Allosteric Modulation of Native Cochlear P2X Receptors: Insights from Comparison with Recombinant P2X2 Receptors

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Abstract
Extracellular adenosine 5′-triphosphate (ATP)-gated ion channels assembled from P2X receptor subunits exhibit subunit-selective allosteric modulation by protons and divalent cations. In voltage-clamped guinea-pig cochlear outer hair cells (OHC) and Deiters’ cells (DC), H+ and Cu2+, but not Zn2+, enhanced the P2X receptor-mediated inward currents. Acid pH (6.5) potentiated OHC ATP-gated currents by 45%. Co-application of Cu2+ (1–40 µM) with ATP increased the response by 20%. In DCs, ATP-gated currents were potentiated 85% by acid pH, and 70% by Cu2+. Alkaline pH inhibited ATP-gated inward currents by 73% in OHCs and 85% in DCs. Zn2+ was either ineffective (1–10 µM) or inhibitory (40–400 µM). Recombinant rat P2X2 receptor-mediated inward currents in Xenopus oocytes displayed allosteric modulation that was different from the native guinea-pig cochlear P2X receptors. The oocyte ATP-gated inward current was potentiated 450% by shifting from pH 7.5 to pH 6.5, and 130% with 40 µM Cu2+. The enhanced response to ATP with acid pH and Cu2+ is a signature of the P2X2 subunit. In contrast to native guinea-pig cochlear cells, extracellular Zn2+ (40 µM) increased the recombinant ATP-gated inward current by 200% in oocytes. These results suggest that the positive allosteric modulation of cochlear OHC and DC ATP-gated ion channels by protons and Cu2+ arises in part from the P2X2 receptor subunit, with additional regulatory elements.