The hepatic transport of bile acid conjugates was studied in the Eisai hyperbilirubinuria rat, a Sprague-Dawley mutant rat with conjugated hyperbilirubinemia. Serum bile acid levels were increased, bile acid-independent bile flow was decreased and biliary glutathione concentrations were markedly decreased in the Eisai hyperbilirubinuria rat. Biliary excretion of sulfobromophthalein was markedly impaired and almost no glutathione conjugate was excreted in the bile of the Eisai hyperbilirubinuria rat. Biliary excretion of lithocholate-3-O-glucuronide and lithocholate-3-sulfate in the Eisai hyperbilirubinuria rat was markedly delayed, whereas that of lithocholate was only slightly delayed. After [14C]chenodeoxycholate infusion (1 mumol/min/100 gm for 60 min), the increases in bile flow and biliary excretion of isotope in the Eisai hyperbilirubinuria rat were not so prominent as those observed in control rats, and the glucuronide of chenodeoxycholate, which constituted about 15% of biliary chenodeoxycholate in control rats, was not observed in the Eisai hyperbilirubinuria rat. Initial uptake of lithocholate and its glucuronide and sulfate by isolated hepatocytes was not impaired in the Eisai hyperbilirubinuria rat; the profiles of cytosolic bile acid binding proteins in Eisai hyperbilirubinuria rat liver were identical to those in control liver. These data indicate that the Eisai hyperbilirubinuria rat has excretory impairment of organic anions, bile acid glucuronide and sulfate and that it has characteristics very similar to those of the hyperbilirubinemic mutant Wistar rats TR- and GY.

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.

We previously reported that high-dose infusion of ursodeoxycholate into rats caused its extensive glucuronidation. In this study, the glucuronidation of various bile acids after high-dose infusion into rats was examined and the effects of coinfusion of bile acids on the glucuronidation of a trace dose of [14C]deoxycholate were also studied. Sixty minutes after infusion of 14C-bile acids at a rate of 1 mumol/min/100 gm, the glucuronidation of the labeled bile acid in the bile was 31%, 15%, 8% and 3% for deoxycholate, ursodeoxycholate, chenodeoxycholate and cholate, respectively. The infusion of a trace dose of [14C] deoxycholate resulted in only 2% glucuronidation, and coinfusion of taurochenodeoxycholate or tauroursodeoxycholate at a rate of 1 mumol/min/100 gm did not change the percentage of glucuronidation of [14C]deoxycholate. However, coinfusion of chenodeoxycholate, ursodeoxycholate or cholate at the same rate markedly increased the percentage of the [14C]deoxycholate-glycuronide in the bile (35%, 18% and 36%, respectively). Thus glucuronidation of bile acids by high-dose infusion into rats occurs predominantly with deoxycholate and is not specific for ursodeoxycholate, and glucuronidation depends on the unconjugated bile acid load, which might be regulated by the capacity of amidation by the liver.

No abstract

No abstract