Muscle oxidative metabolism accelerates with mild acidosis during incremental intermittent isometric plantar flexion exercise

BACKGROUND: It has been thought that intramuscular ADP and phosphocreatine (PCr) concentrations are important regulators of mitochondorial respiration. There is a threshold work rate or metabolic rate for cellular acidosis, and the decrease in muscle PCr is accelerated with drop in pH during incremental exercise. We tested the hypothesis that increase in muscle oxygen consumption ([Image: see text] o(2mus)) is accelerated with rapid decrease in PCr (concomitant increase in ADP) in muscles with drop in pH occurs during incremental plantar flexion exercise. METHODS: Five male subjects performed a repetitive intermittent isometric plantar flexion exercise (6-s contraction/4-s relaxation). Exercise intensity was raised every 1 min by 10% maximal voluntary contraction (MVC), starting at 10% MVC until exhaustion. The measurement site was at the medial head of the gastrocnemius muscle. Changes in muscle PCr, inorganic phosphate (Pi), ADP, and pH were measured by (31)P-magnetic resonance spectroscopy. [Image: see text] o(2mus )was determined from the rate of decrease in oxygenated hemoglobin and/or myoglobin using near-infrared continuous wave spectroscopy under transient arterial occlusion. Electromyogram (EMG) was also recorded. Pulmonary oxygen uptake ([Image: see text] o(2pul )) was measured by the breath-by-breath gas analysis. RESULTS: EMG amplitude increased as exercise intensity progressed. In contrast, muscle PCr, ADP, [Image: see text] o(2mus), and [Image: see text] o(2pul )did not change appreciably below 40% MVC, whereas above 40% MVC muscle PCr decreased, and ADP, [Image: see text] o(2mus), and [Image: see text] o(2pul )increased as exercise intensity progressed, and above 70% MVC, changes in muscle PCr, ADP, [Image: see text] o(2mus), and [Image: see text] o(2pul )accelerated with the decrease in muscle pH (~6.78). The kinetics of muscle PCr, ADP, [Image: see text] o(2mus), and [Image: see text] o(2pul )were similar, and there was a close correlation between each pair of parameters (r = 0.969~0.983, p < 0.001). CONCLUSION: With decrease in pH muscle oxidative metabolism accelerated and changes in intramuscular PCr and ADP accelerated during incremental intermittent isometric plantar flexion exercise. These results suggest that rapid changes in muscle PCr and/or ADP with mild acidosis stimulate accelerative muscle oxidative metabolism.