Case Studies in Physiology: Skeletal Muscle Mitochondrial Protein Synthesis and Respiration in Response to the Energetic Stress of an Ultra-Endurance Race

The 2016 Colorado Trail Race (CTR) was an ultra-endurance mountain bike race where competitors cycled for up to 24 hrs/day between altitudes of 1675 to 4025 meters to complete 800 kilometers and 21,000 meters of elevation gain. In one athlete, we had the unique opportunity to characterize skeletal muscle protein synthesis and mitochondrial respiration in response to a normal activity control period (CON) and the CTR. We hypothesized that mitochondrial protein synthesis would be elevated and mitochondrial respiration would be maintained during the extreme stresses of the CTR. Titrated and bolus doses of ADP were provided to determine substrate-specific oxidative phosphorylation (OXPHOS) and electron transport system (ETS) capacities in permeabilized muscle fibers via high-resolution respirometry. Protein synthetic rates were determined by daily oral consumption of deuterium oxide (²H₂O). The endurance athlete had OXPHOS (226 pmol·s^-1·mg tissue^-1) and ETS (231 pmol·s^-1·mg tissue^-1) capacities that rank amongst the highest published to date in humans. Mitochondrial (3.2 fold), cytoplasmic (2.3 fold), and myofibrillar (1.5 fold) protein synthesis rates were greater during CTR compared to CON. With titrated ADP doses, the apparent Km of ADP, OXPHOS, and ETS increased after the CTR. With provision of ADP boluses after the CTR, the addition of fatty acids (-12 and -14%) mitigated the decline in OXPHOS and ETS capacity during carbohydrate-supported respiration (-26 and -31%). In the face of extreme stresses during the CTR, elevated rates of mitochondrial protein synthesis may contribute to rapid adaptations in mitochondrial bioenergetics.

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