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Diinosine pentaphosphate (IP 5 I) is a potent antagonist at recombinant rat P2X 1 receptors

1999; Wiley; Volume: 128; Issue: 5 Linguagem: Inglês

10.1038/sj.bjp.0702876

1476-5381

Brian F. King, M Liu, Jesús Pintor, Javier Gualix, M. Teresa Miras‐Portugal, Geoffrey Burnstock,

Restless Legs Syndrome Research

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 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., Ap 5 A into Ip 5 I) using 5′‐adenylic deaminase, and purified using reverse‐phase chromatography. Against ATP‐responses at rP2X 1 receptors, the potency order for antagonism was (pIC 50 ): Ip 5 I (8.5)>Ip 4 I (6.3)>Ip 3 I (>4.5). Ip 5 I (10–100 n M ) 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 Ip 5 I is not a competitive antagonist. Blockade by micromolar concentrations of Ip 5 I was not surmountable. Ip 4 I also behaved as a non‐surmountable antagonist. Against ATP‐responses at rP2X 3 receptors, the potency order for antagonism 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 micromolar concentrations. Diinosine polyphosphates failed to inhibit ATP‐responses at rP2X 2 receptors, whereas agonist responses at rP2X 4 were reversibly potentiated by Ip 4 I and Ip 5 I. None of the parent diadenosine polyphosphates behave as antagonists at rP2X 1–4 receptors. Thus, Ip 5 I acted as a potent and relatively‐selective antagonist at the rP2X 1 receptor. This dinucleotide pentaphosphate represents a high‐affinity antagonist for the P2X 1 receptor, at which it acts in a competitive manner at low (100 n M ) concentrations but has more complex actions at higher (>100 n M ) concentrations. British Journal of Pharmacology (1999) 128 , 981–988; doi: 10.1038/sj.bjp.0702876