Fig. 4. Hydrolytic activity of Na⁺/K⁺ ATPase after exposure to different oxygen concentrations and manipulation of cellular redox state. Cells were exposed for 15 min to 21, 5 or 1% O₂ in the presence or in the absence of 1 mmol l^-1 CDNB, 5 mmol l^-1 N-acetyl cysteine or GSH. After incubation, the cells were immediately destroyed by repetitive freezing-thawing and ouabain-sensitive production of P_i measured in cell homogenate in the presence of 3 mmol l^-1 ATP. Data are means ± S.E.M. of five independent experiments. ^*P<0.05.