Agonist and antagonist characterization of the P₂‐purinoceptors in the guinea pig ileum

When adenine nucleotides were administered to isolated guinea pig ileum longitudinal muscle, two immediate effects were observed: a contractile effect and a concomitant inhibition of the responses elicited by transmural nerve stimulation. At concentrations up to 10^‐4m the order of potency for the contractile effect was α,β‐MeADP =α,β ‐MeATP > ADP = ATP = AMPPNP =β,t‐MeATP > 2′‐deoxy AMP = 2′‐deoxy ADP. AMP and adenosine did not show any contractile effect, whereas both compounds dose‐dependently and reversibly inhibited the nerve‐induced contractile responses. ADP, ATP, β,t‐MeATP and AMPPNP also inhibited contractile responses to transmural nerve stimulation, whereas 2′‐deoxy AMP, 2′‐deoxy ADP, α,β ‐MeADP and α,β ‐MeATP showed but weak inhibitory effects, 2′‐deoxyadenosine, IMP, IDP, ITP, 8‐BrATP and TDP lacked significant contractile effects and did not exert a significant inhibition on nerve‐induced contractions. p‐chloromercuribenzene sulphonic acid (PCMBS) irreversibly antagonized the contractile effects of ADP, ATP and the α,β ‐methylene derivatives, whereas dantrolene sodium, tetrodotoxin, scopolamine and 8‐p‐sulphophenyltheophylline were without effect on nucleotide‐induced contractions. The contractile effect of ADP or ATP was unaffected by indomethacin, whereas the contractile effect by α,β ‐methylene derivatives was abolished by indomethacin. ADP, ATP and α,β ‐MeADP enhanced contractile responses to exogenous acetylcholine, α,β‐MeADP being most effective. This enhancement was blocked by indomethacin. We suggest that ADP and ATP contracted the guinea pig ileum by an action at postjunctional P₂‐purinoceptors with different characteristics from prejunctional P₁‐purinoceptors. The α,β‐methylene analogues seemed to act at least at one different excitatory site and promotion of prostaglandin biosynthesis was critical for their effect.