Zn2+ modulation of ATP-responses at recombinant P2X2 receptors and its dependence on extracellular pH.
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 microM) 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.
Wildman
S S
SS
Department of Anatomy & Developmental Biology, University College London.
King
B F
BF
Burnstock
G
G
D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D004047:Diethyl Pyrocarbonate / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D018048:Receptors, Purinergic P2 / Q000187:drug effects* / Q000378:metabolism; D011994:Recombinant Proteins / Q000187:drug effects / Q000378:metabolism; D013498:Suramin / Q000494:pharmacology; D014982:Xenopus laevis; D015032:Zinc / Q000494:pharmacology*
D018048:Receptors, Purinergic P2; D011994:Recombinant Proteins; D013498:Suramin; D000255:Adenosine Triphosphate; D015032:Zinc; D004047:Diethyl Pyrocarbonate