Prolonged Exposure to Hypoxia and Microgravity

Abstract

PURPOSE: Skeletal muscle oxidative metabolism was evaluated in 12 young males (age: 27±5 years [mean±SD]) prior to (CTRL) and following a 21-day horizontal bed rest (simulation of microgravity), carried out in normobaric normoxic (PIO2=133mmHg; NBR) and hypoxic (PIO2=90mmHg; HBR) environments. HBR was aimed to simulate planetary habitats. The effects of a 21-day hypoxic ambulatory confinement (PIO2=90mmHg; HAMB) were also assessed. The study was conducted at the Olympic Sports Centre Planica (Rateče, Slovenia), situated at an altitude of 940m. METHODS: Pulmonary O2 uptake (V’O2) and vastus lateralis fractional O2 extraction (changes in deoxygenated hemoglobin+myoglobin concentration, Δ[deoxy(Hb+Mb)]; near-infrared spectroscopy) were evaluated during an incremental one-leg knee extension exercise to the limit of tolerance, carried out in normoxic environment. Mitochondrial respiration was evaluated ex vivo by high-resolution respirometry in permeabilized vastus lateralis fibers. RESULTS: V’O2peak was lower (P<0.05) after NBR (0.98±0.13 L/min) and after HBR (0.96±0.17) vs. CTRL (1.05±0.15). Δ[deoxy(Hb+Mb)]peak decreased (P<0.05) after NBR (63±13% of limb ischemia) and after HBR (62±13) vs. CTRL (72±11). No changes were observed after HAMB, in both variables. Maximal ADP-stimulated mitochondrial respiration (OXPHOS) was lower (P<0.05) in all conditions (43±10 pmol/s/mg ww in NBR, 38±10 in HBR; 41±12 in HAMB) vs. CTRL (51±10). Non-phosphorylating resting respiration (LEAK) and respiratory control ratio (the ratio between OXPHOS and LEAK) were not significantly affected by any condition. CONCLUSIONS: The impairment of the mitochondrial oxidative potential, described after all conditions, was not evident in muscle oxidative function in vivo after HAMB, presumably for adaptive changes in O2 delivery, peripheral O2 diffusion, intramuscular matching of O2 delivery/O2 utilization. Both ex vivo and in vivo, hypoxia does not exacerbate, nor attenuate, the impairment of skeletal muscle oxidative metabolism described following bed rest alone. EU VII Framework Programme (PlanHab project, grant No. 284438).