Induction of NO by EPO in TrHBMECs. (A) An increase in NO production was observed at one hour in cells cultured at 21% and 2% O₂ in the presence of EPO (5 U/mL) (▪) compared with no EPO (□) (^***P < .001; ^*P < .05), and was inhibited by L-NAME (1 mM; ▨), an inhibitor of NO synthase activity. (B) NO production was determined after one hour of treatment with different concentrations of EPO (1, 5, and 10 U/mL) at 21%, 5%, and 2% O₂. Response was most pronounced at 2% O₂. EPO induction of NO was maximal at 5 U/mL. (C) NO production at 2% O₂ with EPO after 30 minutes and 4 hours of EPO stimulation was also maximal at 5 U/mL EPO (^***P < .001). Error bars represent SD.