Interference with hepatocellular substrate uptake by 1,1,1-trichloroethane and tetrachloroethylene

Serum bile acids as a sensitive biological marker for evaluating hepatic effects of organic solvents

Serum bile acids (SBAs) are suggested as a potentially sensitive and specific indicator of liver function which, accordingly, could provide an early indication of hepatobiliary dysfunction. This offers advantages over more traditional parameters of liver integrity/function. Recent studies have shown that occupational exposure to low levels of halogenated aliphatic or non-halogenated aromatic solvents is associated with significant increases in SBA levels. As this has often been evident in the absence of any effect on conventional parameters of hepatobiliary integrity/function, elevated SBA levels may well be regarded as a sensitive biological marker of exposure/effect of these compounds. In addition, it may be considered that they provide an early indicator of solvent-induced changes in hepatobiliary function. Extensive studies with experimental animals have also provided supporting evidence for these observations in solvent-exposed individuals. Investigations of the mechanisms at cellular and subcellular levels by which these increases occur have suggested that these effects are likely to be the result of selective, dose-related and reversible inhibition of bile acid uptake at the sinusoidal domain of the hepatocyte plasma membrane. Increased concentrations of SBA under low levels of exposure to different solvents have been demonstrated to be a short-lived and reversible effect which is not accompanied by any other evidence of liver damage. Therefore, it could be assumed that it is unlikely that there would be pathological sequelae to these effects, although the longer term ramifications of such effects have not been thoroughly investigated. Nevertheless, the available evidence indicates that investigation of SBA in solvent-exposed workers could provide useful indications of exposure and effect.

Increased serum bile acids as a possible biomarker of hepatotoxicity in Brazilian workers exposed to solvents in car repainting shops

The objective was to evaluate total serum bile acids (SBA) as a biological marker of hepatotoxicity in car painters exposed to organic solvents and to compare their performance with classic biochemical parameters of liver function. SBA were analysed in a selected group of workers (n=57) occupationally exposed to a mixture of organic solvents and in a control group (n=51). In addition, aspartate aminotransferase (AST), alanine aminotransferase (ALT), total bilirubin (TB), gamma-glutamyltransferase (GGT) and alkaline phosphatase (ALP) were determined in the two groups. Urinary hippuric acid was measured in all samples. Statistical analysis of the data revealed a significant increase in the concentration of SBA, AST, ALP and TB in exposed workers compared with controls (Mann-Whitney, p</=0.05). However, SBA was the parameter most frequently altered in exposed workers and showed higher significance between the two groups (chi-square test) compared with the upper limit of the reference range (8 micromol l(-1)). In conclusion, SBA can be considered to be a sensitive parameter of hepatotoxicity induced by organic solvents than the traditional tests and it can be used as an biological marker of subclinical liver injury.

Cytotoxicity evaluation after coexposure to perchloroethylene and selected peroxidant drugs in rat hepatocytes

There is evidence of hepatotoxic effects caused by Perchloroethylene (PCE), presumably due to reactive metabolic intermediates; lipid peroxidation is under study as a potential mechanism of toxicity. We aimed to verify if PCE levels comparable to those reached in the blood of exposed subjects can cause cell damage and lipid peroxidation. The association of PCE with lipid peroxidation inducing drugs (cyclosporine A, valproic acid and amiodarone) was also tested on rat isolated hepatocytes. AST and LDH release, MTT test and lipid peroxidation assay showed that PCE determines dose-dependent effects on rat isolated hepatocytes. The toxic potential resulting from our data would be valproic acid < cyclosporine A < amiodarone. While valproic acid and cyclosporine caused a mild toxicity, the effects of amiodarone were more severe; in particular, the association of PCE with amiodarone showed a clear additive effect. The role of lipid peroxidation in the liver toxicity exerted by the tested compounds was confirmed by our data, and resulted relevant after treatment of cells with amiodarone and PCE. Extrapolating these results to human, we can suggest that a subject professionally exposed to PCE, who chronically assumes a lipid peroxidation inducing drug like amiodarone, may be potentially exposed to a higher risk of liver toxicity.

Protective role of vitamin E, 2-deoxy-D-glucose, and taurine on perchloroethylene induced alterations in ATPases

Perchloroethylene (PER) administered by oral gavage for 15 consecutive days, at a dose of 3000 mg/kg body wt. decreased the activities of Na+, K(+)-ATPase and Mg(2+)-ATPase with an increase in the activity of Ca(2+)-ATPase. It also decreased RBC and platelet counts but the WBC count was found to be increased. An investigation of the relative importance of the modulators, vitamin E, 2-deoxy-D-glucose (2DG) and taurine in rendering protection to tissues against PER induced membrane damage was performed. PER administered mice were subjected to vitamin E (400 mg/kg body wt/day), 2DG (500 mg/kg body wt/day by i.p.) and taurine (100 mg/kg body wt/day) administration for 15 days to study their individual effect on ATPase and on certain hematological parameters. Vitamin E, 2DG and taurine treated mice showed a marked reversal of these metabolic changes related to membrane damage caused by PER. These results suggest that PER induced membrane damage may be associated with energy metabolism and hemolysis, which can be effectively prevented by these modulators.

Toluene-induced elevation of serum bile acids: relationship to bile acid transport

Raised concentrations of serum bile acids (SBA) following occupational exposure to a number of halogenated aliphatic hydrocarbon solvents and after in vivo exposure of experimental animals to these substances have been reported in several studies in recent years. However, the widely used nonchlorinated aromatic hydrocarbon solvent, toluene, has not been critically examined for its effect on serum bile acids. Accordingly, the effect of in vivo treatment with toluene on SBA and its direct in vitro effects on the transport of bile acids by isolated rat hepatocytes were investigated in this study. In vivo treatment with toluene (2.3 mmol/kg body weight, ip, on each of 3 consecutive days) resulted in a significant rise in the serum concentrations of total and some individual bile acids while other parameters of hepatobiliary function were unaltered. Administration of a higher dose of solvent (9.2 mmol/kg body weight, i.p.) resulted in a further increase in total SBA levels together with a significant rise in serum activities of some liver enzymes. In vitro application of noncytotoxic doses of toluene in the vapor phase to hepatocytes isolated from untreated rats resulted in a significant inhibition of the initial rate-(V0)-of uptake of cholic acid (CA). Similarly, accumulation of CA and taurocholic acid (TC) over an extended incubation time by hepatocytes exposed to toluene was significantly inhibited. Kinetic analysis revealed a noncompetitive inhibition of CA uptake as suggested by a decline in Vmax and an unaltered K(m). In contrast, the initial rate of efflux of these substates and their continuous efflux from preloaded cells were unaffected by exposure to toluene. Thus, toluene exposure inhibited the transport and accumulation of bile acids by hepatocytes in a manner largely similar to that of halogenated solvents, and this inhibition could explain the raised SBA concentrations following in vivo exposure to this solvent. These findings are consistent with and provide mechanistic data to support previous studies where increased SBA levels (in the absence of any evidence of liver injury as measured by liver enzyme tests) were reported in workers following occupational exposure to this solvent. Additionally, in full agreement with our previous investigations in which SBA levels were found to be a sensitive biological marker of exposure to halogenated aliphatic hydrocarbon solvents, the data support a similar role for SBA on exposure to toluene as well.

In vitro interference with hepatocellular uptake of bile acids by xylene

Occupational exposure to a mixture of two widely used aromatic hydrocarbon solvents, xylene and toluene, has been associated with a significant rise in the concentrations of serum bile acids (SBA). We have recently shown that toluene interferes with the transport of bile acids by hepatocytes and this could explain elevated SBA after occupational exposure or following in vivo administration of this compound to experimental animals. However, it is not known if xylene, like its monomethylated homologue, toluene, could interfere with the processes of bile acid transport by hepatocytes. Therefore, the present studies were undertaken to examine this possibility. Direct addition of a non-cytotoxic dose (2.5 microliters/2.8 x 10(6) cells) of xylene (in vapour phase) to hepatocytes isolated from untreated rats significantly inhibited the initial rates (determined from slope of the lines in the linear range (20-80 s)) of uptake (V0) of 10 microM cholic acid (CA) and-taurocholic acid (TC) by 37 and 48%, respectively (P < 0.05). Similarly, accumulation of these substrates by hepatocytes over an extended incubation time up to 30 min was significantly inhibited to the same extent by xylene exposure. This inhibitory effect was found to be reversible when sufficient time was allowed for the cells to recover. In contrast, the initial rates (V0) of efflux (determined from slope of the lines in the linear range (1-5 min)) of these bile acids (25 microM) and their continuous efflux (up to 30 min) from preloaded cells incubated with a similar dose of xylene were not (except for the 1 min time point) significantly different from those of controls. In conclusion, xylene interferes with the transport of bile acids by hepatocytes in a manner largely similar to that of its monomethylated homologue, toluene. These findings extend our previous observations on aliphatic and aromatic hydrocarbon solvents and provide mechanistic data at a cellular level to support a causal role for xylene (as well as toluene) in raised SBA levels of exposed individuals.

Current concepts of hepatic uptake, intracellular transport and biliary secretion of bile acids: physiological basis and pathophysiological changes in cholestatic liver dysfunction

Hepatic sinusoidal uptake of bile acids is mediated by defined carrier proteins against unfavourable concentration and electrical gradients. Putative carrier proteins have been identified using bile acid photoaffinity labels and more recently using immunological probes, such as monoclonal antibodies. At the sinusoidal domain, proteins with molecular weights of 49 and 54 kDa have been shown to be carriers for bile acid transport. The 49 kDa protein has been associated with the Na(+)-dependent uptake of conjugated bile acids, while the 54 kDa carrier has been involved in the Na(+)-independent bile acid uptake process. Within the hepatocyte, cytosolic proteins, such as the glutathione S-transferase (also designated the Y protein), the Y binders and the fatty acid binding proteins, are able to bind bile acids and possibly facilitate their movement to the canalicular domain. At the canalicular domain a 100 kDa carrier protein has been isolated and it has been shown by several laboratories that this particular protein is concerned with canalicular bile acid transport. The system is ATP-dependent and follows Michaelis-Menten kinetics. Interference with bile acid transport has been demonstrated by several chemicals. The mechanisms by which these chemicals inhibit bile acid transport may explain the apparent cholestatic properties observed in patients and experimental animals treated with these agents. Several studies have shown that Na+/K(+)-ATPase activity is markedly decreased in cholestasis induced by ethinyloestradiol, taurolithocholate and chlorpromazine. However, other types of interference have been described and the cholestatic effects may be the result of several mechanisms. Cholestasis is associated with several adaptive changes that may be responsible for the accumulation of bile acids and other cholephilic compounds in the blood of these patients. It may be speculated that the nature of these changes is to protect liver parenchymal cells from an accumulation of bile acids to toxic levels. However, more detailed quantitative experiments are necessary to answer questions with regard to the significance of these changes and the effect of various hepatobiliary disorders in modifying these mechanisms. It is expected that the mechanisms by which bile acid transport is regulated and efforts to understand the molecular basis for these processes will be among the areas of future research.

Alpha-naphthylisothiocyanate-induced elevation of serum bile acids: lack of causative effect on bile acid transport

In recent years chemicals including chlorinated solvents have been found to interfere with the transport of bile acids (BA) by hepatocytes, which probably accounts for the raised serum bile acids (SBA) after exposure. However, the known cholestatic agent, alpha-naphthylisothiocyanate (ANIT) has never been fully examined for its effect on these processes. Accordingly, the direct effects in vitro and the effects of in vivo treatment on bile acid transport have been investigated in this study. Direct addition of ANIT (5-100 microns) to hepatocytes isolated from untreated rats did not result in any change in uptake or efflux of taurocholic acid (TC), one of the most obviously elevated SBA in ANIT-treated rats. Additionally, accumulation of TC over an extended incubation period was not affected by ANIT. In vivo treatment with ANIT (50 mumol/kg i.p. on each of 3 consecutive days) resulted in a marked elevation of total serum bile acids (TSBA) and a slight increase in the activity of serum alkaline phosphatase (ALP) and a very mild hyperbilirubinemia, while other markers of liver injury were unaltered. In hepatocytes isolated from these rats, Km and Vmax for uptake and V0 for efflux were no different between ANIT and vehicle-treated animals. In conclusion, ANIT showed no effects on transport of BA on in vitro exposure or after treatment in vivo where SBA were clearly elevated. The lack of effects of ANIT on transport of bile acids is consistent with other postulated mechanisms of action. Furthermore, this indicates that the effects noted with solvents are not necessarily replicated by substances known to cause histopathological cholestasis.

Individual serum bile acids in apprentice spray painters in association with solvent exposure

The effects of exposure to solvents on serum bile acids were investigated by comparing a group of apprentice vehicle spray painters (exposed group) with one of apprentice electricians. Apprentice spray painters from the study were subdivided into high- and low-solvent-exposure groups. Concentrations of individual serum bile acids (SBA) were measured and compared with conventional liver function tests (LFTs). Total, free, glycine- and taurine-conjugated SBA were consistently found to be present at higher levels in the spray painters than in the electricians, even at the beginning of the apprenticeship. Total SBA tended to increase in spray painters with increasing years of exposure during the apprenticeship, but this was significant at only one time point. No rises were observed over the sampling period in electricians. The mean values of individual and total SBA concentrations were all found to be higher in the high-exposure group than in the low-exposure group, with some differences reaching statistical significance. None of the routine liver biochemistry parameters was different between spray painters and electricians. gamma-Glutamyl transferase (GGT) was the only enzyme found to be significantly different between the high- and low-exposure groups, but all values were within the normal range. This study suggests that occupational exposure even to low levels of solvent mixtures results in increases in SBA. The increased SBA may be indicative of a subclinical liver dysfunction. Alternatively, they may reflect solvent exposure only, with the raised levels having no pathologic implication or consequence.

Bile acids in the assessment of hepatocellular function

Bile acids, which are synthesized in the liver from cholesterol, are important in the production of bile flow, excretion of cholesterol, and intestinal digestion and absorption of fats and fat-soluble vitamins. Increases and/or alterations in concentrations of bile acids in serum are specific and sensitive indicators of hepatobiliary disorders. Synthesis of bile acids in hepatocytes involves steps in endoplasmic reticulum, cytosol, mitochondria, and peroxisomes. Other important hepatocellular processes involving bile acids include active uptake by the basolateral membrane, intracellular transport, P-450-mediated conjugations and hydroxylations, and canalicular secretion. Hydrophobic bile acids produce hepatotoxicity in vivo and in vitro. In experimental and epidemiologic studies, some of these forms have been identified as causative agents in the development of colon and liver (experimental only) cancer. Conversely, several hydrophilic forms, primarily ursodeoxycholic acid, have demonstrated cytoprotective properties in a variety of clinical and experimental hepatobiliary diseases and disorders. Because bile acids can have dramatically different properties and effects, determination of mechanisms of action of these compounds has become an active area of research. Primary isolated hepatocytes provide an opportunity to investigate bile acid-related functions and effects in well-designed, carefully controlled studies. Short-term cultures have been used to study a variety of issues related to bile acids, including cytotoxicity, synthesis, and hepatocellular processing. With these systems, however, many functions of mature hepatocytes, including those pertaining to bile acids, can be lost when cultures are maintained for more than several days. Recent developments in culture techniques permit long-term maintenance of functionally stable, differentiated cells. Pertaining to bile acid research, these systems remain to be fully characterized but, in appropriate situations, they should provide important alternatives to in vivo studies and short-term in vitro assays.

Trichloroethylene--a review of the literature from a health effects perspective

This report reviews the literature on the impact of exposure to trichloroethylene (TCE) on human health. Special emphasis is given to the health effects reported in excess of national norms by participants in the TCE Subregistry of the Volatile Organic Compounds Registry of the National Exposure Registries--persons with documented exposure to TCE through drinking and use of contaminated water. The health effects reported in excess by some or all of the sex and age groups studied were speech and hearing impairments, effects of stroke, liver problems, anemia and other blood disorders, diabetes, kidney disease, urinary tract disorders, and skin rashes.

Transfer of methyl chloroform, trichloroethylene and tetrachloroethylene to milk, tissues and expired air following intraruminal or oral administration in lactating goats and milk-fed kids

The distribution of methyl chloroform was determined (MCF), trichloroethylene (TRI) and tetrachloroethylene (PCE) in milk, tissues and expired air by intraruminally administering 0.625 ml kg(-0.75) of an equal-volume mixture of the three compounds to lactating goats. The milk secreted during 24 h after the intraruminal administration contained 1.42 mg of MCF, 1.87 mg of TRI, 6.43 mg of PCE and 0.33 mg of trichloroethanol (TCE). MCF, TRI and PCE appeared in the blood less than 30 min after administration. Oral administration of these chemicals to milk-fed kids showed that at 3.5 h post-administration, the liver contained these chemicals in greatest abundance. The adaptation of milk-fed kids to 3 weeks administration of small amounts of propylene glycol stimulated the metabolic conversion of TRI to TCE. There were linear relationships between the blood concentrations of these chemicals and the expiration rates after oral administration of 0.4 ml kg(-1) of each chemical to milk-fed kids. The expiration rates of MCF, TRI and PCE were 605, 122 and 46 microg min(-1) kg(-1) at 2 microg ml(-1) blood concentrations of MCF, TRI and PCE, respectively. These results suggested that MCF is little metabolized, being most readily exhaled in expired air, while PCE demonstrates the greatest tissue-partitioning, being largely secreted into the milk or retained in the liver. TRI can be extensively metabolized to other compounds such as TCE in milk-fed kids.

Individual serum bile acids as early indicators of carbon tetrachloride- and chloroform-induced liver injury

Individual serum bile acids (SBA) are emerging as potentially useful early indicators of liver injury. This study was undertaken to compare the usefulness of individual SBA with the routinely used assays for detecting the effects of the hepatotoxicants carbon tetrachloride (CCl4) and chloroform (CHCl3). Serum samples were assayed for liver injury by determination of alanine aminotransferase (ALT), aspartate amino-transferase (AST), alkaline phosphatase (ALP), bilirubin and total bile acid (by enzymatic kit). These results were compared with levels of individual SBA measured by high performance liquid chromatography (HPLC). Liver samples from CCl4-treated rats were taken for light and electron microscopic examination. The highest dose for each chemical caused increases in serum ALT and AST but not ALP. Chloroform at the highest dose increased bilirubin. Total SBA levels as assayed by the kit were elevated in response to CCl4 and CHCl3 at doses below which serum enzymes and bilirubin were increased. Some individual SBA were increased at a still lower dose for each of these two chlorinated solvents. At the lowest dose of CCl4 tested no consistent light microscopic or ultrastructural changes were found. At all the higher doses periacinar cells displayed typical accumulation of lipid droplets and degranulation and dilation of rough endoplasmic reticulum. The extent of the ultrastructural changes were dose-dependent. Thus individual SBA assayed by HPLC may be considered as a very sensitive indicator of liver injury induced by the classical hepatotoxicants carbon tetrachloride and chloroform.

Effects of hexachloro-1,3- butadiene and 1,1,2,2- tetrachloroethylene on individual serum bile acids

Rats were exposed to hexachlorobutadiene (HCBD) or tetrachloroethylene (TET) in order to determine which of these chemicals was more likely to be responsible for elevations in individual serum bile acids (SBA) found in workers exposed primarily to these two chemicals. Increases in cholic and taurocholic acids were found on exposure to high doses of HCBD. Elevations of SBA occurred right down to low exposures for TET, however, with cholic, chenodeoxycholic, and glycocholic acids being the most sensitive bile acids. Only at high doses for each chemical was there any indication of liver injury as determined by routinely used parameters such as serum enzymes or bilirubin. The data suggest that TET is likely to play a role in the elevated individual SBA in an exposure situation where both this chemical and HCBD are found.

New perspectives in biomonitoring liver function by means of serum bile acids: experimental and hypothetical biochemical basis

The functional activity of the liver and the variety of its responses to injury makes the choice of appropriate tests of function a difficult task. Because of the highly efficient uptake of bile acids by the normal hepatocyte, the determination of serum bile acid (SBA) concentration has been proposed as a test to detect early changes of liver function not associated with cytotoxicity. Several biomonitoring studies have been carried out on subjects occupationally exposed to hepatotoxic substances, by evaluating SBAs as indicators of early liver dysfunction. Even though these studies are not exactly comparable because of the different protocols adopted, most of them show a significant increase in SBA concentrations among the exposed subjects compared with unexposed controls. Furthermore, higher prevalences of subjects with abnormal SBA concentrations occur in those exposed to mixtures of organic solvents. Increased SBA concentrations among the subjects exposed to various xenobiotics have been explained by assuming a change in function of hepatocytes. As regards the nature of the mechanisms involved in the increase in SBA concentrations, recent experimental observations pointed out that some chlorinated aliphatics were able to inhibit cell membrane ATPases and alter cytosolic calcium homeostasis. The lack of any relation, however, between exposure and SBA concentrations remains an important point to clarify and at present prevents the use of measurement of SBA concentrations as an index of effect.

Potentiating effects of chlorinated hydrocarbons on carbon tetrachloride toxicity in isolated rat hepatocytes and plasma membranes

A number of chemicals are known to potentiate the hepatotoxicity of carbon tetrachloride. The halocarbon trichloroethylene was shown in a previous study to enhance both carbon tetrachloride-induced toxicity and lipid peroxidation in isolated hepatocytes. In this study three other chlorocarbons have been investigated in order to determine whether this interaction was peculiar to trichloroethylene or common to chlorinated solvents. Hepatocyte suspensions were exposed to carbon tetrachloride at subthreshold levels of toxicity and various concentrations of 1,1,1-trichloroethane, tetrachloroethylene, and chloroform over an eightfold concentration range. Plasma membrane preparations were exposed to tetrachloroethylene and carbon tetrachloride and effects on Mg(2+)- and Na(+)-K(+)-ATPase activities determined. None of the treatments alone caused statistically significant toxicity. Combined treatments resulted in toxicity as demonstrated by potassium ion, alanine aminotransferase, and lactate dehydrogenase leakage from the cells on coincubation of carbon tetrachloride with each of the other halocarbons studied. Only tetrachloroethylene and chloroform were found to potentiate lipid peroxidation, however. In liver plasma membranes no changes in Na(+)-K(+)-ATPase were observed with any of the treatments and only the highest dose of tetrachloroethylene was able to inhibit Mg(2+)-ATPase activity. There was no increase in this inhibition on coincubation with carbon tetrachloride, which does not support involvement of ATPases in combined halocarbon toxicity. In conclusion, the data suggest a mechanism of action common to this class of chemical although its specific nature remains to be established.

Chemical-induced interference with hepatocellular transport. Role in cholestasis

Transport of endogenous chemicals both into (at the basolateral membrane) and out of (at the canalicular membrane) hepatocytes plays an important role in bile formation. Hence, interference with these processes, for example by chemicals, may result in reduced bile output. Several different systems are available for the study of transport and hence chemicals that may interfere with the process. These have been used to varying degrees with isolated hepatocytes probably being the most popular over recent years. It is likely that hepatocyte couplets and highly purified plasma membrane vesicles will be increasingly employed over the ensuing years. The inhibitory effects of several chemicals on the transport of bile acids have been demonstrated with indications that this may help to account for some aspects of chemical-induced hepatobiliary dysfunction. For example, the inhibition of transport of bile acids by cyclosporin A is consistent with the reported pattern of liver dysfunction in patients on high doses of this immunosuppressant. Investigation into chemical-induced interference with electrolyte transport has yet to receive the same degree of attention. This and other aspects have been suggested as deserving of and likely to be subjected to more intensive experimentation over the next few years.

Elevation of individual serum bile acids on exposure to trichloroethylene or alpha-naphthylisothiocyanate

Rats were treated with trichloroethylene via intraperitoneal (ip) injection or inhalation, or with ip alpha-naphthylisothiocyanate (ANIT). Serum samples were assayed for indices of liver injury including alanine aminotransferase (ALT), sorbitol dehydrogenase (SDH), alkaline phosphatase (AP), and bilirubin. Liver from some rats was examined for histological appearance. These data were compared to levels of individual serum bile acids (SBA) determined by high-performance liquid chromatography. Trichloroethylene and ANIT, each at their highest dose only, caused elevations in ALT, but not SDH or AP. The highest dose of ANIT also caused elevated serum bilirubin and cholangitis in the liver. SBA were also elevated in response to both trichloroethylene and ANIT, but at doses below those at which other parameters of liver function were increased. For both chemicals, taurocholic acid was the most sensitive of the bile acids assayed, being elevated at the lowest doses tested of 10 mumols/kg for trichloroethylene and 5 mumols/kg for ANIT. As the doses were raised more of the individual bile acids showed increases. On exposure to trichloroethylene via inhalation taurocholic acid was one of two SBA to show elevation. Thus, both trichloroethylene and ANIT cause elevation in SBA at doses well below those which cause an increase in standard indicators of liver dysfunction. This suggests that SBA and perhaps taurocholic acid, in particular, may provide a sensitive tool for studying hepatobiliary effects of chemicals.