Characterization of the P1‐purinoceptors mediating contraction of the rat colon muscularis mucosae

Abstract

1 Previous studies had shown that adenosine and adenine nucleotides including adenosine 5′-triphosphate (ATP) caused contraction of the rat colon muscularis mucosae via P₁ and P_2Y-purinoceptors respectively, and that the stable ATP analogue adenylyl 5′-(β, γ-methylene)diphosphonate (AMPPCP) had an unexpected direct action on the P₁-purinoceptors. The P₁-purinoceptors have now therefore been further characterized by use of the adenosine analogues 5′-N-ethylcarboxamidoadenosine (NECA) and N⁶-cyclopropyladenosine (CPA) and the antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), which is selective for the A₁ subtype. The P₂-purinoceptor antagonist suramin was also used, to investigate the selectivity of the P₂ agonists. 2 The order of potency of P₁ agonists for contraction was CPA > NECA > AMPPCP ≥ adenosine, and DPCPX (1 nm) caused greater than two fold shifts to the right of the log concentration-response curves for each of these agonists, although the shifts were not always parallel and Schild analysis of the inhibition of the effect of adenosine resulted in a plot with a slope greater than unity. These results indicate that the P₁-purinoceptor mediating contraction is of the A₁ subtype, as has been found in other tissues in which adenosine causes contraction. 3 The P₂-purinoceptor antagonist suramin (300 μm) had no effect on the responses to adenosine or to AMPPCP, but abolished contractions induced by the related stable ATP analogue adenosine 5′-(α,β-methylene)triphosphonate (AMPCPP). Contractions induced by ATP, which were not affected by DPCPX (10 nm) alone, were only partially inhibited by suramin (300 μm), revealing an A₁ component to its action which could be blocked by DPCPX (10 nm). 4 In conclusion, these results show that the rat colon muscularis mucosae possesses contractile A₁ receptors in addition to the previously characterized P_2Y receptors, and confirms our finding that the stable ATP analogue, AMPPCP, has an unexpected direct action on these A₁ receptors.