Endogenous ATP release regulates Cl⁻secretion in cultured human and rat biliary epithelial cells

Abstract

P2Y receptor stimulation increases membrane Cl− permeability in biliary epithelial cells, but the source of extracellular nucleotides and physiological relevance of purinergic signaling to biliary secretion are unknown. Our objectives were to determine whether biliary cells release ATP under physiological conditions and whether extracellular ATP contributes to cell volume regulation and transepithelial secretion. With the use of a sensitive bioluminescence assay, constitutive ATP release was detected from human Mz-ChA-1 cholangiocarcinoma cells and polarized normal rat cholangiocyte monolayers. ATP release increased rapidly during cell swelling induced by hypotonic exposure. In Mz-ChA-1 cells, removal of extracellular ATP (apyrase) and P2 receptor blockade (suramin) reversibly inhibited whole cell Cl− current activation and prevented cell volume recovery during hypotonic stress. Moreover, exposure to apyrase induced cell swelling under isotonic conditions. In intact normal rat cholangiocyte monolayers, hypotonic perfusion activated apical Cl−currents, which were inhibited by addition of apyrase and suramin to bathing media. These findings indicate that modulation of ATP release by the cellular hydration state represents a potential signal coordinating cell volume with membrane Cl− permeability and transepithelial Cl−secretion.

Expression of mediators of purinergic signaling in human liver cell lines

“…Specifically, purinergic signals contribute to the maintenance of normal liver physiology via regulation of secretory [40,41] and metabolic [15] functions, as well as disease pathogenesis, via modulation of activation mechanisms in multiple hepatic cell types [21,42,43] and of wound healing and inflammatory responses [10,32,44]. Furthermore, the ecto-enzymes that control balance of extracellular nucleotides and nucleosides in the liver are equally important in pathological processes such …”

Cyclic AMP Regulates Bicarbonate Secretion in Cholangiocytes Through Release of ATP Into Bile

“…However, the endogenous pathways that lead to the appearance of ATP in bile under normal conditions have been less clear. Pathological conditions such as cell swelling result in release of ATP from both hepatocytes (28) and cholangiocytes (29). In addition, the therapeutic bile acids ursodeoxycholate and tauroursodeoxycholate induce hepatocytes to secrete ATP into bile (16).…”

Cholangiocyte primary cilia are chemosensory organelles that detect biliary nucleotides via P2Y12 purinergic receptors

“…Taking into account that bile contains ATP and ADP, which are considered extracellular signaling molecules affecting cholangiocyte functions via apically located P2Y purinergic receptors (11,14,15,32,33,43,45), we further hypothesized that some P2Y receptors are expressed in cholangiocyte cilia, providing a chemosensory function.…”