Metabolic substrate change during daily torpor in deer mice

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Abstract
Various metabolic substrates were measured in plasma (glucose, free fatty acids (FFA), ketones, lactate), heart, liver, and skeletal muscle (glycogen, ketones, lactate) of deer mice (Peromyscus maniculatus) in association with their daily torpor cycle. Immediately after feeding, substrate levels were similar to those observed in other mammals. However, within 8–10 h carbohydrate indices (glucose, glycogen) decreased significantly in plasma, liver, and skeletal muscle, with prominent increases in FAA and ketones. Thus the metabolic condition of normothermic deer mice (body temperature 35–38 °C) resembled that seen during acute starvation. Conditions during the actual torpor bout (body temperature 19–24 °C) were similar to those immediately prior to torpor entrance. Glycogen levels in heart were actually highest during daily torpor, possibly as an 'anticipatory' response prior to arousal. Following arousal no repletion of glucose or glycogen was noted, while ketone levels exceeded 5 mM (plasma) and 5 μmol/g (tissues). Adjustments in FFA and ketone utilization may be important in sparing carbohydrate and protein during daily torpor, enabling foraging activity necessary for restoration of endogenous fuel following arousal.

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