Changes in the extracellular cysteine/cystine redox potential in response to t-BuOOH or Aβ treatment. Astrocytes were either untreated (circle or while bar) or treated acutely (square or gray bar) or repeatedly (triangle or balck bar) with 50 μM Aβ (25–35) or 200 μM t-BuOOH. Extracellular cysteine (a), cystine (b), and GSH (e) concentrations were determined in aliquots removed from the culture medium. In (a), the concentration of cysteine in the medium at time 0 (15±5.7 μM) was subtracted from the values at all time points. (c) shows the effect of Aβ versus t-BuOOH treatment on astrocytic xCT expression. (d) The extracellular cysteine/cystine redox potential when astrocytes were treated with 200 μM t-BuOOH or 50 μM Aβ was calculated using the Nernst equation (Fig. 3 legend), using Eo=−250 mV (pH=7.4). Data are the mean±SD of 4 (a, b, d, e) or 2 (c) independent experiments performed on different batches of cells. Asterisks (*p≤0.05, **p≤0.01, ***p≤0.001) denote the level of significance in the comparison between untreated and treated conditions. ns, not significant.