Polarized expression and function of P2Y ATP receptors in rat bile duct epithelia

Extracellular nucleotides may be important regulators of bile ductular secretion, because cholangiocytes express P2Y ATP receptors and nucleotides are found in bile. However, the expression, distribution, and function of specific P2Y receptor subtypes in cholangiocytes are unknown. Thus our aim was to determine the subtypes, distribution, and role in secretion of P2Y receptors expressed by cholangiocytes. The molecular subtypes of P2Y receptors were determined by RT-PCR. Functional studies measuring cytosolic Ca²⁺ (Ca_i²⁺) signals and bile ductular pH were performed in isolated, microperfused intrahepatic bile duct units (IBDUs). PCR products corresponding to P2Y₁, P2Y₂, P2Y₄, P2Y₆, and P2X₄ receptor subtypes were identified. Luminal perfusion of ATP into IBDUs induced increases in Ca²⁺ that were inhibited by apyrase and suramin. Luminal ATP, ADP, 2-methylthio-adenosine 5′-triphosphate, UTP, and UDP each increased Ca_i²⁺. Basolateral addition of adenosine 5′-O-(3-thiotriphosphate) (ATP-γ-S), but not ATP, to the perifusing bath increased Ca_i²⁺. IBDU perfusion with ATP-γ-S induced net bile ductular alkalization. Cholangiocytes express multiple P2Y receptor subtypes that are expressed at the apical plasma membrane domain. P2Y receptors are also expressed on the basolateral domain, but their activation is attenuated by nucleotide hydrolysis. Activation of ductular P2Y receptors induces net ductular alkalization, suggesting that nucleotide signaling may be an important regulator of bile secretion by the liver.