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

P2Y receptor stimulation increases membrane Cl- permeability in biliary epi thelial cells, but the source of extracellular nucleotides and physiologica l relevance of purinergic signaling to biliary secretion are unknown. Our o bjectives were to determine whether biliary cells release ATP under physiol ogical conditions and whether extracellular ATP contributes to cell volume regulation and transepithelial secretion. With the use of a sensitive biolu minescence 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 e xposure. in Mz-ChA-1 cells, removal of extracellular ATP (apyrase) and P2 r eceptor blockade (suramin) reversibly inhibited whole cell Cl- current acti vation and prevented cell volume recovery during hypotonic stress. Moreover , exposure to apyrase induced cell swelling under isotonic conditions. In i ntact normal rat cholangiocyte monolayers, hypotonic perfusion activated ap ical 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 ce ll volume with membrane Cl- permeability and transepithelial Cl- secretion.