Figure 6. Luzindole decreased protective effects of melatonin on retinal pigment epithelial cells against H₂O₂ damage as tested by microculture tetrazoline test. Cultured retinal pigment epithelial (RPE) cells were treated with or without 50 μM luzindole (L). One hour later melatonin (M) was added to the culture medium at concentrations of 10⁻¹⁰ M (M-10), 10⁻⁸ M (M-8), 10⁻⁶ M (M-6), and 10⁻⁴ M (M-4). After 24 h, 0.5 mM H₂O₂ (H) was added and cultures were incubated for 24 h. Cell viability was evaluated by the microculture tetrazoline test and expressed as percentages of cells cultured with H₂O₂ alone (mean±standard deviation [SD] in triplicate tests). Error bars represent SD A: Luzindole significantly decreased melatonin-induced protective effects at 10⁻¹⁰ to 10⁻⁴ M in ARPE-19 cells (an immortal RPE cell line from a 19-year-old donor). B: The same was true in the primary-culture RPE cells. * p<0.05, comparison between cells treated with and without luzindole.