Bile acid structure and intestinal absorption in the animal model

A close structure-activity relationship exists between the transport of bile acids (BA) in the liver and intestine; hepatic and intestinal BA transport can be evaluated and compared by using perfused liver and perfused intestine in the rabbit. The passive intestinal absorption is limited to the unconjugated BA, which occurs throughout the small bowel and colon, and is conditioned by the apical membrane lipid composition. A higher diffusion component is found in the terminal ileum compared to the jejunum, and seems to be related to the higher cholesterol-to-phospholipid ratio of the ileal brush border membranes. The active transport system is well characterized and the brush border membrane receptor, cytosolic BA binding proteins and basolateral anion exchange protein have been identified. Recently, the ileal BA transporter has been cloned from the hamster and human ileum and the main cytosolic BA binding protein was cloned from the rat ileum. In the liver, the active transport predominates on the passive diffusion both for conjugated and unconjugated BA. The maximal transport capacity in the liver is tenfold higher than in the intestine, while the Km values are of the same order of magnitude, i.e. in the millimolar range. Neither system operates at its maximum transport rate with prevalent concentrations of BA in portal blood or luminal content.