Control of respiration and bioenergetics during muscle contraction

H-NMR experiments have determined intracellular O ₂ consumption (V̇ _O2) with oxymyoglobin (MbO ₂) 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, myoglobin (Mb) desaturates rapidly. The desaturation rate constant of ∼30 s reflects the intracellular V̇ _O2. Although Mb desaturates quickly with a similar time constant at all workload levels, its final steady-state level differs. As work increases, the final steady-state cellular P _O2 decreases progressively. After Mb desaturation has reached a steady state, however, V̇ _O2 continues to rise. On the basis of current respiratory control models, the analysis in the present report reveals two distinct V̇ _O2 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 V̇ _O2 shows that oxidative phosphorylation can support up to 36% of the energy cost, a significantly higher fraction than expected. Partitioning of the energy flux shows that a 31% nonoxidative component exists and responds to the dynamic energy utilization-restoration cycle (which lasts for only milliseconds) as postulated in the glycogen shunt theory. The present study offers perspectives on the regulation of respiration, bioenergetics, and Mb function during muscle contraction.