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Fig. 5. Pharmacology of the recombinant
P2X2-3/3 receptor.
(A) Injection of P2X2-3 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.