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 flui d and electrolytes and by modifying the chemical composition of bile b y ion exchange. Thus, secretin stimulation of the biliary epithelium c auses ductular secretion of a bicarbonate-rich fluid. The bicarbonate secretory mechanism includes Cl-/HCO3- exchangers in the apical membra ne of biliary epithelial cells and cyclic AMP-regulated plasma membran e 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 trans port mechanism driving secretin-dependent ductular bicarbonate secreti on may be either Na+,K+-ATPase or vacuolar H+-ATPase, or a combination of the two. Aquaporin CHIP (the channel-forming integral membrane pro tein of 28 kD) governs biliary epithelial water transport. Certain unc onjugated bile acids, such as ursodeoxycholic acid, modulate the volum e and electrolyte composition of bile through their cholehepatic circu lation.