Hydroxyurea increased cAMP and cGMP levels in HUVECs. (A) Hydroxyurea showed dose-dependent induction of cAMP levels in HUVECs after 30 minutes of incubation. (B) Hydroxyurea showed dose-dependent induction of cGMP levels in HUVECs after 30 minutes of incubation, whereas pretreatment with L-NAME inhibited this effect. Values represent mean ± SEM (n = 3); ^**P < .01 compared with cells untreated with hydroxyurea.