P2 receptors in the murine gastrointestinal tract.

The actions of adenosine, adenosine 5′-triphosphate (ATP), 2-methylthio adenosine diphosphate ADP (2-MeSADP), 2-methylthio ATP (2-MeSATP), α,β-methylene ATP (α,β-meATP) and uridine triphosphate (UTP) on isolated segments of mouse stomach (fundus), duodenum, ileum and colon were investigated. The localization of P2Y1, P2Y2, P2Y4, P2X1 and P2X2 receptors and neuronal nitric oxide synthase (NOS) were examined immunohistochemically, and P2Y1 mRNA was examined with in situ hybridization. The order of potency for relaxation of longitudinal muscle of all regions was: 2-MeSADP≥2-MeSATP>α,β-meATP>ATP=UTP=adenosine. This is suggestive of P2Y1-mediated relaxation and perhaps a further P2Y receptor subtype sensitive to α,β-meATP. As ATP and UTP are equipotent, the presence of a P2Y2 receptor is indicated. ATP responses were inhibited by the P2Y1-selective antagonist MRS 2179, and suramin. P2Y1 receptors were visualized immunohistochemically in the smooth muscle of the ileum and in a subpopulation for myenteric neurones, which also stained for NOS. P2Y1 mRNA was localized in neurones in both myenteric and submucosal ganglia in the ileum. Taken together, these results suggest that ATP was acting on non-adrenergic, non-cholinergic inhibitory neurons, which release both nitric oxide (NO) and ATP. Reduced relaxations to 2-MeSADP by tetrodotoxin and Nω-nitro-l-arginine methyl ester, are consistent with this possibility. Adenosine acts via P1 receptors to relax smooth muscle of the mouse gut. Segments of mouse colon (in contrast to the stomach and small intestine) were contracted by nucleotides with the potency order: 2-MeSATP>α,βmeATP>ATP; the contractions showed no desensitization and were antagonized by suramin and PPADS, consistent with responses mediated by P2X2 receptors. Immunoreactivity to P2X2 receptors was demonstrated on both longitudinal and circular muscle of the colon, but not in the other regions of the gut, except for a small subpopulation of myenteric neurones. In summary, neuronal P2Y1 receptors appear to mediate relaxation, largely through NO in all regions of the mouse gut, and to a lesser extent by P2Y1, P2Y2 and a novel P2Y receptor subtype responsive to α,β-meATP in smooth muscle, while P2X2 receptors mediate contraction of colonic smooth muscle.