Comparison of biliary excretion and metabolism of lithocholic acid and its sulfate and glucuronide conjugates in rats

Effect of genipin on cholestasis induced by estradiol-17beta-glucuronide and lithocholate-3-O-glucuornide in rats

AIM

Genipin is reported to stimulate the insertion of multidrug resistance protein 2 (Mrp2) in the bile canalicular membrane, thereby causing choleresis by the increased the biliary excretion of glutathione, which has been considered to be a substrate of Mrp2. In the present study, we examined the effect of genipin on cholestasis induced by estradiol-17beta-glucuronide and lithocholate-3-O-glucuronide, Mrp2 substrates, in rats. Further, the effect of genipin on the biliary excretion of substrates of P-glycoprotein (P-gp), vinblastine and erythromycin, was also studied.

METHODS

The effect of genipin infusion at the rate of 0.5 micromol/min/100 g on cholestasis induced by estradiol-17beta-glucuronide (0.075 micromol/min/100 g for 20 min) and lithocholate-3-O-glucuronide (0.15 micromol/min/100 g for 40 min) was studied. The effect of genipin infusion on the biliary excretion of a tracer dose of vinblastine and erythromycin infused at the rate of 0.1 micromol/min/100 g was also studied.

RESULTS

Genipin relieved estradiol-17beta-glucuronide-induced cholestasis, and cumulative biliary estradiol-17beta-glucuronide excretion for 120 min was increased from 50 +/- 20%-81 +/- 20% dose. In contrast, genipin had no effect on lithocholate-3-O-glucuronide-induced cholestasis. Biliary excretion of a tracer dose of vinblastine and the maximum biliary excretion of erythromycin were significantly decreased by genipin.

CONCLUSIONS

Genipin protected estradiol-17beta-glucuronide-induced cholestasis. The mechanism of the protection of cholestasis by genipin is unknown, but it is speculated to be due to a conformational change of P-gp by genipin, in addition to the stimulation of Mrp2 insertion into the bile canaliculi.

Effect of colestimide on absorption of unconjugated bile acids in the rat jejunum

BACKGROUND

Colestimide is a newly developed bile acid-binding resin in Japan, but its bile acid-binding properties have not been studied.

METHODS

The absorption of unconjugated bile acids (5 mmol/L) in the ligated rat jejunum was compared in the presence and absence of colestimide. Furthermore, bile acid adsorption by colestimide was also studied in vitro.

RESULTS

All bile acids were efficiently absorbed in the jejunum and the cumulative absorption during 120 min was 29-63%. The absorption of chenodeoxycholate, lithocholate, deoxycholate and ursodeoxycholate was dose-dependently inhibited by 2.5 and 5 mg colestimide, whereas the absorption of cholate was not inhibited, even in the presence of 5 mg colestimide. Adsorption of bile acids by colestimide in vitro was approximately 60% for chenodeoxycholate, lithocholate, deoxycholate and ursodeoxycholate, whereas the adsorption of cholate was low (16%).

CONCLUSIONS

Jejunal absorption of ursodeoxycholate was inhibited by colestimide to a similar extent as other dihydroxy bile acids, whereas that of cholate was not inhibited under the same conditions.

Changes in biliary excretory mechanisms in rats with ethinyloestradiol-induced cholestasis

Several excretory pathways for cholephilic compounds have been known. To examine the changes in excretory pathways in cholestasis induced by ethinyloestradiol, various bile acids, organic anions and organic cations were intravenously administered to ethinyloestradiol-treated rats and their biliary excretion was studied. Biliary excretion of taurocholate was slightly delayed, but its excretory maximum was markedly decreased. Biliary excretion of lithocholate-3-O-glucuronide, leukotriene C4, sulphobromophthalein and pravastatin was markedly impaired to a similar extent. Biliary excretion of vinblastine, a P-glycoprotein substrate, was increased, suggesting increased expression of P-glycoprotein. In contrast, biliary excretion of erythromycin, a cationic antibiotic, was markedly impaired. In conclusion, ethinyloestradiol treatment altered the biliary excretion of organic compounds, which may partly be related to changes of the canalicular transporters.

Absorption of unconjugated bile acids and tauroursodeoxycholate in the rat intestine

The absorption of ursodeoxycholate and its tauro-conjugate by the jejunum and the terminal ileum of rat intestine was compared with that of other unconjugated bile acids and taurocholate. In the ligated jejunum, the efficacy of absorption of unconjugated bile acids was in the following order: ursodeoxycholate = deoxycholate > chenodeoxycholate = cholate > lithocholate. This order cannot be explained by the theory that the passive diffusion of bile acids is faster the less hydroxyl bonds in the molecule. These findings on the unconjugated bile acids in the ligated jejunum were further confirmed by perfusion experiments. In the ligated terminal ileum, ursodeoxycholate, cholate and deoxycholate were absorbed as fast as taurocholate or tauroursodeoxycholate, whereas absorption of chenodeoxycholate was significantly slower. The Na+-dependency of the absorption of ursodeoxycholate and cholate in the terminal ileum was confirmed by perfusion studies. In conclusion, intestinal absorption of ursodeoxycholate was efficient in both the jejunum and ileum and these results may contribute to the high availability of ursodeoxycholate in various hepatobiliary diseases.

Changes in biliary excretory mechanisms in bile duct-ligated rat

The effects of bile duct ligation on biliary excretion of bile acids, glutathione, and lipids were studied in the rat. The bile duct of the rat was ligated for three days. The biliary bile acid excretion after bile duct cannulation was higher at first, but after 90 min became lower than that in the control rat. The bile flow in the bile duct-ligated rat was higher after bile duct cannulation and gradually decreased to the same level as in the control rat. Biliary glutathione excretion, which has been suggested to be a driving force for the bile acid-independent canalicular bile flow, was markedly decreased in the bile duct-ligated rat. The mannitol clearance was increased and the bile ductules showed proliferation in the bile duct-ligated rat, suggesting an increase in the ductular bile flow. Biliary excretion of lithocholate glucuronide was more markedly impaired than that of taurocholate. When taurocholate was infused at higher rates, which increases bile flow and biliary excretion of bile acid and lipids in the control rat, biliary bile acid and lipid excretion remained constant in the bile duct-ligated rat. These findings indicate that, in the bile duct-ligated rat, the ductular bile flow was increased and bile acid-independent canalicular bile flow was decreased and that, although the biliary excretion of bile acids was not as impaired as that of organic anions, the capacity of bile acid and lipid excretion was markedly decreased.

Mechanisms of biliary excretion of lithocholate-3-sulfate in Eisai hyperbilirubinemic rats (EHBR)

Biliary excretion of lithocholate-3-sulfate is markedly impaired in EHBR. To examine the mechanism of biliary lithocholate-3-sulfate excretion in EHBR, the effects of colchicine treatment, a vesicular transport inhibitor, and infusion of taurocholate and organic anions were studied in EHBR and Sprague-Dawley rats. Colchicine treatment and taurocholate infusion had no effect of biliary lithocholate-3-sulfate excretion in EHBR, suggesting that biliary lithocholate-3-sulfate excretion is not mediated by the vesicular transport or by the bile acid excretory pathway. In control Sprague-Dawley rats, both sulfobromophthalein and dibromosulfophthalein infusion inhibited biliary lithocholate-3-sulfate excretion. In contrast, in EHBR dibromosulfophthalein infusion inhibited biliary lithocholate-3-sulfate excretion but BSP infusion did not. Indocyanine green and pravastatin infusion did not affect biliary lithocholate-3-sulfate excretion but pravastatin infusion had no effect in EHBR. These findings indicate that, whether physiologically important or not, two of more excretory pathways for organic anions exist at the canalicular membrane other than the ATP-dependent one.

Intestinal absorption of lithocholate and its sulfate and glucuronide in rats

The absorption of lithocholate and its sulfate and glucuronide in rat jejunum and terminal ileum was studied. Tracer amounts of radiolabelled bile acids were administered to the ligated intestinal segments, and their absorption was monitored by biliary excretion through a bile duct catheter. Absorption of lithocholate was faster in the terminal ileum than in the jejunum. Although the sulfation reduced lithocholate absorption in the jejunum, it did not affect lithocholate absorption in the terminal ileum. This was due to the Na+-dependency of ileal absorption of lithocholate-sulfate assessed by perfusion studies. In contrast, the glucuronidation markedly reduced lithocholate absorption both in the jejunum and the terminal ileum. These findings indicate that the glucuronidation is more effective than sulfation in detoxifying lithocholate as far as the prevention of its intestinal absorption is concerned.

Binding of lithocholate and its glucuronide and sulfate by human serum albumin

In the present study, the binding affinities of lithocholate sulfate and glucuronide by human serum albumin were compared to that of lithocholate by equilibrium dialysis, and the binding sites of these bile acids on those of various fluorescent probes and bilirubin were also studied. The dissociation constants for the primary binding sites for lithocholate sulfate and glucuronide on human serum albumin were 0.057 and 0.24 microM, respectively, which were lower than that for lithocholate (0.82 microM). Lithocholate sulfate and glucuronide, as well as lithocholate, did not simply inhibit the binding of the site II and III fluorescent probes or bilirubin to albumin. Inhibition by these bile acids of the site I fluorescent probe binding to albumin suggested that the secondary binding sites of these bile acids are equal to site I. The results of simultaneous equilibrium dialysis using [3H]lithocholate and [14C]lithocholate sulfate indicated that these compounds have the same primary binding site. In conclusion, sulfation and glucuronidation may increase the binding affinities of bile acids to human serum albumin without changing their binding site.

Colchicine inhibits lithocholate-3-O-glucuronide-induced cholestasis in rats

BACKGROUND/AIMS

It has been suggested that vesicular transport of bile acids in hepatocytes occurs, especially at high-dose loads.

METHODS

The effect was studied of colchicine, a vesicular transport inhibitor, on lithocholate-3-O-glucuronide-induced cholestasis in rats. Cholestasis was induced by an intravenous infusion of lithocholate-3-O-glucoronide at the rate of 0.1 mumol.min-1.100 g-1 for 40 min.

RESULTS

Colchicine treatment almost completely inhibited cholestasis and increased biliary excretion of lithocholate-3-O-glucoronide, whereas lumicolchicine had no effect. Treatment with vinblastine, another vesicular transport inhibitor, also reduced the cholestasis. Colchicine did not affect biliary excretion of taurocholate infused at the rate of 0.3 mumol.min-1.100 g-1 for 40 min, but markedly inhibited its biliary excretion when infused at the rate of 1.5 mumol.min-1.100 g-1 for 40 min. Colchicine had no effect on biliary excretion of tauroursodeoxycholate (1.5 mumol.min-1.100 g-1 for 40 min), lithocholate-3-sulfate (0.3 mumol.min-1.100 g-1 for 40 min), or a trace amount of lithocholate-3-O-glucuronide.

CONCLUSIONS

These findings indicate that lithocholate-3-O-glucoronide-induced cholestasis is caused by its increased access to the vesicular transport pathway, possibly beyond the capacity of the transport by the cytosolic binders, and that the transport of lithocholate-3-O-glucoronide via the vesicular pathway induces cholestasis. Furthermore, the contribution of the vesicular pathway to hepatic transport may be different among bile acids, and lithocholate-3-O-glucuronide seems to have higher accessibility to this transport system.

Lithocholate-3-O-glucuronide-induced cholestasis. A study with congenital hyperbilirubinemic rats and effects of ursodeoxycholate conjugates

The mechanism of lithocholate-3-O-glucuronide-induced cholestasis is unknown. In this study, we investigated the cholestatic effects of this agent in a congenital hyperbilirubinemic rat, EHBR. We also studied the effects of ursodeoxycholate-3-O-glucuronide and tauroursodeoxycholate on lithocholate-3-O-glucuronide-induced cholestasis in rats. Lithocholate-3-O-glucuronide, administered at the rate of 0.1 mumol/min/100 g for 40 min, a cholestatic dose in control rats, failed to cause cholestasis in EHBR, and biliary lithocholate-3-O-glucuronide excretion was delayed. Biliary concentrations of this agent did not correlate with the severity of cholestasis. Both tauroursodeoxycholate and ursodeoxycholate-3-O-glucuronide, infused at the rate of 0.2 mumol/min/100 g for 120 min, completely inhibited cholestasis induced by lithocholate-3-O-glucuronide administered at the rate of 0.1 mumol/min/100 g for 40 min. Only tauroursodeoxycholate enhanced biliary lithocholate-3-O-glucuronide excretion. These findings indicate that lithocholate-3-O-glucuronide-induced cholestasis is induced by damage at the level of the bile canalicular membrane. Ursodeoxycholate-3-O-glucuronide inhibits this cholestasis, possibly by inhibiting the access of lithocholate-3-O-glucuronide to the bile canalicular membrane.

Effects of ursodeoxycholate, its glucuronide and disulfate and beta-muricholate on biliary bicarbonate concentration and biliary lipid excretion

We previously reported that high-dose ursodeoxycholate (UDC) infusion in rats resulted in extensive glucuronidation of UDC, and speculated that the glucuronidation causes bicarbonate-rich hypercholeresis induced by UDC (Takikawa, H., Sano, N., Narita, T. and Yamanaka, M. Hepatology 1990; 11: 743-749). To test this hypothesis, UDC, UDC-3-O-glucuronide, UDC-3,7-disulfate and beta-muricholate were separately and intravenously infused into rats (1 mumol/min per 100 g), and biliary bicarbonate concentration was measured. The effects of these bile acids on biliary lipid secretion were also studied. All four bile acids increased bile flow and biliary bile acid excretion. UDC and beta-muricholate significantly increased biliary bicarbonate concentration, whereas UDC glucuronide and disulfate did not. Independence of UDC glucuronide excretion and biliary bicarbonate concentration was also confirmed in EHBR, a hyperbilirubinemic mutant Sprague-Dawley rat. In this case biliary bicarbonate concentration also increased in spite of the absence of UDC glucuronide in the bile after UDC infusion. Biliary phospholipid secretion was increased with UDC, unchanged with beta-muricholate, and decreased with UDC glucuronide and disulfate. Biliary cholesterol secretion was increased with UDC, unchanged with beta-muricholate and UDC glucuronide, and decreased with UDC disulfate. These data indicate that glucuronidation is not the cause of bicarbonate-rich hypercholeresis induced by UDC but that glucuronidation and sulfation change the effect of UDC on biliary lipid secretion.

Cytotoxic effect and uptake mechanism by isolated rat hepatocytes of lithocholate and its glucuronide and sulfate

The hepatotoxicity and uptake mechanism of lithocholate and its glucuronide and sulfate were studied using isolated rat hepatocytes. Cytotoxicity was in the order of lithocholate greater than lithocholate-glucuronide greater than lithocholate-sulfate; their 50% cytotoxic concentrations on hepatocytes were 50, 150 and 700 microM, respectively. Thus, glucuronidation as well as sulfation acted to detoxify lithocholate, not relating to the previously reported higher cholestatic effect of lithocholate-glucuronide than lithocholate. Lithocholate uptake was linear up to 50 microM, whereas the uptakes of lithocholate-glucuronide and sulfate were saturable with an apparent Km and Vmax of 32 microM and 6.4 nmol/min per 10(6) cells for lithocholate-glucuronide and 26 microM and 11.8 nmol/min per 10(6) cells for lithocholate-sulfate. Na+ replacement by choline+ had no effect on the uptake of lithocholate and lithocholate-glucuronide, whereas it slightly inhibited lithocholate-sulfate uptake. Lithocholate-glucuronide uptake was inhibited by lithocholate-sulfate and sulfobromophthalein, whereas lithocholate-glucuronide and sulfobromophthalein had no effect on lithocholate-sulfate uptake. These data indicate that hepatic lithocholate uptake is mediated by simple diffusion, and that hepatic uptake of lithocholate-glucuronide and sulfate is mainly mediated by a Na(+)-independent carrier.

Cholestasis induced by lithocholate and its glucuronide: their biliary excretion and metabolism

We studied the effects of the infusion of lithocholate and lithocholate-3-sulfate and 3-glucuronide in rats (0.29 mumol/min per 100 g body weight for 40 min) on bile flow, together with their biliary excretion and metabolism. Lithocholate-glucuronide had a higher cholestatic effect than lithocholate, whereas lithocholate-sulfate had almost no effect on bile flow. Lithocholate was mainly converted to taurine or glucuronide conjugates in the bile, serum and liver and hydroxylation of the tauro-conjugate proceeded. Lithocholate-sulfate was almost completely excreted in the bile, mainly as tauro-conjugate. Lithocholate-glucuronide was excreted in bile almost without conjugation, while some taurine conjugation occurred in the serum and liver. These results suggest that the poor biotransformation of lithocholate-glucuronide is related to its higher cholestatic potency than lithocholate.

The ursodeoxycholate dose-dependent formation of ursodeoxycholate-glucuronide in the rat and the choleretic potencies

The reason for the discrepancy between bile flow and biliary bile acid excretion during ursodeoxycholate infusion in rats is unknown. We found that ursodeoxycholate-glucuronide is formed during ursodeoxycholate infusion at higher doses. Ursodeoxycholate infusion (1 to 3 mumol/min/100 gm body weight) for 90 min caused marked hypercholeresis, and the previously reported discrepancy between bile flow and biliary bile acid excretion was observed when bile acid concentrations were measured by regular enzymatic methods. However, the appearance of ursodeoxycholate-glucuronide was observed on thin-layer chromatography analysis and up to 30% of the ursodeoxycholate in bile was found to be glucuronidated when determined by the enzymatic method after beta-glucuronidase treatment. The choleretic activity of ursodeoxycholate-glucuronide (25.2 microliters/mumol) was about 3 times higher than that of ursodeoxycholate (8.9 microliters/mumol) when infused at 0.25 mumol/min/100 gm body weight and ursodeoxycholate-glucuronide also stimulated higher biliary bicarbonate excretion than ursodeoxycholate. These results indicate that the discrepancy between bile flow and biliary bile acid excretion caused by high-dose infusion of ursodeoxycholate into rats can be explained by glucuronide conjugation of ursodeoxycholate that cannot be detected by the regular enzymatic method. The glucuronidation of ursodeoxycholate might also be important in the ursodeoxycholate-induced increase in biliary bicarbonate excretion.