Myoglobin desaturation with exercise intensity in human gastrocnemius muscle

The present study evaluated whether intracellular partial pressure of O-2 (P-O2) modulates the muscle O-2 uptake ((V) over dot (O2)) as exercise intensity increased. Indirect calorimetry followed (V) over dot (O2), whereas nuclear magnetic resonance (NMR) monitored the high-energy phosphate levels, intracellular pH, and intracellular P-O2 in the gastrocnemius muscle of four untrained subjects at rest, during plantar flexion exercise with a constant load at a repetition rate of 0.75, 0.92, and 1.17 Hz, and during postexercise recovery. (V) over dot (O2) increased linearly with exercise intensity and peaked at 1.17 Hz (15.1 +/- 0.37 watts), when the subjects could maintain the exercise for only 3 min. (V) over dot (O2) reached a peak value of 13.0 +/- 1.59 ml O-2. min(-1).100 ml leg volume(-1). The P-31 spectra indicated that phosphocreatine decreased to 32% of its resting value, whereas intracellular pH decreased linearly with power output, reaching 6.86. Muscle ATP concentration, however, remained constant throughout the exercise protocol. The H-1 NMR deoxymyoglobin signal, reflecting the cellular P-O2, decreased in proportion to increments in power output and (V) over dot (O2). At the highest exercise intensity and peak (V) over dot (O2), myoglobin was similar to 50% desaturated. These findings, taken together, suggest that the O-2 gradient from hemoglobin to the mitochondria can modulate the O-2 flux to meet the increased (V) over dot (O2) in exercising muscle, but declining cellular P-O2 during enhanced mitochondrial respiration suggests that O-2 availability is not limiting (V) over dot (O2) during exercise.