Separation of Hydrogen Ion Currents in Intact Molluscan Neurones

Abstract

The amplitude and rate of activation of the voltage-dependent H+ pathway in intact Helix neurones were investigated using standard two-electrode voltageclamp techniques. Na+ and K+ currents were inhibited by a Na+-free, tetraethyl-ammonium (TEA2+) (low-Cl−) saline and by use of Cs+-filled electrodes. Ca+ currents were abolished by holding the membrane at −15 to −10 mV. Depolarizing voltage pulses from these low potentials activated outward currents whose tail current reversal potential shifted with changes in intracellular and extracellular pH, but not with changes in external KC1; thus these remaining currents are carried by hydrogen ions. Furthermore, the amplitude of the voltage-dependent outward current increased as the outward gradient for H+ was increased and a rise in pHi shifted the activation towards negative potentials. At physiological pH levels, H+ currents were typically 60 nA at 30 mV (cell diameter 200–250 μm). H+ currents were rapidly activated; the time to half maximal current at 30 mV was less than 5 ms in the pHi range tested (7.4-6.9) (pHe 7.4). The H+ pathway will therefore be activated by individual action potentials and may play an important role in pH homeostasis during intense neural activity.