Intrinsic factor receptor activity and cobalamin transport in bile duct-ligated rats

The intrinsic factor (IF)-cobalamin (Cbl) receptor activity in the mucosal homogenates progressively decreased after bile duct ligation in the rat, and 80% of the receptor activity was decreased in 96 h after ligation. The activity was restored to normal values of 5.5-6 pmol of IF-[57Co]Cbl bound/g mucosa when the assays were performed with both conjugated and unconjugated bile acids. When [57Co]Cbl bound to intrinsic factor was orally administered, the tissue levels of [57Co]Cbl were decreased by 75-80% in bile duct-ligated rats. The apical membrane receptor activity was also decreased after bile duct ligation; however, the activity was stimulated twofold by the addition of ileal cytosol and threefold with the addition of both ileal cytosol and taurocholate (1 mM). Enhanced binding of IF-[57Co]Cbl to the apical ileal brush-border membrane occurred with the use of dialyzed ileal cytosol but not with cytosol isolated from duodenal or proximal jejunal mucosa. The enhanced binding obtained with ileal cytosol was abolished after its treatment with trypsin. These results suggest that luminal bile acids optimize the binding of IF-Cbl by the ileal membrane receptor via interactions with a cytosolic factor and thus influence the gastrointestinal absorption of cobalamin.