholyl-lysylfluorescein (cholyl-lys-F), was synthesised so that it retained both an intact steroid ring and a side chain structure with an unblocked carboxyl group. Its biliary kinetics and hepatic extraction were studied in Wistar rats and in the isolated perfused rat liver, respectively. The synthetic method used excess N-epsilon-CBZ-l-lysine methyl ester hydrochloride (7 mmol) and cholic acid (5 mmol) via EEDQ with a yield of 94% for cholyl-lys. Cholyl-lys-F was synthesized employing equimolar amounts of cholyl-lys (sodium salt) and fluorescein isothiocyanate (FITC) in bicarbonate buffer (pH 9.5) over 16 h at room temperature (21 degrees C) with a yield of 70%. The fluorescent property of cholyl-lys-F was similar to fluorescein with a strong apple-green fluorescence. In bile-fistula rats under pentobarbital anaesthesia, the cumulative 20 min biliary excretion as a percentage of injected dose were as follows: cholyl-lys-F, 94.4 +/- 0.3%, [14C]cholylglycine (CG), 93.1 +/- 1.2% and fluorescein (F), 34.8 +/- 0.5. Furthermore the single-pass hepatic extraction of cholyl-lys-F was 64.1 +/- 3.9%, [14C]CG was 66.1 +/- 1.2% and F was 16.5 +/- 2%. The similarity in biliary output and hepatic extraction of cholyl-lys-F to that of the natural bile acid cholylglycine suggest that both compounds are handled in a similar fashion. The greater biliary excretion and hepatic extraction of cholyl-lys-F relative to free fluorescein further suggest that conjugation with a bile salt may be an efficient way of targeting compounds to the liver.

Increasing the in vitro bile acid binding capacity of diethylaminoethylcellulose by quaternization

Diethylaminoethylcellulose (DEAE-cellulose) was quaternized with methyl iodide (DEAE-cellulose-CH3I), and its in vitro binding capacity for sodium glycocholate, at room temperature, in water, Tris-HCl buffer (0.0015-0.0050 M, pH 7.0), and aqueous NaCl (0.0025 M) was determined by reversed-phase HPLC. Quaternization increased the in vitro bile salt binding capacity of DEAE-cellulose. On a molar basis, the binding capacity was greater than that of cholestyramine, a cholesterol-lowering agent. Increasing the ionic strength of the medium decreased the binding capacities, as expected if ionic interactions are important. However, conversion of DEAE-cellulose-CH3I to its chloride form did not change the binding capacity. The bile salt binding capacity of DEAE-cellulose-CH3I was similar for both sodium cholate and sodium glycocholate.

Effect of a taurine-supplemented diet on conjugated bile acids in biliary surgical patients

The effect of a taurine-supplemented diet on the level of conjugated bile acids in postoperative patients was investigated during two consecutive 5-day period. Eighteen hepatobiliary patients with choledochostomies and a specific T-tube insertion were collected and divided randomly into two groups. In group 1, an ordinary postoperative soft diet was prescribed for the first 5 postoperative days and then followed with a taurine-supplemented soft diet (40 mumol/kg per day) for 5 consecutive days. In group 2, the taurine-supplemented diet was prescribed in reverse. At the end of the two periods, on days 5 and 10, bile was collected via a T-tube with an inflatable balloon and low-pressure motor suction. Analysis of conjugated bile acids was done by high-performance liquid chromatography. The results showed that a taurine-supplemented diet increased the concentration of taurocholic acid, glycocholic acid, taurochenodeoxycholic acid, glycochenodeoxycholic acid, and total bile acid from 0.5, 1.9, 0.3, 1.4, and 4.7 mg/mL (on day 5) to 1.1, 3.5, 1.0, 2.6, and 8.9 mg/mL, respectively, on day 10 in group 1. Similar findings were noted in group 2. These results indicate that a taurine-supplemented diet may enhance the conjugation and secretion of bile acid in hepatobiliary patients.