Hypothermia Decreases O2 Cost for Ex Vivo Contraction in Mouse Skeletal Muscle

Introduction Evidence suggests that the energy efficiency of key ATPases involved in skeletal muscle contractile activity is improved in a hypothermic condition. However, it is unclear how a decrease in temperature affects skeletal muscle O₂ consumption (mVO₂) induced by muscle contraction. Methods Isolated mouse extensor digitorum longus (EDL) muscles were incubated in a temperature-controlled (37°C or 25°C) bath that included an O₂ probe. EDL muscles from one limb were subjected to the measurement of resting mVO₂, and the contralateral EDL muscles were used for the measurement of mVO₂ with electrically stimulated contraction. For the resting protocol, muscles were suspended at resting tension for 15 min with continuous O₂ recordings. For the contraction protocol, EDL muscles underwent 10 electrically stimulated isometric contractions with continuous O₂ recordings for 15 min. The rate of O₂ disappearance was quantified as micromoles of O₂ per minute and normalized to the wet weight of the muscle. Results Resting mVO₂ 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 mVO₂ at both 37°C and 25°C, which was sustained for 3 min postcontraction. During that period, mVO₂ was elevated approximately fivefold at both 37°C and 25°C. Greater contraction-induced mVO₂ at 37°C compared with 25°C occurred despite lower force generated at 37°C than at 25°C. Conclusions Together, O₂ cost for muscle contraction (force-time integral per O₂ 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 with 25°C. In conclusion, these results indicate that muscle contraction that occurs at subnormal temperature requires less O₂ than at 37°C.