Comparative studies on the affinities of ATP derivatives for P_2X‐purinoceptors in rat urinary bladder

Radioligand binding assays have been used to determine the affinities of a series of ATP derivatives with modifications of the polyphosphate chain, adenine and ribose moieties of the ATP molecule for [³H]‐α,β‐methylene ATP ([³H]‐α,β‐MeATP) binding sites in rat urinary bladder. The replacement of the bridging oxygen in the triphosphate chain of ATP (pIC₅₀ = 5.58) with a methylene or imido group markedly increased the affinity (691 fold in IC₅₀ values for β,γ‐imidoATP, 15 fold for β,γ‐methylene ATP), and the replacement of an ionized oxygen on the γ‐phosphate with a sulphur (ATPγS) also led to increased affinity (5623 fold in IC₅₀ values). Modifications at N⁶, N¹, and C‐8 positions on the purine base usually reduced the affinity of ATP (a decrease of 2.8 fold in IC₅₀ values for N⁶‐methylATP and 8.9 fold for 8‐bromo ATP), while the attachment of an alkylthio group to the C‐2 position greatly increased the affinity for P_2X‐purinoceptors (from 3.5 to 98 fold increase in IC₅₀ values). Replacement of the 3′‐hydroxyl group on the ribose with substituted amino or acylamino groups produced more potent P_2X‐purinoceptor agonists (an increase of 447 fold in IC₅₀ values for 3′‐deoxy‐3′‐benzylamino ATP and 28 fold for 3′‐deoxy‐3′‐(4‐hydroxyphenylpropionyl)amino ATP. Diadenosine polyphosphates (Ap[n]A) were also shown to displace the [³H]‐α,β‐MeATP binding. The rank order of potency was Ap6A > Ap5A > Ap4A >> Ap3A >> Ap2A. Suramin, PPADS, and reactive blue 2 could competitively displace the binding of [³H]‐α,β‐MeATP to P_2X‐purinoceptors, with pIC₅₀ values of 6.26, 5.35, and 6.22, respectively. 1994 British Pharmacological Society