Objective:To investigate the role of the vasodilator systems heme oxygenase-1/heat shock protein 32 (HO-1/HSP32) and nitric oxide synthase-II (NOS-II), generating carbon monoxide and nitric oxide respectively, as modulators of liver injury in an experimental model of reversible hemorrhagic shock.Design:Prospective controlled laboratory study.Setting:University research laboratory.Subjects:Male Sprague-Dawley rats weighing 250-350 g.Interventions:Animals were anesthetized and assigned to a hemorrhagic shock (mean arterial pressure, 35-40 mmHg for 60 mins) or a sham protocol. On the basis of the time course of gene expression, HO-1/HSP32 or NOS-II was blocked 5 hrs after onset of resuscitation. To assess the role of the antioxidative properties of the heme oxygenase (HO) pathway in additional experiments, Trolox, a potent antioxidant, was administered at the time of blockade of HO. Liver injury was assessed morphometrically and by plasma α-glutathione-S-transferase (α-GST) release 11 hours after onset of resuscitation.Measurements and Main Results:Hemorrhage and resuscitation increased HO-1/HSP32 messenger RNA and protein primarily in parenchymal cells, and a faint induction of NOS-II, restricted to nonparenchymal cells, was observed. Inhibition of the HO pathway with tin protoporphyrin-IX (SnPP-IX) increased the incidence of pericentral necrosis (intact acini: shock/vehicle 68.8%; shock/SnPP-IX 42.6%) and α-GST levels (sham 94 ± 24 μg/L; shock/vehicle 377 ± 139 μg/L; shock/SnPP-IX 1708 ± 833 μg/L), whereas blockade of NOS-II with S-methylisothiourea did not affect liver injury. Coadministration of Trolox failed to attenuate the aggravation of necrosis associated with blockade of HO, whereas α-GST levels were reduced (intact acini: shock/vehicle/Trolox 82.1%, shock/SnPP-IX/Trolox 42.7%; α-GST: shock/vehicle/Trolox 202 ± 55 μg/L; shock/SnPP-IX/Trolox 236 ± 61 μg/L).Conclusions:These data suggest that HO-1/HSP32, but not the alternative cyclic guanosine monophosphate-generating enzyme NOS-II, is induced after hemorrhage and resuscitation and protects against hepatocellular injury. Both metabolites generated by the heme oxygenase pathway, e.g., carbon monoxide (a vasodilator) and biliverdin (an antioxidant) seem to contribute to the salutary effects of induction of HO-1/HSP32.