Adenosine triphosphate release and purinergic regulation of cholangiocyte transport.

The discovery of purinergic receptors on almost every cell type studied to date suggests that purinergic signaling is a fundamental process regulating cell and organ level functions. Purinergic receptors have been found on all principal liver cell types, including liver parenchymal cells, or hepatocytes, and biliary epithelial cells, or cholangiocytes. Both hepatocytes and cholangiocytes are capable of the regulated release of adenosine triphosphate (ATP), and both cell types express a range of purinergic receptors to mediate cellular processes. The role of extracellular nucleotides in liver function is presently being elucidated. Extracellular ATP, in addition to autocrine regulation of liver cell volume, has recently been shown to play an important role in paracrine signaling to coordinate specific hepatocyte and cholangiocyte cellular responses. The findings that (1) cholangiocytes are capable of the regulated release of ATP into bile, (2) ATP is present in bile in concentrations capable of stimulating purinergic receptors, and (3) P2 receptor stimulation results in brisk Cl(-) channel activation and fluid secretion suggest an important role of extracellular ATP in the regulation of bile formation. This article highlights important developments in our understanding of the role of purinergic signaling in cholangiocyte transport and bile formation.