Figure 5. Melatonin dose-dependently protected retinal pigment epithelial cells against H₂O₂ damage as tested by microculture tetrazoline test, compared with cells not treated with H₂O₂. Retinal pigment epithelial (RPE) cells were pretreated with melatonin (M) 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 cultured for 24 h. Cells not treated with H₂O₂ were used as negative controls (0). Cell viability was evaluated by the microculture tetrazoline test and expressed as percentages of negative controls (mean±standard deviation [SD] in triplicate tests). Error bars represent SD A: Cell viability of cell treated with melatonin and H₂O₂ still significantly lower than that in cells cultured without H₂O₂ in the 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, compared with the negative controls (cells treated without H₂O₂).