Impact of melatonin on HK-2 cell antioxidant capacity and viability. Cells in each group were treated for 12 h with 4 mmol/L Ox (except the NC group). Comparisons of (a) cell viability, (b) LDH content, (c) MDA content, (d) T-AOC content, (e) GSH content, (f) SOD content, and (g) CAT content. Data are presented as the means ± SEM from three independent experiments. *P < 0.05, **P < 0.01, ***P < 0.001,****P < 0.0001, versus the NC group; #P < 0.05, ##P < 0.01, ###P < 0.001, ####P < 0.0001, versus the Ox group