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‐purinoceptor involved in ATP release and contraction evoked by α,β‐methylene ATP (α,β‐mATP) and other P2‐agonists in guinea‐pig ileum. 2 α,β‐mATP 100 μM produced a transient and steep contraction followed by ATP release from tissue segments. These maximum responses appeared with different time‐courses and their ED50 values were 5 and 25 μM, respectively. The maximum release of ATP by α,β‐mATP was markedly reduced by 250 μM suramin, 30 μM pyridoxal‐phosphate‐6‐azophenyl‐2′,5′‐disulphonic acid (PPADS) and 30 μM 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 α,β‐mATP was strongly diminished by Ca2+‐removal and nifedipine, and also by tetrodotoxin and atropine at 0.3 μM, the release of ATP was virtually unaffected by these procedures. 4 UTP, β,γ‐methylene ATP (β,γ‐mATP) and ADP at 100 μM elicited a moderate release of ATP. The release caused by UTP was virtually unaffected 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 α,β‐mATP>UTP>β,γ‐mATP>ADP. 6 In superfusion experiments with cultured smooth muscle cells from the ileum, α,β‐mATP (100 μM) enhanced the release of ATP 5 fold above the basal value. This evoked release was inhibited by RB‐2. 7 These findings suggest that ATP release and contraction induced by P2‐agonists such as α,β‐mATP in the guinea‐pig ileum result mainly from stimulation of different 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. British Journal of Pharmacology (1997) 121, 1744–1748; doi:10.1038/sj.bjp.0701329