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

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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 P2Y-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 P2Y receptors, and confirms our finding that the stable ATP analogue, AMPPCP, has an unexpected direct action on these A1 receptors.