2′, 3′‐<i>O</i>‐(2,4,6,Trinitrophenyl)‐ATP and A‐317491 are competitive antagonists at a slowly desensitizing chimeric human P2X₃ receptor

(1) Rapid desensitization of ligand-gated ion channel receptors can alter the apparent activity of receptor modulators, as well as make detection of fast-channel activation difficult. Investigation of the antagonist pharmacology of ATP-sensitive homomeric P2X3 receptors is limited by agonist-evoked fast-desensitization kinetics. (2) In the present studies, chimeric receptors were created using the coding sequence for the N-terminus and the first transmembrane domain of either the nondesensitizing human P2X2a or fast-desensitizing P2X3 receptor joined to the sequence encoding the extracellular loop, second transmembrane domain, and C-terminus of the other receptor (designated P2X2-3 and P2X3-2, respectively). These clones were stably transfected into 1321N1 astrocytoma cells for biophysical and pharmacological experiments using both electrophysiological and calcium-imaging methods. (3) Chimeric P2X2-3 and P2X3-2 receptors were inwardly rectifying and agonist responses showed desensitization properties similar to the wild-type human P2X2a and P2X3 receptors, respectively. (4) The P2X2-3 chimera displayed an agonist pharmacological profile similar to the P2X3 wild-type receptor being activated by low concentrations of both ATP and alpha,beta-meATP. In contrast, the P2X3-2 chimera had markedly reduced sensitivity to both agonists. (5) The P2X3 receptor antagonists TNP-ATP and A-317491 were shown to be potent, competitive antagonists of the P2X2-3 chimera (Ki=2.2 and 52.1 nm, respectively), supporting the hypothesis that rapid receptor desensitization can mask the competitive antagonism of wild-type homomeric P2X3 receptors.