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

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; n_H, 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