Oestradiol 17 beta-glucuronide increases tight-junctional permeability in rat liver

Hepatic Bile Formation: Developing a New Paradigm

In 1959, Ivar Sperber contrasted bile formation with that of urine and proposed that water flow into the canalicular conduit is in response to an osmotic, not a hydrostatic, gradient. Early attempts to support the hypothesis using a bile acid, sodium taurocholate, and the hormone secretin to stimulate bile flow led to conflicting data and a moratorium on attempts to further develop the initial proposal. However, current data amplify the initial proposal and indicate both paracellular and transcellular water flow into hepatic ductules and the canalicular conduit in response to an osmotic gradient. Also, the need to further modify the initial proposal became apparent with the recognition that bile acid aggregates (micelles), which form in the canalicular conduit, generate lecithin-cholesterol vesicles that contain water unrelated to an osmotic gradient. As part of this development is the recent introduction of the fluorescent localization after photobleaching technique for direct determination of hepatic duct flow and clarification of the role of biomarkers such as mannitol and polyethylene glycol 900. With the new paradigm, these biomarkers may prove useful for quantifying paracellular and transcellular water flow, respectively. SIGNIFICANCE STATEMENT: It is essential to identify and characterize all the sites for water flow during hepatic bile formation to obtain more precision in evaluating the causes and possible therapeutic approaches to cholestatic syndromes. Updating the Sperber proposal provides a new paradigm that addresses the advances in knowledge that have occurred.

ERK1/2 and p38 MAPKs are complementarily involved in estradiol 17ß-D-glucuronide-induced cholestasis: crosstalk with cPKC and PI3K

Objective

The endogenous, cholestatic metabolite estradiol 17ß-d-glucuronide (E₂17G) induces endocytic internalization of the canalicular transporters relevant to bile formation, Bsep and Mrp2. We evaluated here whether MAPKs are involved in this effect.

Design

ERK1/2, JNK1/2, and p38 MAPK activation was assessed by the increase in their phosphorylation status. Hepatocanalicular function was evaluated in isolated rat hepatocyte couplets (IRHCs) by quantifying the apical secretion of fluorescent Bsep and Mrp2 substrates, and in isolated, perfused rat livers (IPRLs), using taurocholate and 2,4-dinitrophenyl-S-glutathione, respectively. Protein kinase participation in E₂17G-induced secretory failure was assessed by co-administering selective inhibitors. Internalization of Bsep/Mrp2 was assessed by confocal microscopy and image analysis.

Results

E₂17G activated all kinds of MAPKs. The PI3K inhibitor wortmannin prevented ERK1/2 activation, whereas the cPKC inhibitor Gö6976 prevented p38 activation, suggesting that ERK1/2 and p38 are downstream of PI3K and cPKC, respectively. The p38 inhibitor SB203580 and the ERK1/2 inhibitor PD98059, but not the JNK1/2 inhibitor SP600125, partially prevented E₂17G-induced changes in transporter activity and localization in IRHCs. p38 and ERK1/2 co-inhibition resulted in additive protection, suggesting complementary involvement of these MAPKs. In IPRLs, E₂17G induced endocytosis of canalicular transporters and a rapid and sustained decrease in bile flow and biliary excretion of Bsep/Mrp2 substrates. p38 inhibition prevented this initial decay, and the internalization of Bsep/Mrp2. Contrarily, ERK1/2 inhibition accelerated the recovery of biliary secretion and the canalicular reinsertion of Bsep/Mrp2.

Conclusions

cPKC/p38 MAPK and PI3K/ERK1/2 signalling pathways participate complementarily in E₂17G-induced cholestasis, through internalization and sustained intracellular retention of canalicular transporters, respectively.

Phosphoinositide 3-kinase/protein kinase B signaling pathway is involved in estradiol 17β-D-glucuronide-induced cholestasis: complementarity with classical protein kinase C

Estradiol 17β-D-glucuronide (E(2)17G) is an endogenous, cholestatic metabolite that induces endocytic internalization of the canalicular transporters relevant to bile secretion: bile salt export pump (Bsep) and multidrug resistance-associated protein 2 (Mrp2). We assessed whether phosphoinositide 3-kinase (PI3K) is involved in E(2)17G-induced cholestasis. E(2)17G activated PI3K according to an assessment of the phosphorylation of the final PI3K effector, protein kinase B (Akt). When the PI3K inhibitor wortmannin (WM) was preadministered to isolated rat hepatocyte couplets (IRHCs), it partially prevented the reduction induced by E(2)17G in the proportion of IRHCs secreting fluorescent Bsep and Mrp2 substrates (cholyl lysyl fluorescein and glutathione methylfluorescein, respectively). 2-Morpholin-4-yl-8-phenylchromen-4-one, another PI3K inhibitor, and an Akt inhibitor (Calbiochem 124005) showed similar protective effects. IRHC immunostaining and confocal microscopy analysis revealed that endocytic internalization of Bsep and Mrp2 induced by E(2)17G was extensively prevented by WM; this effect was fully blocked by the microtubule-disrupting agent colchicine. The protection of WM was additive to that afforded by the classical protein kinase C (cPKC) inhibitor 5,6,7,13-tetrahydro-13-methyl-5-oxo-12H-indolo[2,3-a]pyrrolo[3,4-c]carbazole-12-propanenitrile (Gö6976); this suggested differential and complementary involvement of the PI3K and cPKC signaling pathways in E(2)17G-induced cholestasis. In isolated perfused rat liver, an intraportal injection of E(2)17G triggered endocytosis of Bsep and Mrp2, and this was accompanied by a sustained decrease in the bile flow and the biliary excretion of the Bsep and Mrp2 substrates [(3)H]taurocholate and glutathione until the end of the perfusion period. Unlike Gö6976, WM did not prevent the initial decay, but it greatly accelerated the recovery to normality of these parameters and the reinsertion of Bsep and Mrp2 into the canalicular membrane in a microtubule-dependent manner.

CONCLUSION

The PI3K/Akt signaling pathway is involved in the biliary secretory failure induced by E(2)17G through sustained internalization of canalicular transporters endocytosed via cPKC.

Dynamic localization of hepatocellular transporters in health and disease

Vesicle-based trafficking of hepatocellular transporters involves delivery of the newly-synthesized carriers from the rough endoplasmic reticulum to either the plasma membrane domain or to an endosomal, submembrane compartment, followed by exocytic targeting to the plasma membrane. Once delivered to the plasma membrane, the transporters usually undergo recycling between the plasma membrane and the endosomal compartment, which usually serves as a reservoir of pre-existing transporters available on demand. The balance between exocytic targeting and endocytic internalization from/to this recycling compartment is therefore a chief determinant of the overall capability of the liver epithelium to secrete bile and to detoxify endo and xenobiotics. Hence, it is a highly regulated process. Impaired regulation of this balance may lead to abnormal localization of these transporters, which results in bile secretory failure due to endocytic internalization of key transporters involved in bile formation. This occurs in several experimental models of hepatocellular cholestasis, and in most human cholestatic liver diseases. This review describes the molecular bases involved in the biology of the dynamic localization of hepatocellular transporters and its regulation, with a focus on the involvement of signaling pathways in this process. Their alterations in different experimental models of cholestasis and in human cholestatic liver disease are reviewed. In addition, the causes explaining the pathological condition (e.g. disorganization of actin or actin-transporter linkers) and the mediators involved (e.g. activation of cholestatic signaling transduction pathways) are also discussed. Finally, several experimental therapeutic approaches based upon the administration of compounds known to stimulate exocytic insertion of canalicular transporters (e.g. cAMP, tauroursodeoxycholate) are described.

Disruption of function and localization of tight junctional structures and Mrp2 in sustained estradiol-17beta-D-glucuronide-induced cholestasis

Estradiol-17beta-D-glucuronide (E(2)17G) induces immediate and profound but transient cholestasis in rats when administered as a single bolus dose. Here, we examined the consequence of sustained E(2)17G cholestasis and assessed the function and localization of the tight junctional proteins zonula occludens-1 (ZO-1) and occludin and of the canalicular transporter multidrug resistance-associated protein-2 (Mrp2). An initial dose of E(2)17G (15 mumol/kg iv) followed by five subsequent doses of 7.5 mumol/kg from 60 to 240 min induced a sustained 40-70% decrease in bile flow. Following their biliary retrograde administration, cholera toxin B subunit-FITC or horseradish peroxidase were detected at the sinusoidal domain, indicating opening of the paracellular route; this occurred as early as 15 min after the first dose as well as 15 min after the last dose of E(2)17G, but not following the administration of vehicle in controls. Localization of ZO-1 and occludin was only slightly affected under acute cholestatic conditions but was severely disrupted under sustained cholestasis, with their appearance suggesting a fragmented structure. Endocytic internalization of Mrp2 to the pericanalicular region was apparent 20 min after a single E(2)17G administration; however, Mrp2 was found more deeply internalized and partially redistributed to the basolateral membrane under sustained cholestasis. In conclusion, acute E(2)17G-induced cholestasis increased permeability of the tight junction, while sustained cholestasis provoked a significant redistribution of ZO-1, occludin, and Mrp2 in addition to increased permeability of the tight junction. Altered tight junction integrity likely contributes to impaired bile secretion and may be causally related to changes in Mrp2 localization.

Hepatic pharmacokinetics of taurocholate in the normal and cholestatic rat liver

The disposition kinetics of [3H]taurocholate ([3H]TC) in perfused normal and cholestatic rat livers were studied using the multiple indicator dilution technique and several physiologically based pharmacokinetic models. The serum biochemistry levels, the outflow profiles and biliary recovery of [3H]TC were measured in three experimental groups: (i) control; (ii) 17 alpha-ethynylestradiol (EE)-treated (low dose); and (iii) EE-treated (high dose) rats. EE treatment caused cholestasis in a dose-dependent manner. A hepatobiliary TC transport model, which recognizes capillary mixing, active cellular uptake, and active efflux into bile and plasma described the disposition of [3H]TC in the normal and cholestatic livers better than the other pharmacokinetic models. An estimated five- and 18-fold decrease in biliary elimination rate constant, 1.7- and 2.7-fold increase in hepatocyte to plasma efflux rate constant, and 1.8- and 2.8-fold decrease in [3H]TC biliary recovery ratio was found in moderate and severe cholestasis, respectively, relative to normal. There were good correlations between the predicted and observed pharmacokinetic parameters of [3H]TC based on liver pathophysiology (e.g. serum bilirubin level and biliary excretion of [3H]TC). In conclusion, these results show that altered hepatic TC pharmacokinetics in cholestatic rat livers can be correlated with the relevant changes in liver pathophysiology in cholestasis.

Biliary secretion of glutathione in estradiol 17beta-D-glucuronide-induced cholestasis

Estradiol-17beta-D-glucuronide (E2-17G) induces an acute but reversible inhibition of bile flow after its intravenous administration to rats, due in part to the endocytic retrieval of the canalicular multidrug resistance-associated transporter protein 2 and the bile salt export pump, transporters that contribute to bile flow. Decreased bile salt-independent bile flow (BSIF) is also involved and persists during the phase of recovery from cholestasis. Because glutathione and HCO3- are major contributors to BSIF, we evaluated changes in their biliary excretion and the hepatic content of total glutathione during E2-17G-induced cholestasis. E2-17G acutely decreased bile flow and biliary excretion of total glutathione by about 80%; glutathione excretion was still inhibited at 80 min and 120 min, even though bile flow was partially and totally restored, respectively. Neither liver glutathione content nor the proportions of oxidized glutathione in bile and liver were affected by E2-17G at any time. HCO3- concentrations in bile were unchanged, so that secretion paralleled variations in bile flow. In the isolated perfused liver, addition of E2-17G decreased both bile flow and the biliary concentration of glutathione, whereas addition of its noncholestatic isomer estradiol-3-D-glucuronide (E2-3G) did not inhibit bile flow, but significantly reduced the concentration of glutathione in bile. The bile:liver concentration ratios of glutathione were significantly decreased in vivo by E2-17G and in the perfused liver by E2-17G and E2-3G. These data indicate that E2-17G cis-inhibits the canalicular transport of glutathione and thus contributes to the cholestatic effect by inhibiting BSIF.

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.

Bile duct obstruction is not a prerequisite for type I biliary epithelial cell hyperplasia

Biliary obstruction, produced by common bile duct ligation or alpha-naphthylisothiocyanate (ANIT) treatment in rats, has been associated with the development of type I biliary epithelial cell (BEC) hyperplasia. However, the exact mechanism(s) by which bile duct obstruction lead(s) to this proliferative lesion are not clear. The present studies were designed to determine if cholestasis, in the absence of biliary obstruction, would result in type I BEC hyperplasia. Male Sprague-Dawley rats were given a single oral dose of 150 mg/kg ANIT or i.v. doses of estradiol glucuronide (E2-17G; 21 mumol/kg/h for 48 h) to produce obstructive and non-obstructive cholestasis, respectively. E2-17G treatment resulted in cholestasis that was comparable in extent and duration to that observed following ANIT treatment. E2-17G and ANIT treatments produced comparable increases in serum bile acids (55- to 60-fold) and activities of ALT (36- to 38-fold), ALP (4- to 5-fold), and 5'-nucleotidase (7- to 11-fold), respectively, compared to controls. Both ANIT and E2-17G also increased serum bilirubin concentrations. ANIT treatment resulted in significant increases in biliary glucose concentrations that were associated with BEC damage/necrosis and obstruction of the bile duct lumen. Conversely, no evidence of BEC damage was observed in E2-17G-treated rats. Nonetheless, BEC hyperplasia was observed in the majority of rats following treatment with either ANIT or E2-17G, assessed by light microscopy and by BrdU immunohistochemistry. These data indicate that E2-17G treatment produces nonobstructive cholestasis and type I BEC hyperplasia, suggesting that biliary obstruction is not a prerequisite for type I BEC hyperplasia in rats. Differences in the time of onset of hyperplasia were observed: hyperplasia was noted immediately following 48 h of E2-17G-induced cholestasis but occurred several days after ANIT-induced cholestasis had subsided. Since the magnitude/duration of cholestasis was similar in the two models but the temporal association between cholestasis and type I BEC hyperplasia were different, these data suggest that the proliferative stimulus may be different in the two models and that E2-17G-induced type I BEC hyperplasia may not be attributed solely to cholestasis.

Effect of tauro-alpha-muricholate and tauro-beta-muricholate on oestradiol-17 beta-glucuronide-induced cholestasis in rats

The effect of tauro-beta-muricholate (beta MC-tau) and tauro-alpha-muricholate (alpha MC-tau) on oestradiol-17 beta-glucuronide (E217G)-induced cholestasis was compared with that of tauroursodeoxycholate (UDC-tau) in rats. Like UDC-tau, alpha MC-tau and beta MC-tau infused at the rate of 0.2 mumol/min per 100 g bodyweight (BW) completely inhibited the cholestasis induced by E217G infused at the rate of 0.06 mumol/min per 100 g BW for 20 min. These findings indicate that beta MC-tau and alpha MC-tau are useful in protecting against various types of experimental cholestasis, as well as against bile acid-induced cholestasis.

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.

Cholestatic potentials of alpha-naphthylisothiocyanate (ANIT) and beta-naphthylisothiocyanate (BNIT) in the isolated perfused rat liver

Previous studies in rats have shown that a single oral dose of alpha-naphthylisothiocyanate (ANIT), but not the regioisomer beta-naphthylisothiocyanate (BNIT), results in intrahepatic cholestasis. The present studies were designed to evaluate the intrinsic cholestatic potential of ANIT and BNIT in the isolated perfused rat liver. Livers from male Sprague-Dawley rats (300-450 g) were isolated and perfused with Krebs-Henseleit buffer supplemented with 50 microM taurocholate and ANIT or BNIT (0, 5, 15 or 50 microM). Rates of bile flow, bile acid uptake and bile acid excretion were monitored for up to 70 min. Permeability of tight junctions also was evaluated. At concentrations of 5 microM, neither ANIT nor BNIT altered hepatobiliary function or tight junction permeability. In contrast, perfusion with 50 microM ANIT or BNIT for 35 min resulted in decreases in bile flow rates of 19 +/- 8 and 13 +/- 4%, respectively. After 70 min of perfusion with ANIT or BNIT, rates of bile flow were decreased by 78 +/- 5 and 71 +/- 4%, respectively. Bile acid excretion also was decreased following perfusion with 50 microM ANIT or BNIT. Perfusion with 50 microM ANIT or BNIT decreased bile acid uptake by 51 +/- 13 and 46 +/- 6%, respectively, at 60 min. Bile/plasma (B/P) ratios of [3H]sucrose were not affected by ANIT or BNIT at any time during perfusion, indicating that changes in bile flow and bile acid excretion in the isolated perfused liver were not associated with increased hepatocyte tight junction permeability. These data demonstrate that the direct portal infusion of a 50 microM concentration of either ANIT or BNIT produced marked decreases in bile flow, indicating that these isomers have a comparable intrinsic cholestatic potential in the isolated perfused liver.

Estradiol-17 beta-glucuronide-induced cholestasis. Effects of ursodeoxycholate-3-O-glucuronide and 3,7-disulfate

The effect of the co-infusion of ursodeoxycholate and its taurine conjugate, 3-O-glucuronide and 3,7-disulfate on estradiol-17 beta-glucuronide-induced cholestasis was examined. Estradiol-17 beta-glucuronide was intravenously administered to bile-drained rats at a rate of 0.075 mumol/min/100 g for 20 min. Co-infusion of ursodeoxycholate and its conjugates was simultaneously begun at a rate of 0.2 mumol/min/100 g and continued for 120 min. Ursodeoxycholate failed to improve and tauroursodeoxycholate only partially improved estradiol-17 beta-glucuronide-induced cholestasis between 20 and 40 min, although both bile acids increased bile flow after 80 min. Tauroursodeoxycholate increased biliary estradiol-17 beta-glucuronide excretion. Ursodeoxycholate-3-O-glucuronide completely inhibited cholestasis induced by estradiol-17 beta-glucuronide without changing biliary estradiol-17 beta-glucuronide excretion. Although ursodeoxycholate-3,7-disulfate had only a minor effect on cholestasis, it increased biliary excretion of estradiol-17 beta-glucuronide. In the Eizai hyperbilirubinuria rat (EHBR), a hyperbilirubinemic mutant Sprague-Dawley rat, the same dose of estradiol-17 beta-glucuronide failed to induce cholestasis with a marked delay in biliary excretion of estradiol-17 beta-glucuronide. In summary, ursodeoxycholate-3-O-glucuronide is more effective than tauroursodeoxycholate in inhibiting estradiol-17 beta-glucuronide-induced cholestasis and ursodoexycholate-3,7-disulfate had little effect. However, the unexpected effects of ursodeoxycholate-3-O-glucuronide and 3,7-disulfate on excretion of estradiol-17 beta-glucuronide suggest that the interaction of these anions at the canalicular membrane is complicated, with interaction occurring at more than two pathways of the biliary excretion of these anions.

Steroid UDP glucuronosyltransferases

The glucuronidation of steroids is a major process necessary for their elimination in the bile and urine. In general, steroid glucuronides are biologically less reactive than their parent steroids. However, in some cases often associated with disease and steroid therapy, more reactive or toxic glucuronides may be formed. The concentrations of specific steroid glucuronides in the blood may also indicate hormonal imbalances and may funnction as diagnostic markers of genetic defects in steroid synthesis and metabolism. In this review, the forms of UDP glucuronosyltransferase involved in steroid glucuronidation are described in terms of their specificities, functional domains and regulation. The available evidence suggests that steroid glucuronidation is mainly carried out by members of the UGT2B subfamily which are encoded by genes containing 6 exons. Members of this subfamily exhibit a regioselectively in their glucuronidation of steroids that is mediated by domains in the amino-terminal half on the protein encoded by exons 1 and 2. Although much of this review will describe studies in the rat, preliminary evidence indicates that a similar situation may exist in humans.

Inhibition of biliary cholesterol and phospholipid secretion by cefmetazole. The role of vesicular transport and of canalicular events

A number of organic anions selectively inhibit the biliary secretion of cholesterol and phospholipids without affecting bile acid secretion. We studied the effect of cefmetazole, a third-generation cephalosporin, on biliary lipid secretion in the rat. Injection of cefmetazole at a dose of 200 mumol/kg body wt. induced a choleretic effect and a significant decrease in the biliary output of cholesterol and phospholipid, without changes in bile acid secretion. The decrease was more marked for cholesterol than for phospholipid secretion, with a significant decrease in their molar ratio in bile. The effects were apparently unrelated to an inhibition of intracellular vesicular transport because, after injection of horseradish peroxidase, both the time course and total amount secreted of the protein did not significantly differ between control animals and those receiving cefmetazole. The secretory rate of the lysosomal marker acid phosphatase was not affected by cefmetazole administration. Biliary outputs of the plasma-membrane enzymes alkaline phosphatase and gamma-glutamyltransferase were significantly decreased by the antibiotic. These results point to an effect of cefmetazole at the level of the canalicular membrane.

Menadione increases hepatic tight-junctional permeability. Its effect can be decreased by butylated hydroxytoluene and verapamil

Infusion of menadione at two different doses [2.7 mg and 5.5 mg in 100 microliters of dimethyl sulphoxide (DMSO)] into perfused rat livers for 30 min caused no or a 6-fold increase respectively in junctional permeability to horseradish peroxidase as compared with controls receiving 100 microliters of DMSO alone. The total glutathione (GSH) contents in these livers measured at the end of the experiments were 115% and 53%, compared with the controls. The free-radical scavenger butylated hydroxytoluene (BHT) (final concn. 5 microM) protected against the GSH depletion caused by the higher dose of menadione and partially decreased the menadione-induced increase in junctional permeability. Verapamil, a Ca2(+)-channel blocker which was added into the perfusion medium (final concn. 40 microM) 10 min before the infusion of 5.5 mg of menadione, completely abolished the effect of menadione on junctional permeability. Menadione exposure therefore increases tight-junctional permeability in the liver; this may involve a depletion of GSH and a subsequent increase in intracellular Ca2+.

Cellular mechanisms of intrahepatic cholestasis

Most forms of intrahepatic cholestasis are caused by a failure of hepatocytes to secrete osmotically active bile constituents into the minute channels of bile canaliculi. This overall vectorial bile secretory process is dependent upon a variety of polarised active transport functions at the basolateral (sinusoidal and lateral) and canalicular plasma membrane domains, as well as upon the coordinated vectorial movement of intracellular vesicles. Although considerable progress has been made in recent years in the identification, characterisation and exact localisation of a number of polarised hepatocellular transport systems, the primary mechanisms and targets leading to defective bile secretion and cholestasis are still not completely understood. For example, not all reported experimental data are compatible with the concept that estrogen-induced cholestasis represents a predominant sinusoidal disease process. In addition, the pathophysiological significance of disturbed transcytotic pathways and/or disrupted intracellular calcium homeostasis are not yet clear. For many forms of cholestasis, it remains uncertain as to whether leaky tight junctions represent a primary cause rather than a secondary phenomenon of the cholestatic state. However, the ongoing progress in the understanding of the normal mechanisms involved in the establishment, maintenance and regulation of ion homeostasis and polar transport functions in hepatocytes will, undoubtedly, improve our knowledge of the pathogenesis of intrahepatic cholestasis and, it is hoped, lead to better therapeutic strategies in the near future.