Hypoxemia and blunted hypoxic ventilatory responses in mice lacking heme oxygenase-2

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Abstract

Heme oxygenase (HO) catalyzes physiological heme degradation and consists of two structurally related isozymes, HO-1 and HO-2. Here we show that HO-2-deficient (HO-2−/−) mice exhibit hypoxemia and hypertrophy of the pulmonary venous myocardium associated with increased expression of HO-1. The hypertrophied venous myocardium may reflect adaptation to persistent hypoxemia. HO-2−/− mice also show attenuated ventilatory responses to hypoxia (10% O2) with normal responses to hypercapnia (10% CO2), suggesting the impaired oxygen sensing. Importantly, HO-2−/− mice exhibit normal breathing patterns with normal arterial CO2 tension and retain the intact alveolar architecture, thereby excluding hypoventilation and shunting as causes of hypoxemia. Instead, ventilation–perfusion mismatch is a likely cause of hypoxemia, which may be due to partial impairment of the lung chemoreception probably at pulmonary artery smooth muscle cells. We therefore propose that HO-2 is involved in oxygen sensing and responsible for the ventilation–perfusion matching that optimizes oxygenation of pulmonary blood.

Section snippets

Materials and methods

Mice. HO-2−/− mice with C57/BL6J × 129/Sv mixed genetic background were generated as previously described [26]. Male homozygous HO-2−/− mice were backcrossed with female C57/BL6J mice. After the backcross for 6 generations, heterozygous HO-2 mice were intercrossed and their littermates were used for this study. HO-2 genotyping was determined by a pair of PCR primers, which spans neomycin-resistant cassette: 5-GCTTGGGTGGAGAGGCTATTC-3 and 5-CAAGGTGAGATGACAGGAGATC-3 using genomic DNA prepared

Hypoxemia in HO-2−/− mice

The identity of the HO-2−/− mice was confirmed by the genotype analysis (data not shown). We also analyzed the expression of HO-2 protein in the testis, where HO-2 protein is abundantly expressed [34]. We prepared an antibody specific to a mouse HO-2 peptide for the immunohistochemical analysis, because a commercially available anti-HO-2 antibody caused high background staining. Western blot analysis detected HO-2 protein only in the tissue extracts from the testes of wild-type littermates (HO-2

Discussion

Here we have identified hitherto unrecognized phenotypic alterations related to oxygen sensing in HO-2−/− mice. The small magnitude of the hypoxemia may be accounted for by the notions that oxygen homeostasis is essential for all living organisms and must be maintained by multiple factors through separate mechanisms. Generally, the phenotypes of the gene-deficient mice reflect the consequences of various compensatory adaptations that occur during embryogenesis to assure their survival. In this

Acknowledgements

We thank Dr. Poss K.D. for providing HO-2−/− mice and Dr. Lusis A.J. for helping backcross these mice. This work was supported in part by Grants-in-Aid for Scientific Research (B) (to S.S.), for Exploratory Research (to S.S.), and for Priority Area (to S.S.) from the Ministry of Education, Science, Sports and Culture of Japan (12670683 and 14570682 to K.I.), and by the 21st Century COE Program Special Research Grant “the Center for Innovative Therapeutic Development for Common Diseases” from

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    Both the authors contributed equally to this work.

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