Effects of internal biliary bypass on the regulation of hepatic cholesterol 7 alpha-hydroxylase activity in rats

In order to re-evaluate the importance of the enterohepatic circulation of bile acid in the regulation of the activities of hepatic cholesterol 7 alph a -hydroxylase, bile acid metabolism was examined in internal biliary bypas s models of rats. A polyethylene tube was inserted into the common bile duc t and another side of the tube was placed in the duodenum (DD), lower jejun um (JD), cecum (CeD), or transverse colon (CoD) as internal biliary bypass models and in the urinary bladder as an external biliary drainage (ED). Aft er bile diversion for 7 days in each group, hepatic cholesterol 7 alpha -hy droxylase activities, bile acid concentrations in bile, serum, and portal v ein, biliary bile acid compositions, and intestinal absorption rates of inf used labeled taurocholic acid were analyzed. Hepatic cholesterol 7 alpha -h ydroxylase activity was similar in the JD group compared with the DD group, however, it was significantly up-regulated in the CeD (227% of the DD grou p), Coo (312%), and ED groups (316%). Biliary, serum, and portal bile acid concentrations were not significantly changed in the DD, JD, and CeD groups but those were significantly lower in the Coo and ED groups compared with the DD group. The proportion of the secondary bile acids was significantly increased in the CeD group and was decreased in the Coo and ED groups. The absorption rate of taurocholic acid was almost 100%, 56%, and 23% in the JD , CeD, and the Coo group, respectively. As the cholesterol 7 alpha -hydroxy lase activity was not significantly changed in the JD group and the predomi nance of secondary bile acids did not suppress the enzyme activity in the C eD group, the luminal factor, which is absorbed in the presence of bile aci ds, and the bile acid metabolites are not likely the regulatory factor. The cholesterol 7 alpha -hydroxylase activity seems to be primarily regulated by the intestinal absorption of bile acids and partly by the intestinal muc osal factor which is linked to the intestinal bile acid absorption. (C) 200 1 Elsevier Science Inc. All rights reserved.