Biliary epithelial cells (cholangiocytes) are responsible for rapid regulation of bile volume and alkalinity. Secretin and other hormones raising intracellular cyclic adenosine monophosphate (cAMP) concentrations promote biliary HCO3 secretion by stimulating apical Cl- channels and Cl-/HCO3- exchange (AE2). Cholangiocyte ion transport may also be stimulated by locally acting mediators; for example, adenosine 5'-triphosphate (ATP), a secretagogue that can be released into the bile by hepatocytes and cholangiocytes, activates Cl- conductances and Na+/H+ exchange (NHE) in cholangiocyte cell lines. To further explore the role of extracellular ATP in the paracrine regulation of carrier mechanisms regulating cholangiocyte H+/HCO3- secretion, we investigated the effects of nucleotides on intracellular pH regulation (measured by microfluorimetry with 2'7'-bis(2- carboxyethyl)-5,6,carboxyfluorescein [BCECF]) in human (MZ-ChA-1) and rat (NRC-1) cholangiocyte cell lines. In MZ-ChA-1 cells, 10 mol/L ATP, uridine 5'-triphosphate (UTP), and ATPγs significantly increased NHE activity. The pharmacological profile of agonists was consistent with that anticipated for receptors of the P2Y2 class. ATP did not increase AE2 activity, but, when given to cells pretreated with agents raising intracellular cAMP, had a synergistic stimulatory effect that was inhibited by amiloride. To assess the polarity of purinergic receptors, monolayers of NRC-1 cells were exposed to apical or basolateral nucleotides. Apical administration of purinergic agonists, but not adenosine, increased basolateral NHE activity (ATPγS > UTP > ATP). Basolateral administration of purinergic agonists induced a weaker activation of NHE, which was instead strongly stimulated by adenosine and by adenosine receptor agonists (NECA = R-PIA = S-PIA). In conclusion, this study demonstrates that, consistent with the proposed role for biliary ATP in paracrine and autocrine control of cholangiocyte ion secretion, extracellular ATP stimulates cholangiocyte basolateral NHE activity through P2Y2 receptors that are predominantly expressed at the apical cell membrane.
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