Adaptations to Counter Diffusional Constraints in Muscle of Channichthyid Icefishes

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Antarctic fish species have evolved over 40 million years at nearly constant body temperatures of about 0 C. Exposure to chronic cold has required adaptations to overcome thermal sensitivities of cellular processes and to permit retention of characteristics that would be lethal at warmer body temperatures. Such features will be addressed in this project which focuses on a hemoglobinless icefish family. Although it is well established that these icefishes lack hemoglobin, the presence/absence of the intracellular respiratory pigment, myoglobin has not been resolved as a familial characteristic. Preliminary evidence is presented that suggests myoglobin is expressed exclusively in one tissue of a single icefish species. Validity of this conclusion will be tested using immunological methods to establish definitively whether myoglobin is expressed in oxidative muscles of channichthyid species. It will also be determined whether point mutations in the protein encoding sequence of the myoglobin gene account for lack of the protein's expression mRNA in tissues that do/do not express myoglobin. Characteristics in protein structure that are necessary for function of the intracellular fatty acid binding protein at cold cell temperatures will be examined; this protein enhances transcellular movement of metabolically important fatty acids and serves a cytoprotectant function. Coordinated ultrastructural and biochemical measurements of icefish glycolytic muscle will identify adaptations that have occurred in the tissues of icefish. These experiments will test the proposed paradigm that adaptations to overcome diffusional limitations are fundamental to ensure life at cold cellular temperatures. The studies will add insight into adaptive responses of muscle in higher vertebrates and increase understanding of the metabolism of antarctic fishes, a group important to the trophic structure of the Southern Ocean.