Proton-activated currents in chick spinal motoneurons.

Proton-activated currents were examined in patch-clamp recordings from embryonic chick motoneurons. Rapid application of protons evoked a large inward current that peaked and then decayed, presumably due to channel inactivation. A pH shift from 7.4 to 7.1 was sufficient to evoke detectable currents. The shift from pH 7.4 required for half-maximal current amplitude (EC50) was to pH 6.8. In single-channel recordings, activation was achieved within 6 ms at pH 7. The average channel open time was 1.4 ms; the closed-state time constants were 1.0 and 6.2 ms. At pH 6.5, the single-channel conductance was 22 pS, and the reversal potential was similar to the calculated Na+ equilibrium potential. Current amplitude declined by 49% following addition of Ni2+ and increased by 58% as Ca2+ was lowered from 2 to 0.1 mM. Inactivation time constants ranged from 90 to 200 ms as pH varied from 6 to 7; these values did not depend on membrane potential. The reactivation time constant was 22 s. Proton- and glutamate-activated currents summated. Thus, transient decreases in extracellular pH can evoke large inward currents that decay rapidly and reactivate slowly. These currents may occur under pathological conditions that affect extracellular pH.