Adenosine nucleotides in bile

Abstract

Activation of purinergic receptors by ATP stimulates Cl- efflux in biliary epithelial cells. To determine whether purinergic agonists are present under physiological conditions, we have assayed mammalian bile for nucleotides and assessed whether hepatoma and cholangiocarcinoma cell lines are capable of nucleotide release. Bile samples were collected from human, rat, and pig donors and assayed for nucleotide concentrations by luminometry. ATP, ADP, and AMP were present in bile from each species, and the average total nucleotide concentration in human bile was 5.21 +/- 0.91 microM (n = 16). In an in vitro model of HTC rat hepatoma cells or Mz-ChA-1 cholangiocarcinoma cells on a superfused column, nucleotides were present in the effluent from each cell type. Addition of alpha, beta-methyleneadenosine 5'-diphosphate (50 microM) to inhibit 5'-nucleotidase activity increased AMP concentrations two- to threefold. Exposure to forskolin (100 microM) or ionomycin (2 microM) stimulated nucleotide release from cholangiocarcinoma but not hepatoma cells. These studies indicate that adenosine nucleotides are present in bile in concentrations sufficient to activate purinergic receptors. Purinergic receptor activation by local nucleotide release might constitute an autocrine and/or paracrine mechanism for modulation of biliary secretion.

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

“…ATP is a partial agonist for P2Y 12 and may also act as a relatively low-affinity antagonist for this receptor (8). ATP is released in bile by both hepatocytes and cholangiocytes via unknown mechanisms, whereas biliary ADP is likely a product of degradation of ATP (11,32,33). We previously demonstrated that ATP, ATP-␥S, and ADP affect [Ca 2ϩ ] i signaling in rat cholangiocytes via P2Y 1 , P2Y 2 , and P2Y 4 purinergic receptors localized to the apical plasma membrane (14).…”

“…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.…”

“…In contrast, P2Y 11 is associated with both [Ca 2ϩ ] i and cAMP signaling (1,10,56). Among four P2Y receptors (i.e., P2Y [11][12][13][14] ) associated with the cAMP signaling pathway, P2Y 12 and P2Y 13 are most suitable for a chemosensory function of cholangiocyte cilia in rodents. This conclusion is based on three critical observations.…”

“…First, rat and mouse are P2Y 11 -deficient animals (1, 56); thus P2Y 11 could not be considered as a potential ciliary-associated purinergic receptor in rats. Second, P2Y 12 and P2Y 13 are activated by ADP and ATP, i.e., two nucleotides that are extracellular signaling molecules in bile (1,11,56); thus these two P2Y purinergic receptors could be potentially involved in a chemosensory function of cholangiocyte cilia. Third, P2Y 14 is activated by UDP-glucose but not by ATP or ADP (56).…”

The role of purinergic signaling in the liver and in transplantation: effects of extracellular nucleotides on hepatic graft vascular injury, rejection and metabolism

“…In biliary epithelium ATP is stored in vesicles and is released in response to cell swelling (23); concentrations of canalicular adenine nucleotides in bile samples are estimated to be 5±0.9µM (148,149). Extracellular nucleotides entering the bile ducts are potent stimuli for secretion of bile fluid including anions, and canalicular contraction, which is in part explained by interaction and communication between hepatocytes and bile duct cells via local ATP release (25,(149)(150)(151). These effects are mediated and coordinated by apical P2Y2 receptors and NTPDase8 (71,83,89).…”