Viability loss of glioblastoma cells after a photodynamic challenge. (a) ALA-treated U87 cells were exposed to the indicated light fluences in the absence (○) or presence of 25 μM 1400W (□) or 25 μM cPTIO (△). After irradiation, cells were switched to serum-containing medium, and after 20 h of dark incubation, examined for viability by MTT assay. (b) U251 cells were sensitized identically to U87 cells, irradiated in the absence (○) vs. presence of 25 μM 1400W (□) or 1 mM L-NAME (△), and then examined for viability after 20 h in the dark. Also represented in (a) and (b) are ALA-only or light-only controls (x). Plotted values in (a) and (b) are means ± SEM (n=3); W, cP, and N denote 1400W, cPTIO, and L-NAME, respectively.