Primary canalicular bile undergoes a process of fluidization and alkalinization along the biliary tract that is influenced by several factors, including hormones, innervation/neuropeptides and biliary constituents. The excretion of bicarbonate at both the canaliculi and the bile ducts is an important contributor to the generation of bile-salt independent flow. Bicarbonate is secreted from hepatocytes and cholangiocytes through parallel mechanisms, which involve chloride efflux through activation of chloride channels and further bicarbonate secretion via AE2 (also designated SLC4A2)-mediated chloride/bicarbonate exchange. The AE2 protein regulates pH, chloride concentration, cell volume and transepithelial ion transport in many tissues. Gene silencing of AE2 causes a marked inhibition of unstimulated and secretin-stimulated chloride/bicarbonate exchange, which maintains the bile acid pool that is crucial for secretin to induce bicarbonate-rich choleresis.
