BILE SECRETION

Bile is an isotonic aqueous solution of bile acids, cholesterol, phospholipids, bile pigments and inorganic electrolytes. It is secreted by the hepatocytes into the bile canaliculi and modified in the bile ductules or ducts. The three main processes identified in the generation of bile flow are: 1. Active transport of bile acids from blood into bile canaliculi. This is responsible of the bile acid-dependent canalicular bile flow. Coupling between water flow and bile acid secretion is probably effected mainly through an osmotic mechanism. There is evidence that water flows (at least in part) through the interhepatocytic junctions. The bile acid-dependent flow accounts for 30 to 60% of spontaneous basal bile flow. 2. A canalicular, bile acid-independent secretion, probably due to transport into bile of organic solutes (glutathione) and inorganic electrolytes. This fraction of bile flow is stimulated by phenobarbital. It represents 30 to 60% of basal bile flow. Normal canalicular bile flow also depends on the integrity of intracellular cytoskeletal organelles, mostly microfilaments. 3. Reabsorption and secretion of fluid and inorganic electrolytes by the ductules and ducts. Secretion chiefly occurs in response to secretin and represents 30% of basal bile flow. Although several ion transport systems have been identified on the biliary epithelial cells (in particular a Na+/H+ exchange, a Na+:HCO3- symport and a Cl-/HCO3- exchange), the cellular mechanism of secretion is not known. Abnormalities of bile duct function may account for the liver disease of cystic fibrosis, but these abnormalities have not been characterized.