ATP RELEASE AND CONTRACTION MEDIATED BY DIFFERENT P2-RECEPTOR SUBTYPES IN GUINEA-PIG ILEAL SMOOTH-MUSCLE

1 The present study was addressed to clarify the subtypes of P2-purino ceptor involved in ATP release and contraction evoked by alpha,beta-me thylene ATP (alpha,beta-mATP) and other P2-agonists in guinea-pig ileu m. 2 alpha,beta-mATP 100 mu M produced a transient and steep contracti on followed by ATP release from tissue segments. These maximum respons es appeared with different time-courses and their ED50 values were 5 a nd 25 mu M, respectively. The maximum release of ATP by alpha,beta-mAT P was markedly reduced by 250 mu M suramin, 30 mu M pyridoxal-phosphat e-6-azophenyl-2',5'-disulphonic acid (PPADS) and 30 mu M reactive blue 2 (RB-2), PZ-receptor antagonists. However, the contractile response was inhibited by suramin, tetrodotoxin and atropine, but not by PPADS and RB-2. 3 Although the contraction caused by alpha,beta-mATP was str ongly diminished by Ca2+-removal and nifedipine, and also by tetrodoto xin and atropine at 0.3 mu M, the release of ATP was virtually unaffec ted by these procedures. 4 UTP, beta,gamma-methylene ATP (beta,gamma-m ATP) and ADP at 100 mu M elicited a moderate release of ATP. The relea se caused by UTP was virtually unaffected by RB-2. However, these PZ-a gonists failed to elicit a contraction of the segment. 5 The potency o rder of all the agonists tested for the release of ATP was alpha,beta- mATP>UTP>beta,gamma-mATP>ADP. 6 In superfusion experiments with cultur ed smooth muscle cells from the ileum, alpha,beta-mATP (100 mu M) enha nced the release of ATP 5 fold above the basal value. This evoked rele ase was inhibited by RB-2. 7 These findings suggest that ATP release a nd contraction induced by P2-agonists such as alpha,beta-mATP in guine a-pig ileum result mainly from stimulation of different P2-purinocepto rs, P2Y-like purinoceptors on the smooth muscles and, probably, P2X-pu rinoceptors on cholinergic nerve terminals, respectively. However, the ATP release may also be mediated, in part, by P2U-receptors, because UTP caused RB-2-insensitive ATP release.