Mitochondrial metabolism during fasting-induced daily torpor in mice

During fasting, mice (Mus musculus) undergo daily bouts of torpor, considerably reducing body temperature (T-b) and metabolic rate (MR). We examined females of different laboratory strains (Balb/c, C57/6N, and CD1) to determine whether liver mitochondrial metabolism is actively reduced during torpor. In all strains, we found that state 3 (phosphorylating) respiration rate measured at 37 degrees C was reduced up to 35% during torpor for at least one of the substrates (glutamate and succinate) used to fuel respiration. The extent of this suppression varied and was correlated with T-b at sampling. This suggests that, at the biochemical level, the transition to and from a hypometabolic torpid state is gradual. In fasted non-torpid animals, T-b and MR still fluctuated greatly: T-b dropped by as much as 4 degrees C and MR was reduced up to 25% compared to fed controls. Changes in T-b and MR in fasted, non-torpid animals were correlated with changes in mitochondrial state 3 respiration rate measured at 37 degrees C. This suggests that fasting mice may conserve energy even when not torpid by occasionally reducing T-b and mitochondrial oxidative capacity to reduce MR. Furthermore, proton conductance was higher in torpid compared to non-torpid animals when measured at 15 degrees C (the lower limit of torpid T-b). This pattern is similar to that reported previously for daily torpor in Phodopus sungorus. (C) 2010 Elsevier B.V. All rights reserved.