Comparative studies on the affinities of ATP derivatives for P2X‐purinoceptors in rat urinary bladder

1 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. 2 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). 3 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 P2X-purinoceptors (from 3.5 to 98 fold increase in IC50 values). 4 Replacement of the 3′-hydroxyl group on the ribose with substituted amino or acylamino groups produced more potent P2X-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. 5 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. 6 Suramin, PPADS, and reactive blue 2 could competitively displace the binding of [3H]-α,β-MeATP to P2X-purinoceptors, with pIC50 values of 6.26, 5.35, and 6.22, respectively.