Diinosine pentaphosphate (IP5I) is a potent antagonist at recombinant rat P2X(1) receptors

1 The antagonist activity of a series of diinosine polyphosphates (Ip(n)I, where n = 3, 4, 5) was assessed against ATP-activated inward currents at ra t P2X(1-4) receptors expressed in Xenopus oocytes and studied under voltage -clamp conditions. 2 Diinosine polyphosphates were prepared by the enzymatic degradation of th eir corresponding diadenosine polyphosphates (e.g., Ap(5)A into Ip(5)I) usi ng 5'-adenylic deaminase, and purified using reverse-phase chromatography. 3 Against ATP-responses at rP2X(1) receptors, the potency order for antagon ism was (pIC(50)): Ip(5)I (8.5) > Ip(4)I (6.3) > Ip(3)I (>4.5). Ip(5)I (10- 100 nM) caused a concentration-dependent rightwards displacement of the ATP concentration-response curve without reducing the maximum ATP effect. Howe ver, the Schild plot was non-linear which indicated Ip(5)I is not a competi tive antagonist. Blockade by micromolar concentrations of Ip(5)I was not su rmountable. Ip(4)I also behaved as a non-surmountable antagonist. 4 Against ATP-responses at rP2X(3) receptors, the potency order for antagon ism was (pIC(50)): Ip(4)I (6.0) > Ip(5)I (5.6) > Ip(3)I (>4.5). Blockade by Ip(4)I (pA(2), 6.75) and Ip(5)I (pA(2), 6.27) was surmountable at micromol ar concentrations. 5 Diinosine polyphosphates failed to inhibit ATP-responses at rP2X(2) recep tors, whereas agonist responses at rP2X(4) were reversibly potentiated by I p(4)I and Ip(5)I. None of the parent diadenosine polyphosphates behave as a ntagonists at rP2X(1-4) receptors. 6 Thus, Ip(5)I acted as a potent and relatively-selective antagonist at the rP2X(1) receptor. This dinucleotide pentaphosphate represents a high-affin ity antagonist for the P2X(1) receptor, at which it acts in a competitive m anner at low (less than or equal to 100 nM) concentrations but has more com plex actions at higher (>100 nM) concentrations.