NH₄ ⁺ conductance in Xenopus laevis oocytes. III. Effect of NH₃

Abstract

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.