Zn 2+ modulation of ATP‐responses at recombinant P2X 2 receptors and its dependence on extracellular pH

1 Using recombinant P2X 2 receptors expressed in Xenopus oocytes, the modulatory effects of zinc (Zn 2+ ) on ATP‐responses were studied under voltage‐clamp conditions and at different levels of extracellular pH. 2 Zn 2+ (0.3–300 μ M ) added to the bathing medium potentiated ATP‐activated membrane currents, increasing ATP‐responses by up to 20 fold. This potentiating effect was reversed on washout. Zn 2+ ‐potentiation was reduced in an exponential manner (decaying 1/e in 42 s) as the interval was lengthened between adding Zn 2+ then ATP to the superfusate. 3 The potentiating effect of Zn 2+ was progressively diminished by acidic shifts in extracellular pH (pH e ) which, of itself, also potentiated ATP‐responses at P2X 2 receptors. The maximal potentiating effects of Zn 2+ and H + were not additive. 4 Neither Zn 2+ nor H + potentiation of ATP‐responses was abolished by diethylpyrocarbonate (DEPC, 0.3–3 m M ), which irreversibly denatures histidyl residues. Nine histidyl residues are present in the extracellular loop of P2X 2 receptors. 5 Zn 2+ also enhanced the blocking activity of the P2 receptor antagonist suramin at P2X 2 receptors. Therefore, Zn 2+ also mimics H + in increasing suramin‐activity at P2X 2 receptors. 6 In summary, Zn 2+ and H + potentiate agonist and antagonist activity at P2X 2 receptors but their effects are not wholly alike for receptor agonism. There, the potentiating effects of Zn 2+ are time‐dependent and gradually convert to inhibition while those of H + are time‐independent, persistent and more potent, suggesting that either these modulators interact in a different way with a single allosteric site or with different allosteric sites.British Journal of Pharmacology (1998) 123 , 1214–1220; doi: 10.1038/sj.bjp.0701717