We used a combined immunochemical and molecular approach to ascertain the presence and concentrations of both the intracellular oxygen-binding hemoprotein myoglobin (Mb) and its messenger RNA (mRNA) in 13 of 15 known species of Antarctic channichthyid icefishes. Mb protein is present in the hearts of eight species of icefishes: Chionodraco rastrospinosus, Chionodraco hamatus, Chionodraco myersi, Chaenodraco wilsoni, Pseudochaenichthys georgianus, Cryodraco antarcticus, Chionobathyscus dewitti and Neopagetopsis ionah. Five icefish species lack detectable Mb protein: Chaenocephalus aceratus, Pagetopsis macropterus, Pagetopsis maculatus, Champsocephalus gunnari and Dacodraco hunteri. Mb concentrations range from 0.44+/−0.02 to 0.71+/−0.08 mg Mb g(−)(1)wet mass in heart ventricle of species expressing the protein. A Mb-mRNA-specific cDNA probe was used to quantify mRNA in five Mb-expressing icefishes. Mb mRNA was found in low but detectable amounts in Champsocephalus gunnari, one of the species lacking detectable Mb. Mb mRNA concentrations in heart ventricle from Mb-expressing species ranged from 0.78+/−0.02 to 16.22+/−2.17 pg Mb mRNA microg(−)(1)total RNA). Mb protein and Mb mRNA are absent from the oxidative skeletal muscle of all icefishes. Steady-state concentrations of Mb protein do not parallel steady-state concentrations of Mb mRNA within and among icefishes, indicating that the concentration of Mb protein is not determined by the size of its mRNA pool.

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