Brown fat GDP binding and circulating metabolites during hibernation and arousal

The effect of hibernation and arousal on brown adipose tissue (BAT) cytochrome-c oxidase activity and GDP binding, as well-circulating metabolites, have been studied in the 13-lined ground squirrel. Control animals (warm adapted) were housed continuously at 23 degrees C, while the remaining animals were transferred into a cold room (4 degrees C) for 8 days to induce hibernation. Hibernating animals were killed while deeply hibernating. Aroused animals were manually stimulated to induce arousal or had spontaneously aroused on the day of the experiment. BAT weight as well as mitochondrial mass were increased in both groups of cold-adapted animals, relative to controls. A substantial increase in GDP binding, however, was seen only in aroused animals, an observation confirmed by Scatchard analysis. Arousal was also accompanied by marked alterations in the levels of several circulating metabolites. Plasma free fatty acids declined by approximately 20% despite a three- to fourfold increase in plasma glycerol concentrations. Plasma lactate levels increased eightfold, while concentrations of beta-hydroxybutyrate were five times lower during arousal than hibernation. These data are consistent with the idea that the oxidation of free fatty acids, glucose, and ketone bodies are all increased during arousal. In conclusion, we have found that cold adaptation and subsequent hibernation increases BAT thermogenic capacity in the 13-lined ground squirrel. However, this increase in thermogenic potential is not manifested as a substantial increase in BAT thermogenic activity until arousal is initiated.