Bile duct epithelia regulate biliary bicarbonate excretion in normal rat liver

Area of Science:

• Hepatobiliary physiology
• Gastroenterology
• Cellular transport mechanisms

Background:

• The specific role of bile ducts in bile formation in vivo remains incompletely understood.
• While transporters and channels in cholangiocytes are known, their contribution to bile flow and biliary bicarbonate excretion requires clarification.

Purpose of the Study:

• To determine the precise contribution of cholangiocytes to bile flow and biliary bicarbonate excretion in normal rat liver.
• To elucidate the regulatory mechanisms of biliary bicarbonate secretion by cholangiocytes.

Main Methods:

• Isolated rat livers were perfused via the portal vein and hepatic artery to assess bile flow and biliary bicarbonate.
• Perfusion with secretin or acetylcholine (ACh) modulated cholangiocyte cyclic AMP or Ca(2+), while glucagon or vasopressin targeted hepatocytes.
• Pharmacological inhibitors, including cyclosporin A, DIDS, NPPB, and DPC, were used to probe specific transport pathways.

Main Results:

• Secretin and ACh significantly increased biliary bicarbonate excretion when delivered via the hepatic artery, indicating cholangiocyte-specific regulation.
• Secretin's effect was dose-dependent and augmented by ACh, but not affected by glucagon or vasopressin.
• ACh-induced bicarbonate excretion was sensitive to chloride channel and bicarbonate exchange inhibitors (DIDS, NPPB, DPC), while secretin's was inhibited by NPPB or DPC but not DIDS.

Conclusions:

• Cholangiocytes, rather than hepatocytes, are the primary regulators of biliary bicarbonate in normal rat liver.
• Acetylcholine-stimulated bicarbonate excretion involves both chloride channels and bicarbonate exchange mechanisms.
• Secretin-induced bicarbonate excretion is independent of bicarbonate exchange, suggesting distinct signaling pathways.