Multiple motor effects of ATP and their inhibition by P2 purinoceptor antagonist, pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid in the small intestine of the guinea-pig

Abstract

Adenosine 5′-triphosphate (ATP) may be an important neurotransmitter in the gastrointestinal tract. The present study examined the motor effects of exogenous ATP on longitudinally-oriented preparations of the guinea-pig isolated ileum and the influence of drugs on the ATP-induced responses. High micromolar concentrations of ATP caused two types of contraction, a phasic, cholinergic response and a tonic, tetrodotoxin-resistant contraction. The phasic contraction was reduced by hexamethonium (5×10-5 M), but left uninfluenced by capsaicin tachyphylaxis or tachyphylaxis to α,β-methylene ATP. The tonic response was resistant to atropine, hexamethonium, capsaicin, ω-conotoxin GVIA, or pretreatment with α,β-methylene ATP. Both types of ATP-induced contraction were diminished or abolished by the P2 purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid (PPADS, 3×10-6 and 3×10-5 M, respectively). In the precontracted, atropine-treated ileum ATP (10-6-10-4 M) caused guanethidine-resistant relaxation. This response was not influenced by tetrodotoxin, ω-conotoxin GVIA, or NG-nitro-L-arginine, but was abolished by apamin (10-7 M), and inhibited by PPADS (3×10-5 M) or reactive blue 2 (10-5 M), in a surmountable manner. A high degree of tachyphylaxis was observed with the relaxant effect of ATP (10-5-10-4 M). A high concentration (3×10-4 M) of PPADS failed to influence ileum contractions to exogenous acetylcholine or histamine. It is concluded that, in addition to its direct contractile action in the guinea-pig ileum, ATP can activate (partly preganglionic) cholinergic neurones, an effect whose mechanism is largely different from that of α,β-methylene ATP. ATP also causes relaxation by a direct, probably P2Y-receptor-mediated effect on the smooth muscle. All motor effects of ATP are inhibited by the antagonist PPADS. © Basic & Clinical Pharmacology & Toxicology 2006. All rights reserved.