P2X4-pHluorin123 receptors display pH-dependent fluorescence changes. (A) Representative images of a HEK-293 cell expressing P2X4-pHluorin123 at the indicated pH values. (B) Average traces for pH-dependent changes in the fluorescence of HEK-293 cells expressing P2X4-pHluorin123. (C) Summary data for pH-dependent changes in the fluorescence of P2X4-pHluorin123. The pKa value was ∼7.3, which is similar to past characterization work on pHluorin (see Results).