A voltage-dependent and pH-sensitive proton current in Rana esculenta oocytes

Abstract

Voltage clamp technique was used to study macroscopic ionic currents in Rana esculenta oocytes. Depolarization steps led to the activation of a single type of outward current (I _out) when contaminant potassium and calcium-dependent chloride currents were pharmacologically inhibited. The voltage threshold of I _out activation was 10 mV and this current, which did not inactivate, presented a deactivation the time constant of 73±21 msec (n=26) corresponding to a membrane voltage of −60 mV. Its reversal potential (E _rev) was dependent on the magnitude of the depolarization and also on pulse duration. These changes in E _rev were thought to reflect intracellular ion depletion occurring during activation of the remaining outward current. Furthermore, the activation threshold of I _out was clearly affected by modifications in extracellular and intracellular H⁺ concentrations. Indeed, intracellular alkalinization (evoked by external application of ammonium chloride) or extracellular acidification induced a rightward shift in the activation threshold while intracellular acidification (evoked by external application of sodium acetate) or extracellular alkalinization shifted this threshold toward a more negative value. Lastly, I _out was dramatically reduced by divalent cations such as Cd²⁺, Ni²⁺ or Zn²⁺ and was strongly decreased by 4 Aminopyridine (4-AP), wellknown H⁺ current antagonists already described in many cell types. Therefore, it was suggested that the outward current was prominently carried by H⁺ ions, which may play a key role in the regulation of intracellular pH and subsequent pH dependent processes in Rana oocyte.