Extracellular nucleotides stimulate Cl- currents in biliary epithelia through receptor-mediated IP3 and Ca2+ release.
ABSTRACT Extracellular ATP regulates bile formation by binding to P2 receptors on cholangiocytes and stimulating transepithelial Cl(-) secretion. However, the specific signaling pathways linking receptor binding to Cl(-) channel activation are not known. Consequently, the aim of these studies in human Mz-Cha-1 biliary cells and normal rat cholangiocyte monolayers was to assess the intracellular pathways responsible for ATP-stimulated increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) and membrane Cl(-) permeability. Exposure of cells to ATP resulted in a rapid increase in [Ca(2+)](i) and activation of membrane Cl(-) currents; both responses were abolished by prior depletion of intracellular Ca(2+). ATP-stimulated Cl(-) currents demonstrated mild outward rectification, reversal at E(Cl(-)), and a single-channel conductance of approximately 17 pS, where E is the equilibrium potential. The conductance response to ATP was inhibited by the Cl(-) channel inhibitors NPPB and DIDS but not the CFTR inhibitor CFTR(inh)-172. Both ATP-stimulated increases in [Ca(2+)](i) and Cl(-) channel activity were inhibited by the P2Y receptor antagonist suramin. The PLC inhibitor U73122 and the inositol 1,4,5-triphosphate (IP3) receptor inhibitor 2-APB both blocked the ATP-stimulated increase in [Ca(2+)](i) and membrane Cl(-) currents. Intracellular dialysis with purified IP3 activated Cl(-) currents with identical properties to those activated by ATP. Exposure of normal rat cholangiocyte monolayers to ATP increased short-circuit currents (I(sc)), reflecting transepithelial secretion. The I(sc) was unaffected by CFTR(inh)-172 but was significantly inhibited by U73122 or 2-APB. In summary, these findings indicate that the apical P2Y-IP3 receptor signaling complex is a dominant pathway mediating biliary epithelial Cl(-) transport and, therefore, may represent a potential target for increasing secretion in the treatment of cholestatic liver disease.
- SourceAvailable from: Amal K Dutta[Show abstract] [Hide abstract]
ABSTRACT: Bile formation by the liver is initiated by canalicular transport at the hepatocyte membrane, subsequently leading to an increase in ductular bile flow. Thus, bile duct epithelial cells (cholangiocytes), which contribute to the volume and dilution of bile through regulated Cl(-) transport, are exposed to changes in flow and shear force at the apical membrane. The aim of the present studies was to determine if the mechanical effects of flow itself is a signal regulating cholangiocyte transport. The results demonstrate that in both human and mouse biliary cells fluid-flow or shear increases Cl(-) currents and identifies TMEM16A, a Ca(2+)-activated Cl(-) channel, as the operative channel. Furthermore, the activation of TMEM16A by flow is dependent on PKCα through a process involving extracellular ATP, binding P2 receptors, and increases in [Ca(2+)](i). These studies represent the initial characterization of mechanosensitive Cl(-) currents mediated by TMEM16A. Identification of this novel mechanosensitive secretory pathway provides new insight into bile formation and suggests new therapeutic targets to enhance bile formation in the treatment of cholestatic liver disorders.AJP Gastrointestinal and Liver Physiology 10/2012; · 3.65 Impact Factor
Article: Physiology of Cholangiocytes.[Show abstract] [Hide abstract]
ABSTRACT: Cholangiocytes are epithelial cells that line the intra- and extrahepatic ducts of the biliary tree. The main physiologic function of cholangiocytes is modification of hepatocyte-derived bile, an intricate process regulated by hormones, peptides, nucleotides, neurotransmitters, and other molecules through intracellular signaling pathways and cascades. The mechanisms and regulation of bile modification are reviewed herein. © 2013 American Physiological Society. Compr Physiol 3:541-565, 2013.Comprehensive Physiology. 01/2013; 3(1):541-565.
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ABSTRACT: Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.Purinergic Signalling 12/2013; · 2.64 Impact Factor