Hepatobiliary transport kinetics of the conjugated bile acid tracer 11C-CSar quantified in healthy humans and patients by positron emission tomography

BACKGROUND & AIMS

Hepatobiliary secretion of bile acids is an important liver function. Here, we quantified the hepatic transport kinetics of conjugated bile acids using the bile acid tracer [N-methyl-¹¹C]cholylsarcosine (¹¹C-CSar) and positron emission tomography (PET).

METHODS

Nine healthy participants and eight patients with varying degrees of cholestasis were examined with ¹¹C-CSar PET and measurement of arterial and hepatic venous blood concentrations of ¹¹C-CSar.

RESULTS

Results are presented as median (range). The hepatic intrinsic clearance was 1.50 (1.20-1.76) ml blood/min/ml liver tissue in healthy participants and 0.46 (0.13-0.91) in patients. In healthy participants, the rate constant for secretion of ¹¹C-CSar from hepatocytes to bile was 0.36 (0.30-0.62)min^-1, 20 times higher than the rate constant for backflux from hepatocytes to blood (0.02, 0.005-0.07min^-1). In the patients, rate constant for transport from hepatocyte to bile was reduced to 0.12 (0.006-0.27)min^-1, 2.3times higher than the rate constant for backflux to blood (0.05, 0.04-0.09). The increased backflux did not fully normalize exposure of the hepatocyte to bile acids as mean hepatocyte residence time of ¹¹C-CSar was 2.5 (1.6-3.1)min in healthy participants and 6.4 (3.1-23.7)min in patients. The rate constant for transport of ¹¹C-CSar from intrahepatic to extrahepatic bile was 0.057 (0.023-0.11)min^-1 in healthy participants and only slightly reduced in patients 0.039 (0.017-0.066).

CONCLUSIONS

This first in vivo quantification of individual steps involved in the hepatobiliary secretion of a conjugated bile acid in humans provided new insight into cholestatic disease.

LAY SUMMARY

Positron emission tomography (PET) using the radiolabelled bile acid (¹¹C-CSar) enabled quantification of the individual steps of the hepatic transport of bile acids from blood to bile in man. Cholestasis reduced uptake and secretion and increased backflux to blood. These findings improve our understanding of cholestatic liver diseases and may support therapeutic decisions.

CLINICAL TRIAL REGISTRATION NUMBER

The trial is registered at ClinicalTrials.gov (NCT01879735).

In silico estimation of basic activity-relevant parameters for a set of drug absorption promoters

Finding a balance between a desired drug's potency and its physicochemical properties that are important for its molecule pharmacokinetic or pharmacodynamics profile is still a challenging issue in rational drug discovery. Quantitative assessment of the lipophilic characteristics of potential drug molecules is indispensable for efficient development of Absorption, Distribution, Metabolism, Excretion, Toxicity-tailored structure-activity models; therefore reliable procedures for deriving log P from molecular structure are desirable. In the current work a range of various software log P predictors for estimation of the numerical lipophilic values for a set of cholic acid derivatives were employed and subsequently cross-compared with the experimental parameters. Thus, the empirical lipophilicity (R_M) was compared with the corresponding log P characteristics calculated using alternative methods for deducing the lipophilic features. The mean values of the selected molecular descriptors that were averaged over the chosen calculation methods (consensus clog P) were subsequently correlated with the R_M parameter. As an additional experiment, the iterative variable elimination partial least squares (IVE-PLS) methodology for an ensemble of descriptors retrieved from Dragon 6.0 software was applied for a set of drug transporters. To investigate the variations within the ensemble of cholic acid derivatives principal component analysis (PCA) and self-organizing neural network (SOM) procedures were used to visualize the major differences in the performance of drug promoters with respect to their lipophilic profile.

[N-methyl-11C]cholylsarcosine, a novel bile acid tracer for PET/CT of hepatic excretory function: radiosynthesis and proof-of-concept studies in pigs

Excretion of conjugated bile acids into bile is an essential function of the liver, and impairment of canalicular bile acid secretion leads to cholestatic liver injury. However, hepatic excretory function cannot be quantified in vivo because of the lack of suitable methods. Cholylsarcosine is an analog of the endogenous bile acid conjugate cholylglycine and exhibits characteristics in vivo that led us to hypothesize that the (11)C-labeled form, that is, [N-methyl-(11)C]cholylsarcosine ((11)C-cholylsarcosine), would be a suitable PET tracer for quantification of hepatic excretory function.

METHODS

A method for radiosynthesis of (11)C-cholylsarcosine was developed involving (11)C-methylation of glycine followed by conjugation with cholic acid. Blood-to-liver uptake and liver-to-bile excretion were investigated in vivo by dynamic (11)C-cholylsarcosine PET/CT of 2 anesthetized pigs. In pig 1, a second dynamic (11)C-cholylsarcosine PET/CT examination was preceded by a high dose of the endogenous bile acid conjugate cholyltaurine to investigate possible inhibition of the transhepatocellular transport of (11)C-cholylsarcosine. In pig 2, a second (11)C-cholylsarcosine administration was given to determine the biodistribution of the tracer by means of 5 successive whole-body PET/CT recordings. Possible formation of (11)C-metabolites was investigated by analysis of blood and bile samples from a third pig.

RESULTS

The radiochemical yield was 13% ± 3% (n = 7, decay-corrected) and up to 1.1 GBq of (11)C-cholylsarcosine was produced with a radiochemical purity greater than 99%. PET/CT studies showed rapid blood-to-liver uptake and liver-to-bile excretion of (11)C-cholylsarcosine, with radioactivity concentrations being more than 90 times higher in the bile ducts than in liver tissue. Cholyltaurine inhibited the transhepatocellular transport of (11)C-cholylsarcosine, indicating that the tracer is transported by one or more of the same hepatic transporters as cholyltaurine. (11)C-cholylsarcosine underwent an enterohepatic circulation and reappeared in liver tissue and bile ducts after approximately 70 min. There were no detectable (11)C-metabolites in the plasma or bile samples, indicating that the novel conjugated bile acid (11)C-cholylsarcosine was not metabolized in the liver or in the intestines. The effective absorbed dose of (11)C-cholylsarcosine was 4.4 μSv/MBq.

CONCLUSION

We have synthesized a novel conjugated bile acid analog, (11)C-cholylsarcosine, and PET/CT studies on anesthetized pigs showed that the hepatic handling of tracer uptake from blood and excretion into the bile was comparable to that for the endogenous bile acid cholyltaurine. This tracer may be valuable for future studies of normal and pathologic hepatic excretory functions in humans.

Hypolipidemic effects of selective liver X receptor alpha agonists

Recently, a number of nuclear receptors have been identified as key regulators of cholesterol homeostasis. Two of these, liver X receptor alpha (LXRalpha) (NR1H3) [1] and ubiquitous receptor (UR) (NR1H2) [1], appear to be involved in cholesterol reverse transport and disposal. LXRalpha null gene mice fail to adapt metabolically to high-cholesterol diets. We have recently shown that some 6alpha-hydroxylated bile acid analogs are selective activators of LXRalpha. In this report, we show that these orally administered LXRalpha agonists have an overall hypolipidemic effect in hypercholesterolemic rats, mice and hamsters, which indicates that in these animal models, endogenous LXRalpha agonist is a limiting factor for induction of cholesterol disposal. Furthermore, in animals, these 6alpha-hydroxylated bile acid analogs exhibit a unique pharmacokinetic profile and do not increase the serum triglyceride level; therefore, they may represent a novel class of therapeutic agents for cholesterol management.

Effect of tauroursodeoxycholate and S-adenosyl-L-methionine on 17beta-estradiol glucuronide-induced cholestasis

BACKGROUND/AIMS

S-adenosyl-L-methionine (SAMe) and tauroursodeoxycholate (TUDC) exert an additive ameliorating effect on taurolithocholate (TLC)-induced cholestasis. The aims were to investigate the protective effect of SAMe on 17beta-estradiol-glucuronide (17betaEG) cholestasis and to find out whether SAMe and TUDC may exert an additive, ameliorating effect.

METHODS

Hepatocyte couplet function was assessed by canalicular vacuolar accumulation (cVA) of cholyllysylfluorescein (CLF). Cells were co-treated with 17betaEG and SAMe, TUDC, or both (protection study), or treated with 17betaEG and then with SAMe, TUDC or both (reversion study) before CLF uptake. Couplets were also co-treated with SAMe and dehydroepiandrosterone (DHEA), a competitive substrate for the sulfotransferase involved in 17betaEG detoxification. The effects of 17betaEG, SAMe and TUDC were also examined on intracellular distribution of F-actin.

RESULTS

Both SAMe and TUDC significantly protected against, and reversed, 17betaEG-induced cholestasis, but their effects were not additive. DHEA abolished the protective effect of SAMe. 17BetaEG did not affect the uptake of CLF into hepatocytes at the concentrations used, and also, it did not affect the intracellular distribution of F-actin.

CONCLUSIONS

17BetaEG does not affect the uptake of CLF into hepatocytes. SAMe and TUDC protect and reverse 17betaEG-induced cholestasis, but without an additive effect. Protection by SAMe may involve facilitating the sulfation of 17betaEG.

Visualization of the transport of primary and secondary bile acids across liver tissue in rats: in vivo study with fluorescent bile acids

BACKGROUND/AIMS

Lysyl fluorescein conjugated bile acid analogues (LFCBAA) closely parallel their natural counterparts. To assess LFCBAA as a tool for the visualization of bile acid transport within liver tissue.

METHODS

Wistar rats were administered physiological concentrations of the primary bile acid analogue cholyllysyl fluoroscein (CLF) and of the secondary bile acid analogue lithocholyllysyl fluorescein (LLF) and serial liver biopsies were taken at fixed intervals. Both compounds were also injected retrogradely into the biliary tree. Frozen sections were examined by fluorescence microscopy.

RESULTS

Both CLF and LLF were rapidly taken up from sinusoidal blood but differed significantly in their hepatic handling. CLF was rapidly transported into bile, whereas LLF transport was slower and produced significantly more bile duct fluorescence. LLF clearance showed a lobular gradient with last remaining bile acid being confined largely to zone 3. Both compounds were avidly taken up by cholangiocytes after injection intravenously or retrogradely into the biliary tree.

CONCLUSIONS

Visualization of LFCBAA by fluorescence microscopy may yield further information regarding hepatobiliary bile acid localization during studies of physiological and pathological mechanisms involved in transport of bile acids. The presence of both compounds within cholangiocytes strongly suggests that they may undergo a degree of chole-hepatic recirculation.

Control by signaling modulators of the sorting of canalicular transporters in rat hepatocyte couplets: role of the cytoskeleton

Hormonal control of the restoration of hepatocanalicular polarity in short-term cultured hepatocyte couplets was analyzed. One hour following isolation, couplets were unable to accumulate the fluorescent bile acid analogue, cholyl-lysyl-fluorescein (CLF), and showed a nonpolarized distribution of F-actin and mrp2 over the cell body. A progressive, time-dependent restoration of couplet-polarized function and morphology was reached after 4 hours of culture. Both dibutyryl cyclic adenosine monophosphate (DBcAMP) and the Ca(2+)-elevating compound, thapsigargin, accelerated restoration of normal couplet morphology and function. The DBcAMP-mediated stimulus was inhibited by the Ca(2+) chelator, 1, 2-bis-(o-aminophenoxy)-ethene-N,N,N',N'-tetra-acetate tetra-(acetomethyl)ester (BAPTA/AM), but not by the protein kinase A (PKA) inhibitors, KT5720 or H89, suggesting that Ca(2+) elevation rather than PKA activation is involved. N-(6-aminohexyl-5-chloro-1-napththalenesulfonamide (W-7), a calmodulin inhibitor, and the protein kinase C (PKC) activator, phorbol dibutyrate, inhibited both the basal and the DBcAMP-stimulated recovery of functional polarity, whereas staurosporine and Gö 6976, 2 PKC inhibitors, accelerated the basal recovery of polarized function. Disruption of the microtubule cytoskeleton by colchicine induced only minor changes under basal, but not under DBcAMP-stimulated, conditions. The Golgi complex disruptor, brefeldin A, significantly delayed, and the microfilament-disrupting agent, cytochalasin D, fully blocked, both processes. However, DBcAMP stimulated trafficking of vesicles containing CLF to the pericanalicular region under the last condition. Our results indicate that restoration of couplet polarity following isolation occurs via a Ca(2+)-calmodulin-mediated mechanism, which depends on microfilament, but not on microtubule integrity. A second pathway is activated by DBcAMP activation via Ca(2+)-calmodulin formation, whose requirements with respect to cytoskeletal components are opposite. PKC has a negative regulatory role in both pathways.

Preparation and characterization of cholic acid-derived antimicrobial agents with controlled stabilities

[reaction: see text] Novel cholic acid-derived antimicrobial agents that decompose under mildly basic conditions have been prepared. These compounds range in biological properties from potent antibacterial activity to effective permeabilization of the outer membranes of Gram-negative bacteria.

Plasma elimination of cholyl-lysyl-fluorescein (CLF): a pilot study in patients with liver cirrhosis

BACKGROUND

Cholyl-lysyl-fluorescein (CLF) is a fluorescein-labelled bile acid whose biological behaviour closely resembles that of naturally occurring cholyl glycine.

AIM

The aim of this study was to analyze the CLF plasma elimination in patients with liver cirrhosis.

METHODS

A dose of CLF at 0.02 mg/kg b.w. was administered i.v. in 26 patients with liver cirrhosis and 9 healthy volunteers. Blood samples were collected before injection and then at 10 min intervals over 60 min. Plasma fluorescence was measured by a luminescence spectrometer and residual fluorescence over the time of the study was compared in each group. Routine liver function tests (rLFTs) were performed before each injection.

RESULTS

Plasma elimination of CLF was significantly impaired in patients with cirrhosis compared to healthy subjects with p values <0.0001 at each analyzed time point. CLF test showed 100% sensitivity for liver cirrhosis when residual fluorescence was measured 30, 40, 50 and 60 min after injection. Routine LFTs showed 85% sensitivity for bilirubin, 84% for total bile acids, 69% for aspartate aminotransferase 62% for albumin and 50% for alkaline phosphatase. CLF elimination measured 60 min after injection correlated with Child-Pugh score (r=0.3945; p<0.05) and albumin (rs=0.6451; p<0.001). No adverse reaction or side effects of CLF were observed.

CONCLUSIONS

CLF test clearly distinguished between the two analyzed groups and was more sensitive than routine liver function tests. The test appears safe, simple to perform and analyze and after validation in larger cohorts of patients may have the potential to become a useful dynamic test of liver function.

Methylprednisolone administration in primary biliary cirrhosis increases cholic acid turnover, synthesis, and deoxycholate concentration in bile

As immunosuppressive agents, corticosteroids may be considered an appropriate treatment for primary biliary cirrhosis, even if bone loss and other side effects may occur. We studied biliary lipid metabolism in 10 nonicteric patients, with histologically proven primary biliary cirrhosis (stage I-IV). We administered methylprednisolone (24 mg daily) for 30 days to ascertain its effects on biliary lipid metabolism, which are largely still unknown. All patients underwent a 30-day drug-washout period before entering the trial. The following parameters were studied before and after methylprednisolone treatment: serum biochemistry; cholic acid pool size, kinetics and synthesis; biliary lipid secretion; biliary bile acid pattern; biliary lipid molar percentage; and cholesterol saturation index. Methylprednisolone induced a statistically significant (Wilcoxon rank test) increase in cholic acid turnover (from 0.26+/-0.04 to 0.50+/-0.05 K/day, P = 0.005) and synthesis (from 0.42+/-0.12 to 0.78+/-0.11 mmol/day, P = 0.04), and in bile deoxycholic acid molar percentage (from 19.4+/-2.7 to 30.6+/-4.4% molar, P = 0.01). On the other hand, a significant decrease in biliary cholesterol molar percentage (from 7.9+/-0.7 to 6.4+/-0.5% molar, P = 0.005), cholesterol saturation index (from 1.11+/-0.11 to 0.95+/-0.07, P = 0.05), and biliary cholesterol secretion (from 64.7+/-5.4 to 53.0+/-4.5 micromol/hr, P = 0.005) was observed. These findings show that short-term administration of methylprednisolone in patients with primary biliary cirrhosis does not induce expansion of the cholic acid pool but increases cholic acid synthesis and turnover, as well as intestinal production of deoxycholic acid. If long-term treatment is considered, the beneficial immunosuppressive effects of corticosteroids have to be weighed against the hepatotoxic properties of deoxycholic acid.

Bile acid kinetics in man studied by radio thin-layer chromatography and densitometry coupling

A method based on coupling of the techniques of radioscanning a TLC plate and densitometry has been developed for the determination of pool sizes and fractional turnover rate of bile acids in man after intraduodenal administration of 14C-labelled acid. The validity of the method has been checked by comparison of the results obtained with those of an enzymatic spectrophotometric analysis, and a measurement of the radioactivity by liquid scintillation counting, after elution of the separated bile acid from a TLC plate. Advantages of the proposed method over the previous one include a reduced number of manipulations, the possibility of automation, a better reproducibility, and the possibility of elaborating the radiometric data obtained for the primary bile acid for better characterising its metabolism inside the enterohepatic circulation.

Quantitative liver function tests define the functional severity of liver disease in early-stage cirrhosis

Some patients with early-stage cirrhosis preserve hepatic function, whereas others have little hepatic reserve and rapidly deteriorate. The aim of this study was to use quantitative tests of liver function (QLFTs) to define the degree of functional hepatic impairment in patients with early-stage cirrhosis (Child-Pugh score 5-7) and to determine whether the tests predicted subsequent hepatic decompensation. We recruited 10 cirrhotic (Cr) patients and 10 healthy controls (NI), who were well matched for race, age, weight, and gender. Clearances of caffeine (CF) and antipyrine (AP) after oral administration were measured from timed samples of saliva. The clearance of cholate (CA) was measured from serum samples obtained after simultaneous oral ([2,2,4,4-2H]CA) and intravenous ([24-13C]CA) administration. CA shunt was calculated as (Cl i.v./Clo x 100%). CF elimination rate (Cr v NI, mean +/- SD: 0.03 +/- 0.02 v 0.075 +/- 0.018 h-1, P < .0005) and AP clearance (24 +/- 16 v 40 +/- 7 mL/minute, P < .02) were reduced in Cr patients. CA shunt was increased in Cr patients (43 +/- 18 v 18 +/- 7%, P < .002). Five Cr patients decompensated during follow-up and had the worst CA shunts (76%, 66%, 51%, 48%, and 45%). Three subsequently received successful orthotopic liver transplantation, 1 died of hepatoma, and 1 is on the waiting list for transplantation. In conclusion, QLFTs define the degree of functional impairment in early cirrhosis and may identify Cr patients at greatest risk of decompensation who may require transplantation for survival.

Cholyllysyl fluroscein and related lysyl fluorescein conjugated bile acid analogues

There have been attempts to couple bile acids to fluorescein to permit their visualization during studies of physiology and pathophysiology. Although conjugation has been achieved by many, the product differed in many respects from the parent bile acid congener. We describe lysylfluorescein conjugated bile acid analogues (LFCBAA) synthesized in our laboratory as model divalent "unipolar" molecules. We have determined LFCBAA properties including their water:octanol partition coefficient, HPLC retention time and critical micellar concentration and compared them with their parent bile acid congeners. Cholyl lysylfluorescein (CLF) and lithocholyl lysylfluoroscein (LLF) have properties similar to cholylglycine (CG) and glycolithocholate (GLC), respectively. In human and rat hepatocytes uptake of CLF follows Michaelis-Menten kinetics with K(m) and Vmax similar to CG. Biliary excretion rates of CLF and LLF closely resemble those of CG and GLC in both normal and mutant TR- rats which lack the multiorganic anion transporter (MOAT), strongly supporting the notion that CLF and LLF are substrates for the canalicular bile salt transporter (cBST). The close similarity of hepatocyte uptake and biliary secretion of these LFCBAA and their parent bile acid congeners makes them potentially useful probes for the intracellular visualization of bile salt movement and deposition in various models of bile formation and secretion.

Relationship between structure and intestinal absorption of bile acids with a steroid or side-chain modification

A structure-activity relationship for bile acid (BA) intestinal absorption is known to exist. To better understand the BA structural requirements for optimal BA intestinal absorption, rabbit ileal perfusion studies were performed. Unconjugated BA: Ursodeoxycholic (UDCA) and ursocholic acid (UCA) with methyl (6MUDCA and 6MUCA) or fluoro group (6FUDCA and 6FUCA) in the 6 position and UCA with a methyl group in 23 position (23MUCA) were compared with unconjugated UDCA, UCA, deoxycholic (DCA), chenodeoxycholic (CDCA), hyocholic (HCA), and hyodeoxycholic (HDCA) acid. BA lipophilicity was evaluated by their octanol-water partition coefficient. Conjugated BA: Taurine-conjugated UDCA and UCA with a methyl group in the 23 position (T23MUDCA and T23MUCA) were compared with the corresponding taurine-conjugated natural analogs. An analog of glycine-conjugated UDCA with the C24 amide bond replaced by a -CO-CH2- in the 24 position (24PUDCA) was studied and results were compared with the natural form (GUDCA). Unconjugated BA absorption was dose dependent (i.e., passive) and followed their lipophilicity: DCA > 6MUDCA > CDCA > HDCA > UDCA > HCA > 6FUDCA > 6MUCA > 6FUCA > UCA. Conjugated BA absorption was active, and Vmax was in the following order: TCA > TUDCA > TUCA > T23MUCA > T23MUDCA > 24PUDCA > GUDCA. 24PUDCA transport was also active and higher than GUDCA.

CONCLUSION

Passive transport is dependent on BA lipophilicity. Conjugated BAs are actively transported, and the presence of a 23-C methyl group does not improve transport when compared with the natural analogs. The substitution of the C24 amide bond with a -CO-CH2-still affords interaction of the BA with the intestinal transport carrier.

cAMP stimulates fluorescent bile acid uptake into hepatocytes by membrane hyperpolarization

Elevation of intracellular adenosine 3',5'-cyclic monophosphate (cAMP) hyperpolarizes hepatocytes and increases the uptake rate of bile acids. The purpose of this study was to determine to what extent these two phenomena are linked. Fluorescent bile acid analogues (FBA) were used to probe bile acid transport into whole cell patch-clamped hepatocytes. Na(+)-dependent uptake of cholyl-nitrobenz-2-oxa-1,3-diazol-4-yl-lysine (C-NBD-L), an FBA with a net charge of -1, was shown to be electrogenic, whereas uptake of cholylglycylamidofluorescein (CGamF), an FBA with a net charge of -2, was neutral. Incubation of hepatocytes with 8-bromo-cAMP (8-BrcAMP; 100 microM) increased the uptake rate of the electrogenically transported FBA by 25% (P = 0.002), but had no effect on the uptake rate of the electroneutrally transported FBA. Microelectrode impalements revealed that 8-BrcAMP or forskolin hyperpolarized hepatocytes by 6-8 mV. To determine if hyperpolarization is responsible for the cAMP-induced increase in uptake rate, cAMP was directly introduced into hepatocytes during whole cell patch clamp under voltage-clamp conditions. As long as voltage clamp was maintained at -30 mV there was no stimulation of C-NBD-L uptake. However, when voltage clamp was terminated by either pipette removal or current clamp, cAMP increased the uptake rate by 25-34% (P < 0.002). In both of these protocols, cAMP had no effect on uptake of the electroneutrally transported FBA, CGamF. Finally, in voltage-clamped hepatocytes in the absence of cAMP, a 10-mV hyperpolarization increased the uptake rate of C-NBD-L by 23%. We therefore conclude that short-term cAMP-induced stimulation of fluorescent bile acid uptake in hepatocytes is a direct consequence of membrane hyperpolarization.

[Cholic acid sorption by food fibers]

The ability of five kinds of dietary fibers isolated from different plants for binding of bilious acids (BA) in vitro was estimated. The bile was derived from patients with obstructive hepatitis. The examined kinds of dietary fibers have a capacity to absorb BA. This capacity is a little lower than a pharmaceutical preparation bilignin. The studied dietary fibers may be recommended for preventive diets.

Relationship between the microsomal epoxide hydrolase and the hepatocellular transport of bile acids and xenobiotics

Recently two different bile-acid carriers for the hepatocellular sodium-dependent uptake of taurocholate have been described. The first transport system was isolated and characterized by functional expression cloning in Xenopus laevis oocytes. The corresponding cDNA clone, named Ntcp for Na+/taurocholate co-transporting polypeptide, codes for a protein of 362 amino acids and shows no similarity to previously known sequences. The transport function of this carrier system is well documented by expression in Xenopus laevis oocytes and by transient and stably transfected cell lines. In addition, several lines of evidence implied that the well-known xenobiotic-metabolizing enzyme microsomal epoxide hydrolase (mEH, EC 3.3.2.3) is also able to mediate sinusoidal uptake of taurocholate. Furthermore, it was claimed that the same enzyme also mediates the uptake of the conjugated bile acid into the smooth endoplasmic reticulum (ER). No direct proof of the transport function of mEH by its heterologous expression has yet been published. In the present work we used a stable transfected cell line that expressed high levels of heterologous mEH for uptake studies of various bile acids and the loop diuretic bumetanide. The uptake of the conjugated bile acid taurocholate, of the non-conjugated bile acid cholate and of the organic anion bumetanide was measured in the transfected as well as in the non-transfected parental cell line. These organic anions represent the main substrates of the known transport systems for organic anions in the rat liver. The results show that the microsomal epoxide hydrolase is unable to transport taurocholate, cholate or bumetanide. Furthermore, Western-blot analysis revealed the expression of mEH in hepatoma tumor cell lines, which show no transport activity for these organic anions. These results show that it is unlikely that mEH can mediate the transport of these substrates.

Identification and characterization of a basolateral dicarboxylate/cholate antiport system in rat hepatocytes

The mechanisms and driving forces for the uptake of the unconjugated bile acid cholate were investigated both in cultured rat hepatocytes and in rat liver basolateral (sinusoidal) plasma membrane (BLPM) vesicles. Determination of initial uptake rates of [3H]cholate (0.1 microM) into cultured hepatocytes confirmed that the majority (75%) of the transmembrane transport was mediated by Na(+)-independent mechanisms. This portion of cholate uptake consisted of a pH-sensitive moiety representing nonionic diffusion, which may become quantitatively important at low pH and high cholate concentrations, as well as of a saturable (Michaelis constant 7.4 microM), 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive transport moiety, suggesting the involvement of a carrier. This latter transport system was functionally characterized by 1) inhibition of cellular cholate uptake in the absence of extracellular sodium by the dicarboxylic acid alpha-ketoglutarate (alpha-KG; 1 mM) and by the organic anion p-aminohippurate (PAH; 1 mM); 2) stimulation of cellular cholate uptake by alpha-KG (10 microM) or PAH (1 mM) in the presence of an inwardly directed sodium gradient; 3) lack of sensitivity toward lithium in BLPM vesicles; 4) trans-stimulation of vesicular cholate uptake by alpha-KG or PAH, but not by benzoate; and 5) cis-inhibition of alpha-KG/alpha-KG self-exchange by extravesicular cholate (400 microM), PAH (5 mM), probenecid, or DIDS. Collectively, these data indicate the presence of a Na(+)-dicarboxylate cotransport-coupled organic anion exchanger in the hepatocyte basolateral plasma membrane that may be involved in cholate uptake in the liver.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of sodium in the uptake of ursodeoxycholic acid in isolated hamster hepatocytes

The uptake of ursodeoxycholic acid (UDCA) was studied in isolated hamster hepatocytes. The uptake was rapid and linear up to 60 seconds for each concentration studied. When the uptake rate was plotted against UDCA concentration, the curve was nonlinear, indicating both saturable and nonsaturable uptake mechanisms. The nonsaturable process had a diffusion constant of 0.01 nmol.s-1.g of cell.mumol/L-1. The saturable component was characterized by a maximum rate of uptake (Vmax) of 5.68 nmol.s-1.g of cell-1 and a Michaelis constant (Km) of 224 mumol/L. In the presence of monensin, ouabain, and amiloride, the uptake of UDCA was significantly decreased by 35% to 55%, whereas the sodium-independent uptake of UDCA was not affected by either monensin or amiloride, thereby confirming sodium dependence of UDCA uptake. The sodium-dependent uptake of UDCA was characterized by a Vmax and a Km of 1.57 nmol.s-1.g of cell-1 and 46 mumol/L, respectively. The rate of uptake of UDCA was maximal at extracellular sodium concentrations > or = 20 mmol/L. Furthermore, the uptake of UDCA was competitively inhibited by both taurocholic acid and cholic acid with an inhibitory constant (Ki) of 60 mumol/L and 48 mumol/L, respectively. Finally, 1 mmol/L of 4,4'-diisothiocyano-2,2'-disulfonic stilbene (DIDS) inhibited solely the sodium-dependent uptake of cholic acid and UDCA. These findings confirm that the hepatocellular uptake of UDCA involves, at least in part, a sodium-dependent, ouabain, amiloride, and DIDS-sensitive transporter.

New 6-substituted bile acids: physico-chemical and biological properties of 6 alpha-methyl ursodeoxycholic acid and 6 alpha-methyl-7-epicholic acid

New analogs of ursodeoxycholic acid and 7-epicholic acid containing a 6 alpha-methyl group were synthesized, and their physico-chemical properties were studied and compared with those of their natural analogs. The 6 alpha-methyl group slightly increases the lipophilicity and slightly lowers the critical micellar concentration with respect to the corresponding natural analogs. Simulated bile 50% enriched with 6 alpha-methyl ursodeoxycholic acid, with a total bile acid/phospholipid ratio of 10/1, demonstrated a higher cholesterol-holding capacity and a faster cholesterol gallstone dissolution rate with respect to ursodeoxycholic acid, while 6 alpha-methyl-7-epicholic acid and 7-epicholic acid were much less efficient in these processes. The 6 alpha-methyl analogs were highly stable toward 7-dehydroxylation when incubated with human stool in anaerobic conditions. Their transport, metabolism, and effect on biliary lipid secretion were evaluated both in rats and hamsters after acute intravenous and intraduodenal infusion at a dose of 10 mumol/min per kg. In both species, 6 alpha-methyl ursodeoxycholic acid is efficiently secreted in bile, with a cumulative recovery similar to that of ursodeoxycholic acid. The only metabolites of 6 alpha-methyl ursodeoxycholic acid identified were its glycine and taurine amidated forms. 6 alpha-Methyl-7-epicholic acid was efficiently secreted into bile when infused intravenously, and to a lesser extent when infused intraduodenally, in both rats and hamsters; it was secreted in bile as amidate and also as free acid. When 6 alpha-methyl ursodeoxycholic acid, 6 alpha-methyl-7-epicholic acid, ursodeoxycholic acid, and 7-epicholic acid were chronically administered to hamsters (for 3 weeks, at a dose of 50 mg/kg per day) their accumulation in gallbladder bile was, respectively, 25.1%, 4.0%, 15.2%, and 3.4% of the total bile acids. In conclusion, of the two analogs, only 6 alpha-methyl ursodeoxycholic acid shows potential as a cholesterol gallstone-dissolving agent. In this regard, its most important properties are moderate lipophilicity, good metabolic stability, and better conservation in the enterohepatic circulation, with respect to ursodeoxycholic acid.

A single intravenous high dose of cholic acid to a pregnant ewe does not affect fetal well-being

Cardiovascular variables of both mother and fetus and total bile acid levels were monitored in amniotic fluid and in maternal and fetal plasma after the administration of a single intravenous high dose (29 mg) of cholic acid to a pregnant ewe. We observed the diffusion of significant quantities of cholic acids from the mother to the fetus and amniotic fluid. Nevertheless, cardiovascular parameters (heart rate, blood pressure, PO2, PCO2 and pH) were not affected, neither did intra-amniotic pressure show significant changes compared with the initial experimental conditions. It is concluded that an acute increase in maternal plasma levels of bile acids does not produce important adverse effects in either the mother animal or the fetus and consequently, if bile acids should be deemed responsible for any deleterious effect observed in a preterm fetus in intrahepatic cholestasis, this should be considered as the consequence of a chronic process.

Uptake of taurocholic acid and cholic acid in isolated hepatocytes from rainbow trout

The uptake of the bile acids cholate (CHA) and taurocholate (TCHA) was studied in isolated hepatocytes from rainbow trout (Oncorhynchus mykiss). Both CHA and TCHA were taken up in a concentration- and temperature-dependent manner with optimum temperature at 15 degrees C and a strikingly efficient uptake even at low temperatures (0-5 degrees C). The total uptake was a combination of a saturable [Michaelis-Menten constant (Km) for CHA, 20 microM; Km for TCHA, 19 microM] and a nonsaturable component. The maximal uptake rate of the saturable component was 416 and 805 pmol.mg protein-1.min-1 for CHA and TCHA, respectively. The uptake of both bile acids was shown to be energy dependent, since it was inhibited by the metabolic inhibitors antimycin A, oligomycin and carbonyl cyanide m-chlorophenylhydrazone. The uptake was clearly Na+ independent, since isosmotic replacement of extracellular Na+ by Li+, choline, or K+ did not inhibit the uptake. Furthermore, it seemed to be independent of the presence of extracellular Cl-, since it was not inhibited by replacement of Cl- with sodium gluconate. On the whole, our results show that the hepatocellular uptake of bile acids in rainbow trout is mediated by a Na(+)-independent carrier system, with characteristics resembling the corresponding transport component in mammalian hepatocytes, but with high efficiency even at low temperatures.

Mechanism of trichloroethylene-induced elevation of individual serum bile acids. II. In vitro and in vivo interference by trichloroethylene with bile acid transport in isolated rat hepatocytes

The effects of trichloroethylene (TRI) on bile acid transport in isolated rat hepatocytes have been studied using doses ranging from 0.5 to 4.0 microliters/flask and a 20-min equilibration period. It was found that TRI caused a dose-related suppression of initial rates of uptake of cholic acid (CA) and taurocholic acid (TC) with no significant effect on enzyme leakage and intracellular potassium ion contents. Accumulation over 30 min for each of those two bile acids was also inhibited. A noncompetitive inhibition of bile acid uptake was shown as indicated by a decrease in maximum velocity (Vmax) and unchanged Michaelis constant (Km). Thirty minutes after cessation of TRI exposure in vitro the uptake of bile acids had gradually returned to normal levels. No significant interference of efflux was found in cells preloaded with either CA or TC. After dosing rats with 1 mmol/kg TRI in vivo the inhibition of uptake of CA and TC by subsequently isolated hepatocytes was not detected until 4 hr. By 16 hr uptake had returned to normal. The accumulation of bile acids was also suppressed at 4 and 8 hr. The inhibition of uptake after in vivo treatment was also noncompetitive. The data are consistent with the reversible increase of serum bile acids (SBA) in experimental animals after exposure to TRI. Furthermore, they support the contention that it is an interference with bile acid uptake, rather than actual cell damage, that is responsible for TRI-induced increases in SBA. Thus, the changes in SBA seem to be the result of interference with a physiological process rather than an event associated with significant pathological consequences.

The effects of cyclic imides on lipid absorption from the intestine and on bile lipids and bile acids of Sprague Dawley rats

The cyclic imides, o-(N-phthalimido)acetophenone, 2,3-dihydrophthazine-1,4-dione and N(4-methyl phenyl)diphenimide, were evaluated for their effects on bile lipids, bile acids, small intestinal absorption of cholesterol and cholic acid and liver and small intestinal enzyme activities involved in lipid metabolism. The agent at 20 mg/kg/day orally elevated rat bile excretion of lipids, e.g. cholesterol and phospholipids, and increased the bile flow rate. These agents altered the composition of the bile acids, but there was no significant increase in lithocholic acid which is most lithogenic in rats. The three agents did decrease cholesterol and cholic acid absorption from isolated in situ intestinal duodenum loops in the presence of drug. Hepatic and small intestinal mucosa enzyme activities, e.g. ATP-dependent citrate lyase, acyl CoA cholesterol acyl transferase, cholesterol-7-alpha hydroxylase, sn-glycerol-3-phosphate acyl transferase, phosphatidylate phosphohydrolase, and lipoprotein lipase were reduced. However, the cyclic imides did not accelerate HMG-CoA reductase activity, the regulatory enzyme for cholesterol synthesis, in a manner which would accelerate biliary cholesterol excretion. There was no evidence of hepatic cell damage afforded by the drugs based on clinical chemistry values which would induce alterations in bile acid concentrations after treatment of the rat.

Effect of indomethacin on the uptake, metabolism and excretion of 3-oxocholic acid: studies in isolated hepatocytes and perfused rat liver

3 alpha-Hydroxysteroid dehydrogenase catalyzes the reduction of 3-oxo-bile acids and binds 3 alpha-hydroxy bile acids. Indomethacin is a competitive inhibitor of the enzyme. In incubations of isolated rat hepatocytes, indomethacin delayed the intracellular reduction and the initial uptake of 3-oxocholic acid. Following a tracer dose of 3-oxocholic acid in perfused rat liver, rapid biliary excretion was observed mainly as taurocholic acid. Only 1.1% of the dose was recovered in the caval outflow and nearly all appeared in the first 5 min collection. When the tracer dose was given after initiating a constant infusion of indomethacin (50 microM), a dramatic decrease in biliary excretion was observed, still mainly as taurocholic acid, and 14% of the dose was recovered in the caval effluent: 10% in the first 5 min collection, mainly as 3-oxocholic acid, followed by a steady, slow release of mainly taurocholic acid. The increased intrahepatic retention of bile acids and slow release into perfusate and bile in response to indomethacin are consistent with displacement of bile acids from cytosolic protein.

Equilibration of labelled and endogenous bile acids in patients with liver cirrhosis after administration of (24-14C)cholic and chenodeoxycholic acids

On separate occasions (24-14C)cholic acid and (24-14C)chenodeoxycholic acid were administered intravenously to patients with liver cirrhosis and the isotope excretion in urine and faeces monitored. Bile acids in serum, urine and faeces were extracted and separated into unconjugated bile acids, glycine- and taurine conjugates, glucuronides and sulphates. Individual bile acid conjugates were separated by high-performance liquid chromatography (HPLC) and the unconjugated bile acids were separated by gas-liquid chromatography (GLC) and identified by gas chromatography-mass spectrometry (GC-MS). Individual bile acid conjugates were quantified and their isotope contents determined. In serum, isotope contents declined rapidly during the first day, followed by a markedly slow rate of reduction. In accordance with this, the excretion of isotope from the patients was found to be very slow and the routes of bile acid excretion were changed, which resulted in an increased ratio of urine/faeces isotope excretion. Studies of the ratio of labelled to endogenous bile acid conjugates indicated that a continuous transformation of the labelled compounds occurred during the period of study. As judged from serum bile acids, conjugation to glycine- or taurine conjugates was rapid. The specific activities of labelled sulphate esters were consistently lower than for other conjugates during the 300-min observation period. During the first day, the urinary bile acids contained a high proportion of unconjugated labelled bile acids, which gradually disappeared. Labelled primary bile acids were slowly converted into microbial products, mainly 7-alpha dehydroxylated derivatives. The observed slow transformations resulted in a much delayed equilibration of labelled and endogenous bile acid derivatives, which invalidates isotope techniques for calculation of kinetic data of bile acid turnover. However, the observed very slow turnover of labelled bile acids in cirrhosis, owing to the persistent high rate of intestinal absorption and low capacity for urinary excretion, makes it possible for the intestinal flora to markedly change the composition of the bile acids in the pool. Studies of endogenous urinary and faecal bile acid excretion revealed the changed route of bile acid excretion with a high urinary/faeces ratio and the decreased synthesis of bile acids in cirrhosis.

Hepatobiliary delivery of polyaminopolycarboxylate chelates: synthesis and characterization of a cholic acid conjugate of EDTA and biodistribution and imaging studies with its indium-111 chelate

A conjugate in which the steroid nucleus of cholic acid was linked to EDTA via an 11-atom spacer was obtained by reacting the succinimidyl ester of cholic acid with the amine formed by reaction of a benzyl isothiocyanate derivative of EDTA with N-(tert-butoxycarbonyl)ethylenediamine and subsequent deprotection. Potentiometric titration studies with model complexes showed that the EDTA moiety retained the ability to form 1:1 chelates of high thermodynamic stability, although formation constants were some 3-4 log K units lower for complexes of the conjugate than for the analogous chelates with underivatized EDTA. A complex formed between the cholic acid-EDTA conjugate and 111InIII was clearly rapidly into the liver when injected iv into mice, with subsequent excretion from the liver into the gastrointestinal tract being complete within 1 h of injection. Radioscintigraphic imaging studies conducted in a rabbit given the 111In-labeled conjugate also showed early liver uptake followed by rapid clearance from the liver into the intestine, with good visualization of the gallbladder in images obtained at 20-25 min postinjection. It is concluded that conjugation to cholic acid provides a useful means for the hepatobiliary delivery of EDTA chelates that otherwise exhibit predominantly extracellular distribution and renal clearance.

Biological properties of the 2-fluoro-beta-alanine conjugates of cholic acid and chenodeoxycholic acid in the isolated perfused rat liver

The isolated perfused rat liver was used to examine the hepatic extraction, biliary secretion and effect on bile flow of the 2-fluoro-beta-alanine conjugates of cholic acid and chenodeoxycholic acid. The naturally occurring taurine and glycine conjugates of these bile acids were used for comparisons. The 2-fluoro-beta-alanine conjugates were extracted by the liver to a similar extent as the taurine and glycine conjugates. The biliary secretion rate and increase in bile flow were similar for all the cholic acid conjugates. On the other hand, the maximal biliary secretion rate of the 2-fluoro-beta-alanine conjugate of chenodeoxycholate was similar to that of the glycochenodeoxycholate, but 47% lower than that of taurochenodeoxycholate. In addition, the 2-fluoro-beta-alanine conjugate of chenodeoxycholate produced a decrease in bile flow that was comparable to that observed with the glycochenodeoxycholate (54% vs. 74%), but which was greater than that produced by the taurochenodeoxycholate (12%). In summary, these data demonstrate that the biological properties of the 2-fluoro-beta-alanine conjugates of cholic acid and chenodeoxycholic acid are not markedly different from those of the naturally occurring taurine and glycine conjugates. These data also suggest that the amino acid moiety can influence the biliary secretion and cholestatic properties of chenodeoxycholic acid conjugates.

[Plasma clearance of cholic acid in patients with chronic diseases of the liver]

The diagnostic value of 14C-cholic acid plasma clearance following oral administration was evaluated. 14C-cholic acid clearance was 1223 +/- 267 mL/min. per 1 square meter in the control group without liver disease. Significantly lower values (p less than 0.001) were obtained in the patients with chronic hepatitis (694 +/- 137 mL/min. per 1 square meter) and liver cirrhosis (332 +/- 156 mL/min. per 1 square meter). Sensitivity of the 14C-cholic acid clearance test was 100% while specificity--80%. A 3-year follow-up has shown that this test is of high prognostic value in patients with liver cirrhosis.

On the mechanism of detergent modification of myosin structure and function

The concentration dependences of the activation of myosin subfragment-1 (S1) Mg-ATPase by the detergents CHAPS and C12E8 were determined at 23 degrees C in 25 mM Tris (pH 7.0), 250 microM EDTA, 5 mM MgCl2, and 100 microM ATP. At detergent concentrations expected to bind hydrophobic S1 surface areas equally, C12E8 caused an 8.5-fold greater increase in activity than CHAPS, which suggests that detergent binding to the surface of S1 is not the mechanism of activation. At detergent concentrations above their critical micelle concentrations, C12E8 was also much more effective than CHAPS, suggesting that micelles are not involved. A series of n-alcohols (which do not form micelles) with from 3 to 10 carbons all increased S1 Mg-ATPase activity as much or more than C12E8. The largest increase (5.7-fold) was caused by n-hexanol. The more hydrophobic alcohols activated S1 at lower concentrations. A linear plot of the alcohol concentration that caused 50% of maximum activity versus the number of carbons in the alcohol, indicated the apparent free energy of binding per CH2-group was -0.60 +/- 0.03 kcal/mol. There were two indications that alcohol binding caused an S1 conformational change. The intrinsic fluorescence increase of S1 during steady-state activity was reduced from 17.5 to 12.8%, and the apparent hydrodynamic rotational mobility of fluorescently labeled S1 was decreased 25% by the present of n-hexanol. The data suggest that S1 activation by C12E8 and by n-alcohols is due to hydrophobic binding to S1 at non-surface sites, which causes an S1 structural change.

Cholate uptake in basolateral rat liver plasma membrane vesicles and in liposomes

The mechanism(s) and driving force(s) for hepatocellular uptake of the unconjugated bile acid cholate were investigated in isolated basolateral (sinusoidal) rat liver plasma membrane (blLPM) vesicles and in protein free liposomes. In blLPM vesicles both an inwardly directed Na+ gradient and a transmembrane pH difference (8.0 in/6.0 out) stimulated cholate uptake 2-3-fold above equilibrium uptake values (overshoot). While Na+ gradient driven cholate uptake could be inhibited by the anion transport inhibitor 4,4'-diisothiocyanato-2,2'-disulfonic acid stilbene (DIDS), the pH gradient dependent portion of cholate uptake was insensitive to DIDS, but could be inhibited by furosemide. Furthermore, initial rates (1-s values) of the pH gradient stimulated cholate uptake were linear with increasing substrate concentrations (no saturability). In liposomes a similar inside alkaline pH gradient also induced a transient DIDS insensitive/furosemide inhibitable intravesicular accumulation (approx. 2-fold) of cholate (overshoot). These findings confirm that hepatocellular uptake of cholate occurs in part via the common Na+/bile acid cotransport system. In addition, the data strongly indicate that in isolated membrane vesicles pH gradient driven cholate uptake represents nonionic diffusion rather than a carrier mediated process (Blitzer, B.L., Terzakis, C. and Scott, K.A. (1986) J. Biol. Chem. 261, 12042-12046). Since in the perfused liver DIDS inhibited uptake of both cholate and taurocholate to a similar extent, DIDS-insensitive pH gradient dependent membrane diffusion appears to be of minor significance for cholate uptake in the intact organ.

Transport, metabolism, and effect of chronic feeding of cholylsarcosine, a conjugated bile acid resistant to deconjugation and dehydroxylation

To test the effect in rodents of chronic ingestion of a bile acid resistant to deconjugation, cholylsarcosine was synthesized and its transport, metabolism, and effect on biliary bile acid and biliary lipid composition were determined in rabbits, hamsters, and rats. Cholylsarcosine was shown to be well absorbed from the ileum but underwent little absorption from the jejunum or colon. When cholylsarcosine was administered in the diet at 140 mumol/kg.day, it was well absorbed and underwent little biotransformation during enterohepatic cycling; however, both bacterial deconjugation and dehydroxylation (without deconjugation) occurred to a small extent. With chronic feeding, cholylsarcosine accumulated to compose 24%-29% of circulating bile acids in all 3 rodent species. It was rapidly lost from the enterohepatic circulation, with a daily fractional turnover rate of 75%-150%, depending on the species. Cholylsarcosine caused no change in liver tests or hepatic morphology and did not influence biliary lipid secretion. When cholyltaurine was fed, it was also absorbed, but, in contrast to cholylsarcosine, was rapidly deconjugated and dehydroxylated to form deoxycholic acid. The deoxycholic acid accumulated in the enterohepatic circulation, as evidenced by a slow fractional turnover rate of 26%-40% per day, depending on the species. It is concluded that cholylsarcosine is absorbed from the ileum, has an enterohepatic circulation, does not undergo appreciable deconjugation or dehydroxylation in these rodents, and is nontoxic. In the rodent, the circulating bile acids can be somewhat enriched when a bile acid resistant to deconjugation is ingested; but the effect on the steady state biliary bile acid composition is less than that obtained when cholyltaurine is administered because cholyltaurine is biotransformed to deoxycholic acid, which in turn is absorbed and has its own efficient enterohepatic circulation.

Uptake of bile acids by perfused rat liver: evidence of a structure-activity relationship

The hepatic extraction of unconjugated and taurine-conjugated bile acids, provided with different hydrophilicity values, has been measured in the perfused rat liver, in order to evaluate the role of the bile acid structure and bile acid hydrophilicity on their uptake by the liver. Ursocholic, cholic, ursodeoxycholic and chenodeoxycholic acids, free and taurine-conjugated, were injected into the portal vein in dose response studies, using a nonrecirculating perfusion system. For all of the bile acids, the uptake process showed saturation. In addition, a nonsaturable component was apparent in bile acids provided with the lowest hydrophilicity values, as expressed by the lowest values of the water to octanol partition coefficient. The maximum uptake velocity increased with increasing values of the partition coefficient, which in turn were associated with 7-OH alpha to beta epimerization, the presence of 12-OH in alpha position and taurine conjugation. The ratio of maximum uptake velocity to Km (Km being the half-saturation constant) appeared to be markedly increased by taurine conjugation and by 7-OH alpha to beta epimerization, whereas it was reduced by the presence of 12-OH in alpha position.

Bile acid metabolism in human hyperthyroidism

Decreased levels of serum cholesterol are a well-recognized finding in hyperthyroidism. Since the conversion to bile acids is an important pathway for the elimination of cholesterol, we studied primary bile acid kinetics in seven hyperthyroid patients before and after medical treatment. Pool sizes, fractional turnover and synthesis rates of cholic acid and chenodeoxycholic acid were determined after oral administration of 50 mg [13C]cholic acid and 50 mg [13C]chenodeoxycholic acid. 13C/12C isotope ratios in serum were measured by capillary gas chromatography/electron impact mass spectrometry. Compared with the euthyroid state, serum cholesterol levels were distinctly lower in hyperthyroidism (150 +/- 33 vs. 261 +/- 51 mg per dl, p less than 0.01). Thyroid hormone excess caused a 34% reduction in cholic acid synthesis (5.8 +/- 2.8 vs. 7.9 +/- 4.2 mu moles per kg per day, p less than 0.02), which was associated with a 47% decrease in cholic acid pool size (11.7 +/- 3.4 vs. 22.0 +/- 5.2 mu moles per kg, p less than 0.01). Chenodeoxycholic acid kinetics exhibited no apparent changes. Thus, total primary bile acid synthesis was diminished by 20% in hyperthyroidism. After normalization of thyroid function, the ratio of cholic acid/chenodeoxycholic acid pool size increased in all patients. This was paralleled by a rise in the ratio of concentrations of cholic acid/chenodeoxycholic acid in serum. The depression of cholic acid synthesis in the presence of unaltered subjects is compatible with an inhibition of hepatic 12 alpha-hydroxylation by thyroid hormone. Furthermore, evidence is provided that, in man, the low serum cholesterol levels found during hyperthyroidism are not caused by an increased conversion of cholesterol to bile acid.

Intestinal absorption of bile acid glucuronides in rats

While the intestinal absorption of taurine, glycine, and sulfate conjugates of bile acids has been studied extensively, nothing is known about the absorption of bile acid glucuronides. In the present study, the intestinal phase of the enterohepatic circulation of two bile acid glucuronides was examined. [3 beta-3H]cholic acid 3-O-beta-D-glucuronide or [3 beta-3H]lithocholic acid 3-O-beta-D-glucuronide was perfused through isolated segments of ileum or jejunum with intact blood supply in rats prepared with a biliary fistula. [14C]Taurocholic acid was perfused simultaneously with each glucuronide to compare glucuronide absorption with that of an actively transported bile acid. Intestinal absorption was determined by measuring the rate of secretion of labeled bile acid in bile. The absorption of [3H]cholic acid glucuronide by the ileum and jejunum was one fortieth and one eighth, respectively, that of [14C]taurocholic acid. Comparison of the two glucuronides show that [3H]lithocholic acid glucuronide absorption was 18 and 10 times greater than [3H]cholic acid glucuronide absorption from the jejunum and ileum, respectively. Collectively, the above observations suggest that glucuronidation of bile acids markedly reduces absorption from the small intestine.

Peptide conformations--49(1): synthesis and structure-activity relationships of side chain modified peptides of cyclo(-D-Pro-Phe-Thr-Lys-Trp-Phe.)

Cyclic hexapeptide analogues representing the modified retro sequence of the amino acid residues 7-11 of natural somatostatin are known to protect liver cells from phalloidin poisoning. To determine the influence of steric, lipophilic, and charge effects on (a) the conformation of the backbone and the aromatic side chains and (b) the biological response, the side chains of Phe2, Lys4, and Phe6 of cyclo(-D-Pro1-Phe2-Thr3-Lys(Z)4-Trp5-Phe6-), 1a, one of the most active peptides found so far, were modified by various residues. The discussion of conformationally relevant parameters proves that neither backbone conformations nor populations of aromatic side chain rotamers were altered by these substitutions. The potency of these derivatives in a cytoprotection assay varies by at most one order of magnitude (more or less active than the parent peptide 1a). A qualitative evaluation of lipophilic, steric, and charge effects reveals the dominance of lipophilic effects of aromatic residues; the most potent compounds contain aromatic substructures in the side chain of Lys4.

Determination of cholic acid and chenodeoxycholic acid pool sizes and fractional turnover rates by means of stable isotope dilution technique, making use of deuterated cholic acid and chenodeoxycholic acid

A procedure is described for the simultaneous determination of cholic acid and chenodeoxycholic acid pool sizes and fractional turnover rates. After oral administration of known amounts of 11,12-dideuterated chenodeoxycholic acid and 2,2,4,4-tetradeuterated cholic acid, the ratios of chenodeoxycholic acid-D2/chenodeoxycholic acid and cholic acid-D4/cholic acid are measured in consecutive serum samples, after which fractional turnover rates and pool sizes of chenodeoxycholic acid and cholic acid are determined arithmetically. In 7 healthy volunteers pool sizes for chenodeoxycholic acid and cholic acid were 22.9 +/- 7.8 and 24.1 +/- 11.7 mumol/kg, respectively. The corresponding values for the fractional turnover rates were 0.23 +/- 0.10 and 0.29 +/- 0.12/day. After oral administration of the labelled bile acids in capsule, the obtained pool sizes were significantly higher than after administration in a bicarbonate solution. Bile acid kinetics were also performed in a patient suffering from a cholesterol synthesis deficiency and in a patient very likely suffering from a bile acid synthesis deficiency. Furthermore, the kinetics of the intestinal absorption and hepatic clearance of unconjugated bile acids have been investigated in 2 healthy subjects.

Utilizing bile acid carrier mechanisms to enhance liver and small intestine absorption

On the basis of the enterohepatocycling phenomenon of bile acids involving the intestines, liver, and gallbladder, it was conceptualized that bile acids could serve as a molecular carrier of drugs by taking advantage of the bile acid active transport mechanism. It was further proposed that derivatization or analogation of bile acids at the C3-OH position was the desired route because of the reactive hydroxyl group and, moreover, because of the active transport requirement of retaining the C17 side chain with a single terminal acidic function. Using 3-tosylcholic, 3-benzoylcholic, and 3-iodocholic acids, in situ liver absorption, biliary excretion, and intestinal absorption studies in the rat were successful in establishing the concept that C3-derivatives and analogs of bile acids are, potentially, novel molecular delivery systems for intestinal and liver-site directed absorption.

Ileal absorption of tyrosine-conjugated bile acids in Wistar rats

We recently reported that tyrosine-conjugated bile acids, when injected intravenously into bile-fistula rats, are extracted by the liver and secreted intact into bile with an efficiency similar to that seen for taurocholate. Now the effect of tyrosine and glycyltyrosine conjugation of bile acids on ileal absorption has been studied in Wistar rats. 125I-labelled tyrosine- and glycyltyrosine-conjugated bile acid or [14C]taurocholate was injected in 400 microliters aliquots of physiological saline buffered to pH 7.8 into the ileal lumen of bile-fistula rats. Recovery of bile salts in bile was taken as proof of ileal absorption. In comparison with taurocholate, ileal absorption was about 10% less for cholyltyrosine and chenodeoxycholyltyrosine and about 50% less for deoxycholyltyrosine. Thus, tyrosine-conjugated bile acids are absorbed by the ileum and excreted into bile and may undergo enterohepatic circulation. Low recoveries of deoxycholyltyrosine relative to deoxycholylglycine suggested that side chain structure was important for ileal absorption of 3 alpha,12 alpha-dihydroxy bile acids. Elongation of cholic acid to form cholylglycyltyrosine markedly reduced 90-min cumulative ileal absorption relative to cholyltyrosine. Although initial rates of recovery of cholylglycyltyrosine were comparable to those of the other bile acids, very little further absorption was seen in the last hour of the experiment, suggesting that this compound was rapidly degraded within the intestinal lumen.

Anion transport in basolateral (sinusoidal) liver plasma-membrane vesicles of the little skate (Raja erinacea)

The mechanism(s) of [35S]sulphate transport was investigated in basolateral liver plasma-membrane vesicles of the little skate elasmobranch, Raja erinacea. Imposition of an intravesicular alkaline pH gradient (pH 8.0 in/pH 6.0 out) stimulated sulphate uptake 5-10-fold compared with pH-equilibrated (pH 8.0 in = out) conditions and 2-3-fold over equilibrium sulphate uptake (overshoot). This pH-gradient-stimulated sulphate uptake was temperature-dependent, saturable with increasing concentrations of sulphate and could be inhibited by the protonophore carbonyl cyanide m-chlorophenylhydrazone and the anion-transport inhibitors 4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid (DIDS) and probenecid, cis-Inhibition of pH-gradient-driven sulphate uptake was observed with sulphate, oxalate, cholate and bromosulphophthalein, but not with chloride and taurocholate. In addition, sulphate and oxalate trans-stimulated [35S]sulphate uptake under pH-equilibrated conditions. Although also stimulated by an inside-alkaline pH gradient, transmembrane transport of [3H]cholate was not inhibited by DIDS, suggesting that its pH-gradient-driven uptake is not mediated by an anion-transport 'carrier'. In conclusion, these studies indicate that a basolateral plasma-membrane sulphate-transport system has evolved in skate hepatocytes and is similar to that in mammalian liver cells. This archaic anion-exchange system co-transports certain organic anions such as oxalate and has developed early in vertebrate evolution.

A mechanism for the hypocholesterolaemic activity of saponins

1. Saponins are steroid or triterpene glycosides which occur in a number of important food plants, including such staples as soya beans (Glycine max) and chickpeas (Cicer arietinum). They are known to be hypocholesterolaemic. 2. Some saponins form an insoluble complex with cholesterol which prevents its absorption from the small intestine. Others cause an increase in the faecal excretion of bile acids, an indirect route for elimination of cholesterol. 3. We have investigated the effects of different saponins on absorption of the bile salt sodium cholate from perfused loops of small intestine, in vivo, in the rat. Purified saponins from soapwort (Saponaria officinalis), soya beans and quillaia (Quillaia saponaria) reduced the rate of absorption of the bile salt; soya-bean and soapwort saponins substantially so but quillaia saponin to a much lesser extent. 4. These results were explained by the formation of large mixed micelles by bile acid and saponin molecules in aqueous solution. These aggregates can have molecular weights in excess of 10(6) daltons, consequently the bile acid molecules incorporated in them are not available for absorption. 5. Control of plasma cholesterol and nutrient absorption through dietary saponins could provide substantial health and nutritional benefits in humans.