Modulatory activity of extracellular H+ and Zn2+ on ATP-responses at rP2X1 and rP2X3 receptors.
1 The modulatory activity of extracellular H+ and Zn2+ was examined on ATP-responses at rat P2X1 (rP2X1) and rat P2X3 (rP2X3) receptors expressed in Xenopus oocytes and studied under voltage-clamp conditions. 2 Superfused ATP (0.03-30 microM, at pH 7.5) evoked inward currents at rP2X1 receptors (EC50 value, 300+/-7 nM). ATP potency was reduced 2 fold at pH 6.5, and 6 fold at pH 5.5, without altering the maximum ATP effect. Alkaline conditions (pH 8.0) did not alter ATP activity. 3 Superfused ATP (0.01 - 300 microM, at pH 7. 5) evoked inward currents at rP2X3 receptors (EC50 value, 1.8+/-0.3 microM). ATP activity was affected only at pH 5.5, reducing agonist potency 15 fold without altering the maximum ATP effect. 4 Extracellular Zn2+ inhibited ATP-responses at rP2X1 receptors in a time-dependent manner, a 20 min pre-incubation being optimal (IC50 value, 1.0+/-0.2 microM). However, the Zn2+ effect was pH-independent, suggesting Zn2+- and H+-inhibition of ATP-responses occur through independent processes. 5 Extracellular Zn2+ weakly potentiated ATP-responses at rP2X3 receptors (EC50 value, 11+/-1 microM). The Zn2+ effect was dependent on pre-incubation time and, with 20 min pre-incubation periods, Zn2+ potentiated then inhibited ATP-responses in a concentration-dependent, but pH-independent, manner. 6 In summary, ATP activity at rP2X1 receptors was decreased by both extracellular H+ and Zn2+ and their effects were additive. ATP activity at rP2X3 receptors was less sensitive to H+-inhibition and, in contrast, was potentiated by Zn2+ in a pH-independent manner. These differential effects may help distinguish P2X1 and P2X3 receptors in whole tissues.
Wildman
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SS
Autonomic Neuroscience Institute, Royal Free and University College Medical School, Royal Free Campus, Rowland Hill Street, Hampstead, London NW3 2PF.
King
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BF
Burnstock
G
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D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D004594:Electrophysiology; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials / Q000187:drug effects; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism; D058910:Purinergic P2 Receptor Agonists; D051381:Rats; D018048:Receptors, Purinergic P2 / Q000378:metabolism; D058469:Receptors, Purinergic P2X; D058476:Receptors, Purinergic P2X2; D058477:Receptors, Purinergic P2X3; D058848:Receptors, Purinergic P2X4; D011994:Recombinant Proteins / Q000378:metabolism; D013997:Time Factors; D014982:Xenopus laevis; D015032:Zinc / Q000494:pharmacology*
C546403:P2rx2 protein, rat; C546410:P2rx3 protein, rat; C548228:P2rx4 protein, rat; D058910:Purinergic P2 Receptor Agonists; D018048:Receptors, Purinergic P2; D058469:Receptors, Purinergic P2X; D058476:Receptors, Purinergic P2X2; D058477:Receptors, Purinergic P2X3; D058848:Receptors, Purinergic P2X4; D011994:Recombinant Proteins; D000255:Adenosine Triphosphate; D015032:Zinc