Mechanisms of fluid and electrolyte transport by the biliary epithelium and their contribution to bile formation

The biliary epithelium contributes to bile formation by secreting fluid and electrolytes and by modifying the chemical composition of bile by ion exchange. Thus, secretin stimulation of the biliary epithelium causes ductular secretion of a bicarbonate-rich fluid. The bicarbonate secretory mechanism includes Cl-/HCO3- exchangers in the apical membrane of biliary epithelial cells and cyclic AMP–regulated plasma membrane Cl conductances (cystic fibrosis transmembrane conductance regulator). High conductance anion channels in the apical membrane of bile duct epithelial cells may also be involved. Some of the secreted HCO3- is later exchanged with Cl- in the biliary tree. The primary active transport mechanism driving secretin-dependent ductular bicarbonate secretion may be either Na+, K+-ATPase or vacular H+-ATPase, or a combination of the two. Aquaporin CHIP (the channel-forming integral membrane protein of 28 kD) governs biliary epithelial water transport. Certain unconjugated bile acids, such as ursodeoxycholic acid, modulate the volume and electrolyte composition of bile through their cholehepatic circulation.