Methods

Isolated mouse extensor digitorum longus (EDL) muscles were incubated in a temperature-controlled (37 C or 25 C) bath that included an O 2 probe. EDL muscles from one limb were subjected to the measurement of resting mVO 2, and the contralateral EDL muscles were used for the measurement of mVO 2 with electrically-stimulated contraction. For the resting protocol, muscles were suspended at resting tension for 15 mins with continuous O 2 recordings. For the contraction protocol, EDL muscles underwent ten electrically-stimulated isometric contractions with continuous O 2 recordings for 15 mins. The rate of O 2 disappearance was quantified as μmole O 2 per minute and normalized to the wet weight of the muscle.

Results

Resting mVO 2 was greater at 37 C than at 25 C, consistent with the idea that lower temperature reduces basal metabolic rate. Electrically-stimulated contraction robustly increased mVO 2 at both 37 C and 25 C, which was sustained for~ 3 min post-contraction. During that period, mVO 2 was elevated~ 5-fold at both 37 C and 25 C. Greater contraction-induced mVO 2 at 37 C compared to 25 C occurred despite lower force generated at 37 C than at 25 C.

Conclusion

Together, O 2 cost for muscle contraction (force-time integral per O 2 consumed) was greater at 37 C than at 25 C. Levels of high-energy phosphates were consistent with greater energy demand at 37 C compared to 25 C. In conclusion, these results indicate that muscle contraction that occurs at subnormal temperature requires less O 2 than at 37 C.