Secretin-dependent HCO3−secretion from pancreas and liver

Acetazolamide inhibits stimulated feline liver and gallbladder bicarbonate secretion

Bile acidification is a key factor in preventing calcium carbonate precipitation and gallstone formation. Carbonic anhydrase II (CA II), that is inhibited by acetazolamide, plays a role in regulation of the acid-base balance in many tissues. This study examines the effect of acetazolamide on secretin- and vasoactive intestinal peptide (VIP)-stimulated gallbladder mucosal bicarbonate and acid secretion. Gallbladders in anaesthetized cats were perfused with a bicarbonate buffer bubbled with CO2 in air. In 20 experiments VIP (10 microg kg(-1) h(-1)) and in 10 experiments secretin (4 microg kg(-1) h(-1)) were infused continuously intravenous (i.v.). Hepatic bile and samples from the buffer before and after perfusion of the gallbladder were collected for calculation of ion and fluid transport. During basal conditions a continuous secretion of H+ by the gallbladder mucosa was seen. Intravenous infusion of vasoactive intestinal peptide (VIP) and secretin caused a secretion of bicarbonate from the gallbladder mucosa (P < 0.01). This secretion was reduced by intraluminal (i.l.) acetazolamide (P < 0.01). Bile flow was enhanced by infusion of VIP and secretin (P < 0.01) but this stimulated outflow was not affected by i.v. acetazolamide. The presence of CA II in the gallbladder was demonstrated by immunoblotting. Biliary CA activity has an important function in the regulation of VIP- and secretin-stimulated bicarbonate secretion across the gallbladder mucosa.

Regulation of cholangiocyte bicarbonate secretion

The objective of this review article is to discuss the role of secretin and its receptor in the regulation of the secretory activity of intrahepatic bile duct epithelial cells (i.e., cholangiocytes). After a brief overview of cholangiocyte functions, we provide an historical background for the role of secretin and its receptor in the regulation of ductal secretion. We review the newly developed experimental in vivo and in vitro tools, which lead to understanding of the mechanisms of secretin regulation of cholangiocyte functions. After a description of the intracellular mechanisms by which secretin stimulates ductal secretion, we discuss the heterogeneous responses of different-sized intrahepatic bile ducts to gastrointestinal hormones. Furthermore, we outline the role of a number of cooperative factors (e.g., nerves, alkaline phosphatase, gastrointestinal hormones, neuropeptides, and bile acids) in the regulation of secretin-stimulated ductal secretion. Finally, we discuss other factors that may also play an important role in the regulation of secretin-stimulated ductal secretion.