Direct effects of adenylyl 5′‐(β,γ‐methylene)diphosphonate, a stable ATP analogue, on relaxant P 1 ‐purinoceptors in smooth muscle

1 Previous results obtained with the rat colon muscularis mucosae, which contracts in response to adenosine and adenosine 5′‐triphosphate (ATP), had suggested that adenylyl 5′‐(β,γ‐methylene)diphosphonate (AMPPCP), a stable ATP analogue, acted on P 1 ‐purinoceptors rather than, as expected, on P 2 ‐purinoceptors. This possibility has been examined in two tissues in which adenosine and ATP both cause relaxation, the guinea‐pig taenia caeci and the rat duodenum. 2 ATP, 2‐methylthio‐ATP (2‐MeSATP), AMPPCP, adenosine 5′‐(α,β‐methylene)triphosphonate (AMPCPP) and adenosine each relaxed the taenia caeci and the duodenum, and the order of potency of the nucleotides in each tissue was 2‐MeSATP > ATP > AMPCPP > AMPPCP, indicating that these effects were mediated by P 2Y ‐purinoceptors. 3 The P 1 antagonist 8‐( p ‐sulphophenyl)theophylline (8‐SPT) (100 μ m ) did not affect the responses to ATP, 2‐MeSATP or AMPCPP in either tissue, but inhibited the responses of adenosine and of AMPPCP in both tissues. In the duodenum a lower concentration of 8‐SPT caused a parallel shift to the right of the concentration‐response curve to adenosine and to AMPPCP but to different extents, with AMPPCP being inhibited more powerfully than adenosine. A dose‐ratio of around 5 was observed for adenosine and AMPPCP at concentrations of 8‐SPT of 20 μ m and 2 μ m respectively, but Schild analysis resulted in plots with slopes greater than unity. In the taenia caeci, however, 8‐SPT inhibited adenosine more powerfully than AMPPCP, and a range of concentrations (10–20 μ m ) only caused a two fold shift in the concentration‐response curve for AMPPCP, although the concentration‐response curve to adenosine was shifted in a concentration‐dependent manner and Schild analysis gave a pA 2 value of 5.13 with a slope of 0.90. 4 As has been shown in other tissues, including the guinea‐pig taenia caeci, ATP (100 μ m ) was rapidly dephosphorylated by enzymes present in the rat duodenum, with less than 10% remaining after 20 min incubation, whereas AMPPCP (100 μ m ) was resistant to degradation, with greater than 90% remaining at the same time point. 5 AMPPCP therefore has pronounced but variable agonist actions on P 1 ‐purinoceptors, and appears to act entirely via these receptors on the rat duodenum although in the guinea‐pig taenia caeci this action is less important and it acts largely via P 2Y ‐purinoceptors. These P 1 ‐purinoceptor effects of AMPPCP are direct and are not due to its degradation to adenosine.