Control of respiration and bioenergetics during muscle contraction

H-1-NMR experiments have determined intracellular O-2 consumption ((V) over dot O-2) 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 similar to 30 s reflects the intracellular (V) over dot O-2. 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 PO2 decreases progressively. After Mb desaturation has reached a steady state, however, (V) over dot O-2 continues to rise. On the basis of current respiratory control models, the analysis in the present report reveals two distinct (V) over dot O-2 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 (V) over dot O-2 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.