An integrative evaluation of oxidative metabolism was carried out in 9 healthy young men (age: 24.1±1.7 years [mean±SD]) prior to (CTRL) and following a 10-day horizontal bed rest, carried out in normoxia (N-BR) or hypoxia (FIO2=0.147; H-BR). H-BR was aimed to simulate planetary habitats. Pulmonary O2 uptake (V'O2) and vastus lateralis fractional O2 extraction (changes in deoxygenated hemoglobin+myoglobin concentration, [deoxy(Hb+Mb)]; near-infrared spectroscopy) were evaluated, in normoxia, during an incremental cycle ergometer exercise (CE) and one-leg knee extension exercise (KE) (aimed at reducing cardiovascular constraints to oxidative function). Mitochondrial respiration was evaluated ex vivo by high-resolution respirometry in permeabilized vastus lateralis fibers. During CE V'O2peak and [deoxy(Hb+Mb)]peak were lower (P<0.05) after both N-BR and H-BR vs. CTRL; during KE the variables were lower after N-BR, but not after H-BR. During CE the "overshoot" of [deoxy(Hb+Mb)] during constant work rate exercise was greater in N-BR and H-BR vs. CTRL, whereas during KE a significant difference vs. CTRL was observed only after N-BR. Maximal mitochondrial respiration determined ex vivo was not affected by both interventions. In N-BR, a significant impairment of oxidative metabolism occurred downstream of central cardiovascular O2 delivery and upstream of mitochondrial function, possibly at the level of the intramuscular matching between O2 supply and utilization (see the [deoxy(Hb+Mb)] overshoot) and peripheral O2 diffusion. Superposition of H on BR did not aggravate, and partially reversed, the impairment of muscle oxidative function in vivo induced by BR. The effects of longer exposures will have to be determined.
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