Inhibitory purinergic P2 receptor characterisation in rat distal colon

The aim of this study was to characterise the P2 receptors involved in purinergic relaxant responses in rat distal colon circular muscle. Concentration–response curves for purinergic agonists were constructed on methacholine-precontracted circular muscle strips of rat distal colon in the absence and presence of the nerve blocker TTX and the ecto-nucleotidase inhibitor ARL67156. The effects of the P2 receptor antagonists RB2, PPADS, suramin, MRS2179 and NF279, the NO-synthase inhibitor L-NAME and the small conductance K+ channel blocker apamin were investigated. The localisation of the different P2 receptors was examined immunocytochemically. Immunocytochemistry demonstrated the expression of P2Y1, P2Y6 and P2X1 receptors on smooth muscle cells and P2Y2, P2Y12, P2X2 and P2X3 receptors in the myenteric plexus; almost a quarter of the P2Y2-immunopositive neurons co-expressed nNOS. The P2X-selective agonist αβmeATP and the P2Y-selective agonist ADPβS were the most potent relaxants; their effects were abolished by apamin. The effect of ADPβS was antagonised by the P2Y1-selective antagonist MRS2179 pointing to interaction with the muscular P2Y1-receptors. The relaxant effect of αβmeATP was partially reduced by TTX and concentration-dependently antagonised by PPADS, suramin, RB2 and the P2X1-selective antagonist NF279; this correlates with an interaction with neuronal P2X3 and muscular P2X1 receptors. UTP was the least potent agonist; its effect was markedly increased by ARL67156, nearly abolished by TTX and reduced by L-NAME. This points to interaction with the neuronal P2Y2-receptors inducing relaxation, at least partially, by NO release.