Hemorrhagic shock was induced in male anesthetized rats by intermittently withdrawing blood from an iliac catheter until mean arterial blood pressure (MAP) fell and stabilized within the range of 20-30 mmHg. Survival rate, MAP, and serum and macrophage levels of tumor necrosis factor-alpha (TNF-alpha) were then evaluated. Furthermore, in ex vivo studies, the responsiveness to phenylephrine (PE; 1 nM to 10 microM) was investigated in aortic rings from hemorrhagic shocked rats. Antibodies raised against TNF-alpha (anti-TNF-alpha; 2 mg/kg) or vehicle (phosphate-buffered saline, 1 ml/kg) were injected intravenously 3 h before the bleeding. Vehicle-treated rats, subjected to hemorrhagic shock, exhibited acute and serious hypotension (MAP = 20-30 mmHg) and high levels of serum (790 +/- 47 pg/ml) and macrophage (78 +/- 9 pg/ml) TNF-alpha and died within 30 min. Moreover, aortas from shocked rats showed a marked hypocontractility to PE compared with the reactivity of aortas from a group of sham shocked rats. Anti-TNF-alpha administration significantly improved survival rate and MAP in hypovolemic shocked rats. Furthermore, the hyporesponsiveness to PE was significantly restored in aortic rings. Therefore, these data suggest that TNF-alpha is an important mediator in the pathophysiology of hypovolemic hemorrhagic shock and it might be responsible, at least in part, for the vascular hyporeactivity of this experimental circulatory shock.