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1 Biological Chemistry, University of California Davis, Davis, CA, USA
2 Exercise Science, University of California Davis, Davis, CA, USA
3 GE Medical Systems, Inc., Fremont, CA, USA
* To whom correspondence should be addressed. E-mail: tjue{at}ucdavis.edu.
1H NMR experiments have determined the intracellular VO2 with the oxymyoglobin (MbO2) desaturation kinetics in human calf muscle during plantar flexion exercise at 0.75, 0.92, and 1.17 Hz with a constant load. At the onset of muscle contraction, the myoglobin (Mb) desaturates rapidly. The desaturation rate constant of ~30s reflects the intracellular VO2. Although Mb desaturates quickly with a similar time constant at all workload, its final steady state level differs. As work increases, the final steady state cellular PO2 decreases progressively. After Mb desaturation has reached a steady state, however, VO2 still continues to rise. Based on current respiratory control models, the analysis reveals two distinct oxygen consumption (VO2) phases: an ADP independent phase at the onset of contraction and an ADP dependent phase after Mb has reached a steady state. In contrast to the accepted view, the initial intracellular VO2 shows that oxidative phosphorylation can support up to the 36% of the energy cost, a significantly higher fraction than expected. Partitioning the energy flux shows that a 31% non-oxidative component exists and responds to the dynamic millisecond energy utilization/restoration cycle, as postulated in the glycogen shunt theory. The study casts perspectives on the regulation of respiration, bioenergetics, and Mb function during muscle contraction.
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