Effects of purines on the longitudinal muscle of the rat colon

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Abstract
Adenosine and adenosine 5′‐triphosphate (ATP) have been reported to cause relaxation of the rat colon longitudinal muscle preparation; the purinoceptors mediating this effect were investigated by use of a series of agonists and antagonists. The tissue was precontracted with carbachol (1 μm), and the purines induced reversible relaxations with a potency order of 5′‐N‐ethylcarboxamidoadenosine (NECA) >N6‐cyclopentyladenosine (CPA) = adenosine 5′‐(α,β‐methylene) triphosphonate (AMPCPP) >adenosine = adenylyl 5′‐(β,γ‐methylene) disphosphonate (AMPPCP) = ATP. The P1‐selective antagonist 1,3‐dipropyl‐8‐cyclopentylxanthine (DPCPX) (3 μm) shifted to the right the log concentration‐response curves of all these agonists except for AMPCPP, indicating that they all act via P1‐purinoceptors. The order of potency of the adenosine analogues and the relatively high concentrations of the antagonist required indicated that these receptors are of the A2 subtype. The P2‐selective antagonist suramin (300 μm) inhibited responses to AMPCPP, but not to the other agonists. The dephosphorylation of the nucleotides was studied by high performance liquid chromatography following incubation with the longitudinal muscle preparation for up to 30 min. ATP was rapidly degraded, largely to adenosine, and AMPPCP and AMPCPP were also degraded, although more slowly, to adenosine and adenosine 5′‐(α,β‐methylene) diphosphonate (AMPCP) respectively. AMPCP, like AMPCPP, caused relaxations by acting on P2‐purinoceptors, as it was also inhibited by suramin (300 μm). Incubation of the tissue with adenosine deaminase abolished responses to adenosine, reduced those to ATP and AMPPCP, but had no effect on those to AMPCPP. ATP and AMPPCP therefore appear to be acting on the A2 receptors in this tissue largely via their degradation product adenosine. The longitudinal muscle of the rat colon therefore contains both P1‐ and P2‐purinoceptors, which both mediate relaxation. The P1‐purinoceptors are of the A2 subtype and the P2‐purinoceptors are probably of the P2Y subtype, although the rapid degradation of the nucleotides means that it is difficult to classify them with certainty.