Evidence for two distinct P₂-purinoceptors subserving contraction of the rat anococcygeus smooth muscle

1. The effects of the P₂-purinoceptor agonists, adenosine 5'-triphosphate (ATP), α,β-methylene ATP (α,β-MeATP), β,γ-methylene ATP (β,γ-MeATP), L-β,γ-methylene ATP (L-β,γ-MeATP), adenosine-5'-O-(2-thiodiphosphate) (ADPβS), and 2-methylthio ATP (2-MeSATP) were investigated on the isometric tension of the rat anococcygeus muscle. 2. Non-cumulative additions of ATP (100-1500 μM), α,β-MeATP (1-300 μM), β,γ-MeATP (10-300 μM), L-β,γ-MeATP (3-100 μM) and ADPβS (1-100 μM) produced concentration-dependent contractions, whereas 2-MeSATP (1-100 μM) had no effect. The rank order of potency was α,β-MeATP > L-β,γ-MeATP ≥ ADPβS > β,γ-MeATP >> ATP > 2-MeSATP. 3. Contractions to cumulative additions of ATP, α,β-MeATP, β,γ-MeATP and L-β,γ-MeATP were subject to desensitization whilst those to ADPβS were unaffected. 4. Contractions to ATP, α,β-MeATP, β,γ-MeATP and ADPβS were abolished by the non-selective P(2X)/P(2Y)-purinoceptor antagonist, suramin (100 μM). In contrast, contractions to ATP, α,β-MeATP and β,γ-MeATP were not affected by the non-selective P₁-purinoceptor antagonist 8-(p-sulphophenyl)-theophylline (8SPT, 30 μM). Blockade of P(2X)-purinoceptors with the selective P(2X)-purinoceptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 μM) or desensitization with L-β,γ-MeATP (10 μM) abolished contractions to α,β-MeATP, but enhanced those to ADPβS. The P(2Y)-purinoceptor antagonist, reactive blue 2 (RB2, 100 μM) enhanced contractions to ATP and α,β-MeATP but abolished those to ADPβS. 5. Simultaneous addition of α,β-MeATP and ADPβS produced an additive contraction. 6. The findings suggest that in the rat anococcygeus, smooth muscle cells are endowed with two distinct P₂-purinoceptors which subserve contractions: a P(2X)-purinoceptor activated by ATP and its analogues, and another type of P₂-purinoceptor activated by ADPβS.