Bile acid secretion, bile flow and biliary lipid secretion in humans.

The transport of bile acids through the hepatocyte and the effect of this flux of molecules on bile flow, biliary lipid secretion and bile acid biosynthesis are reviewed. Efficient hepatic clearance of bile acids involves several active systems and passive uptake. Formation of coenzyme A derivatives prevents reflux of lipophilic dihydroxy bile acids. Biotransformation of bile acids during hepatocyte transport involves both type I and type II biotransformations. The major type II biotransformation is reamidation of unconjugated bile acids, but sulfation and glucuronidation also occur. The only major type I biotransformation is oxidoreduction, which converts iso- or 3 oxo bile acids to the preferred 3 alpha-hydroxy form and 7 oxo bile acids to their corresponding 7 alpha-hydroxy derivatives. Secretion of bile acids into the canaliculus is concentrative and induces osmotic flow of plasma water and solutes across the paracellular junctions between the space of Disse and the canaliculus. Bile flow in man is characterized by its low volume and by a paucity of bile acid-independent flow when compared with that of other mammals. Bile acid secretion also induces biliary lipid secretion, but some lipid secretion appears to be bile acid independent. The lipid vesicles secreted in human bile have a much higher cholesterol/phospholipid ratio than those of other mammals. During flow down the biliary tree, vesicles are solubilized by bile acid micelles, and Ca++ ions are complexed by bile acid monomers and micelles. However, the biochemical mechanisms and factors regulating biliary lipid secretion in man are poorly understood.