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

1 Using recombinant P2X(2) 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 mu 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. 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 (pH(c)) which, of itself, also potentiated ATP-responses at P2X(2) 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 P2X(2) receptors. 5 Zn2+ also enhanced the blocking activity of the P2 receptor antagonist suramin at P2X(2) receptors. Therefore, 2n(2+) also mimics H+ in increasing suramin-activity at P2X(2), receptors. 6 In summary, Zn2+ 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 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.