NH4+ conductance inxXenopus laevis oocytes.: III.: Effect of NH3

Exposure of Xenopus laevis oocytes to NH4Cl caused intracellular acidification, cell membrane depolarization and the generation of an inward current. To determine the contribution of uncharged NH3 and positively charged NH4+, the NH4Cl-induced inward current was measured in the presence of increasing [NH3] at constant [NH4Cl] (10 mM) or increasing [NH4Cl] at constant [NH3] (0.045 mM) with pH varying in both cases. At -70 mV, the NH4Cl-induced current was barely detectable at pH 6.5, 0.01 mM NH3, but increased successively at pH 7.5, 0.1 mM NH3 and pH 8.5, 1 mM NH3. In contrast, NH4Cl-associated currents were independent of changes of the [NH4Cl] at constant [NH3] and variable pH. Similar results with respect to acidification, depolarization and inward current in response to concentration and pH changes were obtained with trimethylamine HCl. Increasing concentrations of the weak acid propionate led to a reduction of the NH4Cl-induced current. These data suggest that NH3 entry may induce local alkalinization that, in turn, may trigger the opening of a conductance for NH4+ or trimethylamine-H+ entry.