1 The present study was addressed to clarify the subtypes of P2-purinoceptor involved in ATP release and contraction evoked by a,b-methylene ATP (a,b-mATP) and other P2-agonists in guinea-pig ileum. 2 a,b-mATP 100 mM produced a transient and steep contraction followed by ATP release from tissue segments. These maximum responses appeared with di erent time-courses and their ED 50 values were 5 and 25 mM, respectively. The maximum release of ATP by a,b-mATP was markedly reduced by 250 mM suramin, 30 mM pyridoxal-phosphate-6-azophenyl-2',5'-disulphonic acid (PPADS) and 30 mM reactive blue 2 (RB-2), P2-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 a,b-mATP was strongly diminished by Ca 2+ -removal and nifedipine, and also by tetrodotoxin and atropine at 0.3 mM, the release of ATP was virtually una ected by these procedures. 4 UTP, b,g-methylene ATP (b,g-mATP) and ADP at 100 mM elicited a moderate release of ATP. The release caused by UTP was virtually una ected by RB-2. However, these P2-agonists failed to elicit a contraction of the segment. 5 The potency order of all the agonists tested for the release of ATP was a,b-mATP4UTP4b,gmATP4ADP.6 In superfusion experiments with cultured smooth muscle cells from the ileum, a,b-mATP (100 mM) enhanced the release of ATP 5 fold above the basal value. This evoked release was inhibited by RB-2. 7 These ®ndings suggest that ATP release and contraction induced by P2-agonists such as a,b-mATP in the guinea-pig ileum result mainly from stimulation of di erent P2-purinoceptors, P2Y-like purinoceptors on the smooth muscles and, probably, P2X-purinoceptors 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.
