Effect of reactive oxygen species on NH4+ permeation in Xenopus laevis oocytes

To investigate the effects of reactive oxygen species (ROS) on NH4+ permeation in Xenopus laevis oocytes, we used intracellular double-barreled microelectrodes to monitor the changes in membrane potential (V(m)) and intracellular pH (pH(i)) induced by a 20 mM NH4Cl-containing solution. Under control conditions, NH4Cl exposure induced a large membrane depolarization (to V(m) = 4.0 +/- 1.5 mV; n = 21) and intracellular acidification [reaching a change in pH(i) (DeltapH(i)) of 0.59 +/- 0.06 pH units in 12 min]; the initial rate of cell acidification (dpH(i)/dt) was 0.06 +/- 0.01 pH units/min. Incubation of the oocytes in the presence of H2O2 or beta-amyloid protein had no marked effect on the NH4Cl-induced DeltapH(i). By contrast, in the presence of photoactivated rose bengal (RB), tert-butyl-hydroxyperoxide (t-BHP), or xanthine/xanthine oxidase (X/XO), the same experimental maneuver induced significantly greater DeltapH(i) and dpH(i)/dt. These increases in DeltapH(i) and dpH(i)/dt were prevented by the ROS scavengers histidine and desferrioxamine, suggesting involvement of the reactive species (1)DeltagO2 and.OH. Using the voltage-clamp technique to identify the mechanism underlying the ROS-measured effects, we found that RB induced a large increase in the oocyte membrane conductance (G(m)). This RB-induced G(m) increase was prevented by 1 mM diphenylamine-2-carboxylate (DPC) and by a low Na+ concentration in the bath. We conclude that RB, t-BHP, and X/XO enhance NH4+ influx into the oocyte via activation of a DPC-sensitive nonselective cation conductance pathway.