Extracellular nucleotides stimulate Cl^- currents in biliary epithelia through receptor-mediated IP3 and Ca²⁺ release

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 Ca²⁺ concentration ([Ca²⁺] _i) and membrane Cl^- permeability. Exposure of cells to ATP resulted in a rapid increase in [Ca²⁺]_i and activation of membrane Cl^- currents; both responses were abolished by prior depletion of intracellular Ca²⁺. ATP-stimulated Cl^- currents demonstrated mild outward rectification, reversal at E_Cl-, and a single-channel conductance of ∼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 [Ca²⁺] _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 [Ca²⁺]_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.