Background—Heme oxygenase-1 (H_mox-1) has been implicated in protection of cells against ischemia/reperfusion injury.

Methods and Results—To examine the physiological role of H_mox-1, a line of heterozygous H_mox-1-knockout mice was developed by targeted disruption of the mouse H_mox-1 gene. Transgene integration was confirmed and characterized at the protein level. A 40% reduction of H_mox-1 protein occurred in the hearts of H_mox-1^+/− mice compared with those of wild-type mice. Isolated mouse hearts from H_mox-1^+/− mice and wild-type controls perfused via the Langendorff mode were subjected to 30 minutes of ischemia followed by 120 minutes of reperfusion. The H_mox-1^+/− hearts displayed reduced ventricular recovery, increased creatine kinase release, and increased infarct size compared with those of wild-type controls, indicating that these H_mox-1^+/− hearts were more susceptible to ischemia/reperfusion injury than wild-type controls. These results also suggest that H_mox-1^+/− hearts are subjected to increased amounts of oxidative stress. Treatment with 2 different antioxidants, Trolox or N-acetylcysteine, only partially rescued the H_mox-1^+/− hearts from ischemia/reperfusion injury. Preconditioning, which renders the heart tolerant to subsequent lethal ischemia/reperfusion, failed to adapt the hearts of the H_mox-1^+/− mice compared with wild-type hearts.

Conclusions—These results demonstrate that H_mox-1 plays a crucial role in ischemia/reperfusion injury not only by functioning as an intracellular antioxidant but also by inducing its own expression under stressful conditions such as preconditioning.