Zn2+ modulation of ATP‐responses at recombinant P2X2 receptors and its dependence on extracellular pH

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Abstract
1 Using recombinant P2X2 receptors expressed in Xenopus oocytes, the modulatory effects of zinc (Zn2+) on ATP‐responses were studied under voltage‐clamp conditions and at different levels of extracellular pH. 2 Zn2+ (0.3–300 μM) 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. Zn2+‐potentiation was reduced in an exponential manner (decaying 1/e in 42 s) as the interval was lengthened between adding Zn2+ then ATP to the superfusate. 3 The potentiating effect of Zn2+ was progressively diminished by acidic shifts in extracellular pH (pHe) which, of itself, also potentiated ATP‐responses at P2X2 receptors. The maximal potentiating effects of Zn2+ and H+ were not additive. 4 Neither Zn2+ nor H+ potentiation of ATP‐responses was abolished by diethylpyrocarbonate (DEPC, 0.3–3 mM), which irreversibly denatures histidyl residues. Nine histidyl residues are present in the extracellular loop of P2X2 receptors. 5 Zn2+ also enhanced the blocking activity of the P2 receptor antagonist suramin at P2X2 receptors. Therefore, Zn2+ also mimics H+ in increasing suramin‐activity at P2X2 receptors. 6 In summary, Zn2+ and H+ potentiate agonist and antagonist activity at P2X2 receptors but their effects are not wholly alike for receptor agonism. There, the potentiating effects of Zn2+ 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