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Highly efficient dissociation of oxygen from hemoglobin in Tibetan chicken embryos compared with lowland chicken embryos incubated in hypoxia

C. Liu^*, L. F. Zhang, M. L. Song, H. G. Bao, C. J. Zhao and N. Li^*,,1

^* State Key Laboratory for Agrobiotechnology, China Agricultural University, Beijing 100193, China; College of Animal Science, Zhejiang University, Hangzhou 310029, China; and College of Animal Science and Technology, China Agricultural University, Beijing 100193, China

¹ Corresponding author: ninglbau{at}public3.bta.net.cn

Oxygen is one of the critical determinants for normal embryonic and fetal development. In avian embryos, lack of oxygen will lead to high fetal mortality, heteroplasia, and cardiovascular dysfunction. Tibetan chicken is a breed native to Tibet that could survive and keep higher hatchability regardless of negative effects of hypoxia. Generally, adaptive animals in high altitudes are characterized by higher hemoglobin concentrations and oxygen affinity. In the present study, the capacity of oxygen supply in late chick embryo (including d 17, 19, and 21) was compared between Tibetan chicken and a lowland breed, Dwarf White chicken, by determining the hemoglobin concentrations and oxygen equilibrium curves in both hypoxic (13% O₂) and normoxic (21% O₂) conditions. The results showed that a higher level of hemoglobin concentration was induced by hypoxia in Tibetan chicken embryos, and the hemoglobin could perform with better cooperativity and deliver oxygen to tissues more easily. Further investigation revealed that the carbonic anhydrase II mRNA in red blood cells of Tibetan chicken was increasingly induced to a higher level in hypoxia than that of the lowland breed. These results suggested that the stronger capacity of oxygen dissociation was an important characteristic of Tibetan chicken embryo to survive in hypoxia and the upregulating mode of carbonic anhydrase II mRNA might assist this dissociation. Therefore, for avian at high altitudes, the efficient dissociation of oxygen might reveal another aspect associated with the hypoxia adaptability.

Key Words: hypoxia • Tibetan chicken • hemoglobin • oxygen equilibrium curve • carbonic anhydrase II