HU exerts its effects via an NO/cGMP–dependent pathway. WBC (A) rolling, (B) adhesion, and (C) extravasation in H₂O-treated C57BL/6 mice after administration of HU (250 mg/kg IV) or vehicle in the presence of ODQ (15 mg/kg IP) or PTIO (1 mg/kg IV) or vehicle. *P < .05 and ***P < .001 compared with vehicle. n = 22 to 33 venules from 3 to 4 mice.