The interaction of diadenosine polyphosphates with P 2X ‐receptors in the guinea‐pig isolated vas deferens

The site(s) at which diadenosine 5′,5′′′‐P 1 , P 4 ‐tetraphosphate (AP 4 A) and diadenosine 5′, 5′′′‐P 1 , P 5 ‐pentaphosphate (AP 5 A) act to evoke contraction of the guinea‐pig isolated vas deferens was studied by use of a series of P 2 ‐receptor antagonists and the ecto‐ATPase inhibitor 6‐N, N‐diethyl‐ D ‐β,γ‐dibromomethyleneATP (ARL 67156). Pyridoxalphosphate‐6‐azophenyl‐2′,4′‐disulphonic acid (PPADS) (300 n M –30 μ M ), suramin (3–100 μ M ) and pyridoxal‐5′‐phosphate (P‐5‐P) (3–1000 μ M ) inhibited contractions evoked by equi‐effective concentrations of AP 5 A (3 μ M ), AP 4 A (30 μ M ) and α,β‐methyleneATP (α,β‐meATP) (1 μ M ), in a concentration‐dependent manner and abolished them at the highest concentrations used. PPADS was more potent than suramin, which in turn was more potent than P‐5‐P. PPADS inhibited AP 5 A, AP 4 A and α,β‐meATP with similar IC 50 values. No significant difference was found between IC 50 values for suramin against α,β‐meATP and AP 5 A or α,β‐meATP and AP 4 A, but suramin was more than 2.5 times more potent against AP 4 A than AP 5 A. P‐5‐P showed the same pattern of antagonism. Desensitization of the P 2X1 ‐receptor by α,β‐meATP abolished contractions evoked by AP 5 A (3 μ M ) and AP 4 A (30 μ M ), but had no effect on those elicited by noradrenaline (100 μ M ). ARL 67156 (100 μ M ) reversibly potentiated contractions evoked by AP 4 A (30 μ M ) by 61%, but caused a small, significant decrease in the mean response to AP 5 A (3 μ M ). It is concluded that AP 4 A and AP 5 A act at the P 2X1 ‐receptor, or a site similar to the P 2X1 ‐receptor, to evoke contraction of the guinea‐pig isolated vas deferens. Furthermore, the potency of AP 4 A, but not AP 5 A, appears to be inhibited by an ecto‐enzyme which is sensitive to ARL 67156. British Journal of Pharmacology (1997) 121 , 57–62; doi: 10.1038/sj.bjp.0701099