Objective    To observe the effects of RhoA/Rac balance on vascular reactivity in rats following hemorrhagic shock (HS).     Methods    SD rats after hemorrhagic shock (40 mmHg for 0 or 2 h)  were adopted to observe the effects of RhoA/Rac agonists or inhibitors separately on the mean arterial pressure (MAP) and the caliber of superior mesenteric artery (SMA) after the contraction initiated by norepinephrine (NE).     Results    NE-induced pressure response and contraction of SMA following HS were significantly increased at early shock (P0.01)

and then significantly decreased at late shock (P0.01). RhoA inhibitor C3 enzyme and Rac agonist PDGF decreased the pressure response and SMA contraction after NE induction

while RhoA agonist U-46619 and Rac inhibitor NSC23766 had opposite effect in these events

and further increased the contraction of SMA. C3 enzyme and NSC23766 abolished U-46619- and PDGF-induced  change of pressure response and SMA contraction initiated by NE.     Conclusion    Regulation of RhoA/Rac balance can modulate the biphasic vascular reactivity following HS

suggesting that RhoA activity is increased and Rac activity is decreased at early shock

and their corresponding activities is opposite at later shock. Balancing of RhoA/Rac activity takes part in the biphasic reactivity regulation of vessels following HS.