Fig. 5. Pharmacology of the recombinant P2X_2-3/3 receptor. (A) Injection of P2X_2-3 and P2X₃ mRNAs in a ratio of 1:2 into Xenopus oocytes resulted in expression of ATP-gated inward currents with a broad sensitivity to ATP agonists. (B) Concentration-response curves for ATP at pH 7.5 show an EC₅₀ of 0.4 µM for these ATP-gated ion channels. Acidification to pH 6.5 produced a leftward shift in the concentration-response curve, to reduce activation thresholds to <10 nM ATP. This is attributable to positive allosteric modulation by protons acting through the P2X₂ subunit.