Enhancement of ATP-activated current by protons in dorsal root ganglion neurons

The effect of pH on ATP-activated current in bullfrog dorsal root ganglion neurons was studied using the whole-cell patch-clamp technique. ATP-activated current amplitude was highly dependent upon extracellular pH. An acid pH increased, whereas alkaline pH decreased, ATP-activated current amplitude. The half-maximal pH (EC50) for potentiation of 2.5 micro;M ATP-activated current was 7.2. Acidification alone did not activate detectable current and, at an acid pH, ATP-activated current was abolished by suramin. Proton-induced enhancement of ATP-activated current was not sensitive to membrane potential between -80 and +40 mV, and did not involve a shift in reversal potential. Lowering pH from 7.2 to 6.5 or elevating pH from 7.2 to 8.0 shifted the ATP concentration/response curve to the left or right, respectively, without changing the maximal response to ATP. Protons increased the time constant of deactivation without affecting the time constant of activation or desensitization of ATP-activated current. Alteration of patch-pipette (intracellular) pH did not affect the enhancement of ATP-activated current by extracellular protons. Diethylpyrocarbonate (DEP), dithiothreitol (DTT), 5, 5'-dithio-bis-(2-nitro-benzoic acid) (DTNB), or N-ethylmaleimide (NEM) did not affect enhancement of ATP-activated current by protons. The results suggest that extracellular protons, at physiological concentrations, can regulate the function of P2X purinoceptors by modulating the affinity of the ATP-binding site.