Figure 4. Melatonin dose-dependently protected retinal pigment epithelial cells against H₂O₂ damage as tested by microculture tetrazoline test. 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 treated with H₂O₂ alone were used as the controls (H). Cell viability was evaluated by the microculture tetrazoline test and expressed as percentages of the controls (mean±standard deviation [SD] in triplicate tests). Error bars represent SD. Pretreatment with melatonin showed dose-dependent protective effects on ARPE-19 cells (an immortal RPE cell line from a 19-year-old donor) against H₂O₂ damage (A). Studies in primary culture of human RPE cells isolated from the donor eye showed similar results (B). *p<0.05, compared with the controls (cells treated with H₂O₂ alone).