A VOLTAGE-DEPENDENT AND PH-SENSITIVE PROTON CURRENT IN RANA-ESCULENTAOOCYTES

Voltage clamp technique was used to study macroscopic ionic currents i n Rana esculenta oocytes. Depolarization steps led to the activation o f a single type of outward current (I-out) when contaminant potassium and calcium-dependent chloride currents were pharmacologically inhibit ed. The voltage threshold of I-out activation was 10 mV and this curre nt, which did not inactivate, presented a deactivation the time consta nt 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(r ev) were thought to reflect intracellular ion depletion occurring duri ng activation of the remaining outward current. Furthermore, the activ ation threshold of I-out was clearly affected by modifications in extr acellular and intracellular H+ concentrations. Indeed, intracellular a lkalinization (evoked by external application of ammonium chloride) or extracellular acidification induced a rightward shift in the activati on threshold while intracellular acidification (evoked by external app lication of sodium acetate) or extracellular alkalinization shifted th is threshold toward a more negative value. Lastly, I-out was dramatica lly reduced by divalent cations such as Cd2+, Ni2+ or Zn2+ and was str ongly decreased by 4 Aminopyridine (4-AP), well-known H+ current antag onists already described in many cell types. Therefore, it was suggest ed that the outward current was prominently carried by H+ ions, which may play a key role in the regulation of intracellular pH and subseque nt pH dependent processes in Rana oocyte.