Modulation of ATP‐responses at recombinant rP2X4 receptors by extracellular pH and zinc

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Abstract
The modulatory effects of extracellular H+ and Zn2+ were tested against ATP‐responses at rat P2X4 (rP2X4) receptors expressed in Xenopus oocytes under voltage‐clamp conditions. ATP (0.1–100 μM, at pH 7.5), evoked inward currents via rP2X4 receptors (EC50 value, 4.1±0.98 μM; nH, 1.2±0.1). ATP potency was reduced 2 fold, at pH 6.5, without altering maximal activity. ATP potency was reduced by a further 4 fold, at pH 5.5, and the maximal activity of ATP was also reduced. Alkaline conditions (pH 8.0) had no effect on ATP‐responses. Zn2+ (100 nM–10 μM) potentiated ATP‐responses at the rP2X4 receptor by 2 fold, whereas higher concentrations (30 μM–1 mM) inhibited ATP‐responses. Zn2+ potentiation was due to an increase in ATP potency, whereas its inhibitory action was due to a reduction in ATP efficacy. Zn2+ modulation of ATP‐responses was pH‐dependent. At pH 6.5, the bell‐shaped curve for Zn2+ was shifted to the right by 1 log unit. At pH 5.5, Zn2+ potentiation was abolished and its inhibitory effect reduced considerably. Suramin (50 μM) also potentiated ATP‐responses at rP2X4 receptors. Neither H+ (pH 6.5 and 5.5), Zn2+ (10–100 μM) or a combination of both failed to reveal an inhibitory action of suramin at rP2X4 receptors. In conclusion, H+ and Zn2+ exerted opposite effects on the rP2X4 receptor by lowering and raising agonist potency, respectively. H+ (3 μM) and Zn2+ (30 μM) also reduces agonist efficacy by lowering the number of rP2X4 receptors available for activation. The striking differences between the modulatory actions of H+ and Zn2+ at rP2X4 and rP2X2 receptors are discussed. British Journal of Pharmacology (1999) 126, 762–768; doi:10.1038/sj.bjp.0702325