We employ electrophysiological approaches to characterize the membrane ion channels involved in biliary secretion and liver bile formation. Defining the mechanisms responsible for bile formation will serve as a basis for the development of new strategies to stimulate cholangiocyte secretion for the treatment of childhood liver disorders.
Intrahepatic bile duct epithelial cells, or cholangiocytes, represent an important component of the bile secretory unit. Bile formation is initiated at the hepatocyte canalicular membrane, and cholangiocytes modify the volume and composition of bile through regulated secretion of ions and water.
Cl– channels in the apical membrane of cholangiocytes provide the driving force for ductular secretion. We have identified two types of Cl– channels:
- CFTR is activated by cAMP and is found on cholangiocytes in medium- and large-sized bile ducts. It is regulated via binding of the hormone secretin to basolateral receptors, by increases in [cAMP]i and by PKA-dependent channel phosphorylation.
- TMEM16A is activated by Ca2+ and is found on cholangiocytes in small-, medium-, and large-sized bile ducts; we are currently characterizing the physiologic events that mediate its regulation.
Our studies provided the first evidence that TMEM16A is the operative channel responsible for Ca2+-activated Cl– secretion in response to extracellular nucleotides.
These studies represent the first molecular identification of an alternate, non-CFTR Cl– channel in cholangiocytes, and demonstrate that TMEM16A is a potential target to modulate bile formation in the treatment of cholestatic liver disorders.
Using electrophysiological techniques, we have characterized the biophysical properties and molecular identities of the membrane ion channels mediating the secretory response to ATP, including the Ca2+-activated K+ channels, SK2 and IK1, and Cl–channels.