Mitochondrial function and increased convective O2 transport: Implications for the assessment of mitochondrial respiration in vivo
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Although phosphorus magnetic resonance spectroscopy ( 31 P-MRS)- based evidence suggests that in vivo peak mitochondrial respiration rate in young untrained adults is limited by the intrinsic mitochondrial capacity of ATP synthesis, it remains unknown whether a large, locally targeted increase in convective O2 delivery would alter this interpretation. Consequently, we examined the effect of superimposing reactive hyperemia (RH), induced by a period of brief ischemia during the last minute of exercise, on oxygen delivery and mitochondrial function in the calf muscle of nine young adults compared with free-flow conditions (FF). To this aim, we used an integrative experimental approach combining 31 P-MRS, Doppler ultrasound imaging, and near-infrared spectroscopy. Limb blood flow [area under the curve (AUC), 1.4 Â± 0.8 liters in FF and 2.5 Â± 0.3 liters in RH, P < 0.01] and convective O2 delivery (AUC, 0.30 Â± 0.16 liters in FF and 0.54 Â± 0.05 liters in RH, P < 0.01), were significantly increased in RH compared with FF. RH was also associated with significantly higher capillary blood flow (P < 0.05) and faster tissue reoxygenation mean response times (70 Â± 15 s in FF and 24 Â± 15 s in RH, P < 0.05). This resulted in a 43% increase in estimated peak mitochondrial ATP synthesis rate (29 Â± 13 mM/min in FF and 41 Â± 14 mM/min in RH, P < 0.05) whereas the phosphocreatine (PCr) recovery time constant in RH was not significantly different (P = 0.22). This comprehensive assessment of local skeletal muscle O2 availability and utilization in untrained subjects reveals that mitochondrial function, assessed in vivo by 31 P-MRS, is limited by convective O2 delivery rather than an intrinsic mitochondrial limitation. Â© 2013 the American Physiological Society.
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