40, 41 Extracellular Cl− then stimulates Cl−/HCO exchange over the membrane.42 Extracellular ATP has also been shown to up-regulate the inflammatory

cytokine interleukin-6 in bile duct epithelium in a cAMP- and Ca++-dependent way through the activation of purinergic receptors43 and to recruit macrophages, possibly through induction of intercellular cell adhesion of molecules on the epithelium.44 P2Y receptors on the basolateral membrane are probably involved in recognition of extracellular ATP due to destruction of neighboring cells. The activation of receptors on the apical membrane leads to a net alkalinization of bile through Ca++- and/or cAMP-mediated stimulation of Cl− secretion.33 Forskolin-induced GSK458 ic50 HCO secretion was shown to be dependent on luminal ATP. In isolated bile

ductules, ATP hydrolyzing apyrase reduced forskolin- and thus cAMP-induced HCO secretion by ≈80%, and P2Y blockade with suramin abolished intracellular Ca++ increase and HCO secretion.33 Hepatocytes and cholangiocytes release ATP in a paracrine/autocrine GSK3235025 mouse fashion by yet unresolved molecular mechanisms.35, 45 Release through undefined ATP channels and CFTR-mediated ATP release35, 46 but also ATP release through exocytosis of ATP-enriched vesicles47 or even maxi-anion channels described in macula densa cells of the rabbit kidney, rat cardiomyocytes, and mouse astrocytes48 have been discussed. Whatever the molecular

mechanisms of hepatobiliary ATP release are, it is attractive to speculate that cholestatic injury per se may hamper biliary medchemexpress ATP release and thereby ductular HCO secretion as targeting of vesicles, vesicular exocytosis, and membrane insertion of transport proteins into their target membrane are impaired in cholestatic liver.49, 50 Thus, cholestatic injury might even be a primary culprit for a defective biliary HCO umbrella. In this regard, the recent finding of bile flow–induced mechanosensitive Ca++- and PKCζ-dependent ATP release and associated ATP-dependent Cl− secretion from human biliary cells as well as rat cholangiocytes deserves particular attention.51 It is attractive to link mechano-sensitive ATP and Cl− secretion to the stabilization of the biliary HCO umbrella through stimulation of Cl−/HCO exchange when higher amounts of bile salts as a major driving force of stimulated hepatocellular bile flow reach the cilia of cholangiocytes. Given their putative role in stabilization of the biliary HCO umbrella, but also their potential proinflammatory effects, ATP levels in bile have to be tightly controlled. Alkaline phosphatase catalyzes the dephosphorylation of ATP to ADP, AMP, and eventually adenosine in various tissues.