Metabolism and transport of bile salts in the intestine

Abstract

A specific sodium-dependent active transport system for conjugated and unconjugated bile salts is located in the distal half of the small intestine. Considerations of physical-chemical factors suggest that passive diffusion of unconjugated bile salts and possibly of glycine-conjugated bile salts also occurs throughout the small bowel and colon. Studies in animals designed to assess the extent of this passive absorption have been complicated by species variation. These considerations have prompted questions in normal man, dealing with the physiologic significance of the ileal transport system in the maintenance of enterohepatic bile salt circulation, and the relative importance of other intestinal sites and mechanisms for bile salt absorption. Hydrolysis of conjugated bile salts occurs regularly during their enterohepatic circulation in normal man, resulting in unconjugated primary and secondary bile salts that can be absorbed by passive processes in regions other than the ileum. The site and extent of intestinal absorption of unconjugated bile salts is dependent upon the presence of micoorganisms capable of deconjugating bile salts and a luminal milieu compatible with these enzymatic alterations. Present evidence indicates that such microorganisms exist chiefly in the colon and to a lesser extent in the terminal ileum of normal man. It is likely that the major increment of the daily unconjugated bile salt reabsorption occurs via passive absorption in the colon. The kinetics of this absorptive system(s) are unknown, and quantification of the daily traffic of unconjugated bile salts is quite complex. However, from investigations described in detail in the text, an approximation of 15 per cent of the daily circulating bile salt seems reasonable. Despite these uncertainties, it is clear that the major site of bile salt reabsorption in normal man is via ileal transport of conjugated bile salts. The efficiency of the intestinal bile salt transport systems in the maintenance of their enterohepatic circulation is essential to the accomplishment of several functions of bile salts in the intestinal lumen. These functions include participation in the hydrolysis of triglycerides, solubilization of the products of pancreatic lipolysis, and the well documented function of bile salts in the absorption of lipid soluble vitamins and cholesterol.