Carbon monoxide paradoxically protects against hypoxia

Abstract

Background: Tissue hypoxia is fundamental to the development of cellular injury caused by multiple processes, including hemorrhagic shock. Others and we have previously demonstrated that endogenous production of carbon monoxide (CO) by heme oxygenase or exogenous administration of CO can protect against organ injury induced by hemorrhage. The purpose of these experiments was to test the hypotheses that CO abrogates the development of tissue hypoxia in vivo and diminishes effects of hypoxia in vitro.Methods: Male C57/BL6 mice were hemorrhaged to a mean arterial pressure of 25 mmHg. Control animals underwent the same procedure but were not hemorrhaged (sham). Inhalational CO exposure (250 ppm) was initiated concurrently with hemorrhage Mice were administered EF5 (10 μl/g; ip), which is an agent that irreversibly binds to intracellular proteins under hypoxic conditions and can then be detected by immunohistochemistry. Mice were sacrificed after 75 minutes of shock and livers were harvested. For in vitro experiments, primary mouse hepatocytes were exposed to normoxia (21% O2) or hypoxia 1% O2) with and without CO (250 ppm). Cells were harvested and assayed for viability, ATP content, or VEGF production. Results: Untreated hemorrhaged mice had a 17-fold increase in EF5 staining compared to shams. Hepatic hypoxia was most pronounced around the central veins. CO-treated hemorrhaged mice had a 79±13% reduction in EF5 staining compared to untreated hemorrhaged mice (P < 0.05). Additionally, CO-treated sham mice demonstrated no significant liver hypoxia. In vitro, CO significantly protected against hypoxia-induced cell death (P < 0.05). Hepatocytes exposed to hypoxia demonstrated a 54 ± 4% reduction in intracellular ATP levels after 6 hours, whereas CO treatment of hypoxic cells resulted in only a 9 ± 3% reduction in ATP (P < 0.01). Likewise, CO significantly inhibited VEGF elaboration by hypoxic hepatocytes. Conclusions: Inhaled low dose CO can paradoxically abrogate liver hypoxia during hemorrhage. Additionally, CO diminishes the consequences of hypoxia in vitro. CO may exert these effects by decreasing cellular metabolic activity and oxygen requirements, though further investigation is required. CO may prove to be a useful clinical adjunct in the treatment of hemorrhagic shock.