Diinosine pentaphosphate (IP5I) is a potent antagonist at recombinant rat P2X1 receptors

Abstract

The antagonist activity of a series of diinosine polyphosphates (Ip_nI, where n=3, 4, 5) was assessed against ATP‐activated inward currents at rat P2X_1–4 receptors expressed in Xenopus oocytes and studied under voltage‐clamp conditions. Diinosine polyphosphates were prepared by the enzymatic degradation of their corresponding diadenosine polyphosphates (e.g., Ap₅A into Ip₅I) using 5′‐adenylic deaminase, and purified using reverse‐phase chromatography. Against ATP‐responses at rP2X₁ receptors, the potency order for antagonism was (pIC₅₀): Ip₅I (8.5)>Ip₄I (6.3)>Ip₃I (>4.5). Ip₅I (10–100 nM) caused a concentration‐dependent rightwards displacement of the ATP concentration‐response curve without reducing the maximum ATP effect. However, the Schild plot was non‐linear which indicated Ip₅I is not a competitive antagonist. Blockade by micromolar concentrations of Ip₅I was not surmountable. Ip₄I also behaved as a non‐surmountable antagonist. Against ATP‐responses at rP2X₃ receptors, the potency order for antagonism was (pIC₅₀): Ip₄I (6.0)>Ip₅I (5.6)>Ip₃I (>4.5). Blockade by Ip₄I (pA₂, 6.75) and Ip₅I (pA₂, 6.27) was surmountable at micromolar concentrations. Diinosine polyphosphates failed to inhibit ATP‐responses at rP2X₂ receptors, whereas agonist responses at rP2X₄ were reversibly potentiated by Ip₄I and Ip₅I. None of the parent diadenosine polyphosphates behave as antagonists at rP2X_1–4 receptors. Thus, Ip₅I acted as a potent and relatively‐selective antagonist at the rP2X₁ receptor. This dinucleotide pentaphosphate represents a high‐affinity antagonist for the P2X₁ receptor, at which it acts in a competitive manner at low (100 nM) concentrations but has more complex actions at higher (>100 nM) concentrations. British Journal of Pharmacology (1999) 128, 981–988; doi:10.1038/sj.bjp.0702876