Diinosine pentaphosphate (IP5I) is a potent antagonist at recombinant rat P2X1 receptors.
1. The antagonist activity of a series of diinosine polyphosphates (IpnI, 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. 2. 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. 3. 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. 4. 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. 5. 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 rP2X1 - 4 receptors. 6. 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.
King
B F
BF
Autonomic Neuroscience Institute, Royal Free & University College Medical School, Royal Free Campus, Rowland Hill Street, Hampstead, London NW3 2PF, UK. b.king@ucl.ac.uk
Liu
M
M
Pintor
J
J
Gualix
J
J
Miras-Portugal
M T
MT
Burnstock
G
G
D058892:Adenosine Deaminase Inhibitors; D000818:Animals; D001230:Aspergillus / Q000201:enzymology; D002851:Chromatography, High Pressure Liquid; D015226:Dinucleoside Phosphates / Q000494:pharmacology*; D004558:Electric Stimulation; D004594:Electrophysiology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009865:Oocytes / Q000187:drug effects; D058919:Purinergic P2 Receptor Antagonists; D051381:Rats; D058469:Receptors, Purinergic P2X; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors; D014982:Xenopus laevis
D058892:Adenosine Deaminase Inhibitors; D015226:Dinucleoside Phosphates; D058919:Purinergic P2 Receptor Antagonists; D058469:Receptors, Purinergic P2X; D011994:Recombinant Proteins; C109479:diinosine pentaphosphate