Modulation of ATP-responses at recombinant rP2X4 receptors by extracellular pH and zinc.
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 microM, at pH 7.5), evoked inward currents via rP2X4 receptors (EC50 value, 4.1+/-0.98 microM; 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 microM) potentiated ATP-responses at the rP2X4 receptor by 2 fold, whereas higher concentrations (30 microM - 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 microM) also potentiated ATP-responses at rP2X4 receptors. Neither H+ (pH 6.5 and 5.5), Zn2+ (10-100 microM) 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+ (> or = 3 microM) and Zn2+ (> or = 30 microM) 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.
Wildman
S S
SS
Autonomic Neuroscience Institute, Royal Free Hospital School of Medicine, University College London, England.
King
B F
BF
Burnstock
G
G
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D005260:Female; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials / Q000187:drug effects; D009865:Oocytes; D058919:Purinergic P2 Receptor Antagonists; D051381:Rats; D018048:Receptors, Purinergic P2 / Q000187:drug effects*; D058848:Receptors, Purinergic P2X4; D011994:Recombinant Proteins / Q000187:drug effects; D013498:Suramin / Q000494:pharmacology; D014982:Xenopus laevis; D015032:Zinc / Q000494:pharmacology*
C548228:P2rx4 protein, rat; D058919:Purinergic P2 Receptor Antagonists; D018048:Receptors, Purinergic P2; D058848:Receptors, Purinergic P2X4; D011994:Recombinant Proteins; D013498:Suramin; D000255:Adenosine Triphosphate; D015032:Zinc