Fig. 6. Inhibitory effects of ROS on I_pump. I_pump was recorded in cortical neurons at –60 mV before and after an oxidant insult. (A) Hydrogen peroxide (0.25 mM) showed a marked inhibitory effect on I_pump after a few minutes application. This suppression of the Na⁺, K⁺-pump activity was markedly prevented in the presence of catalase (250 U/ml) or pyruvate (5 mM) added 5 minutes before and during the exposure. Succinate, on the other hand, showed no such effect even at high concentration of 20 mM (n=5-6 for each test). (B) Acute application (10 minutes) of 20 µM menadione, a stimulator of endogenous production of O₂^-, suppressed I_pump; this inhibition was prevented by 25 U/ml SOD or 5 mM pyruvate but not by 20 mM succinate (n6 in each test). (C) Chronic exposure to 8 µM menadione for 9-10 hours also diminished I_pump; both pyruvate (5 mM) and succinate (20 mM) protected the Na⁺, K⁺-pump activity from damage by the endogenous ROS (n=5-7). (D) Acute (20 minutes) and chronic (15-20 hours) applications of DMNQ (5-20 µM) inhibited I_pump, pyruvate (5 mM) or succinate (20 mM) antagonized the inhibition in both conditions (n=5-8).*P<0.05 compared with controls; #P<0.05 compared with hydrogen peroxide, menadione, or DMNQ alone.