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Hepatocytes isolated from frogs hibernating in hypoxic water exhibited a greater metabolic depression than normoxic counterparts. The amount of energy channelled into non-mitochondrial processes was reduced in favour of proton leak, thought to be as a means to prevent oxidative free radical damage when oxygen is limited. Protein synthesis and Na ⁺K⁺ATP-ase activity were suppressed during hibernation, but the relative proportions to total metabolism remained stable. Therefore, long-term metabolic depression in hepatocytes isolated from overwintering frogs is effected by a general suppression of energy-consuming processes in such a way as to maintain metabolic efficiency. Oxyconformation was noted in isolated hepatocytes in response to acute, progressively hypoxic conditions. This ability to oxyconform was reflected at the level of the mitochondria whilst held in the confines of the cellular environment, but was lost completely upon isolation, prompting the suggestion that mitochondria are ‘slaves’ to the cell. This control appears to be regulated by an oxygen sensing system in which the extra-mitochondrial nitric oxide synthase would detect falling localised PO₂ causing the release of the oxygen radicals, superoxide and nitric oxide. These would then act as intermediate signalling molecules activating the mitoK_ATP channel synchronising the suppression of mitochondrial metabolism with the falling oxygen tension of the local environment i.e. effecting oxyconformation. In an attempt to understand the impact that transient anoxic periods during overwintering might have on liver metabolism, isolated hepatocytes were exposed to anoxia. Lactate studies signified an upregulation of glycolytic metabolism during the initial stages of anoxia switching to a more hypometabolic strategy as the insult continued. Studies on hepatocyte oxygen consumption showed that cells were able to recover from an anoxic period exhibiting a reduced rate of respiration upon reoxygenation and recovery i.e. they were metabolically depressed. Preconditioning the cells with the incorporation of a 30min-preconditioning period prior to the longer insult produced an even more pronounced metabolic depression in response to anoxia. Overall, the results presented in this thesis support an important role for the liver in effecting the hypometabolic state of overwintering frogs.

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