Normobaric hypoxia accelerates high-intensity intermittent training-induced mitochondrial biogenesis (PGC-1α)- and dynamics (OPA1)-related protein expressions in rat gastrocnemius muscle.

High-intensity intermittent training (HIIT) in a normobaric hypoxic environment enhances exercise capacity, possibly by increasing the mitochondrial content in skeletal muscle; however, the molecular mechanisms underlying these adaptations are not well understood. Therefore, we investigated whether HIIT under normobaric hypoxia can enhance the expression of proteins involved in mitochondrial biogenesis and dynamics in rat gastrocnemius muscle. Five-week-old male Wistar rats (n = 24) were randomly assigned to the following four groups: (1) sedentary under normoxia (20.9% O₂) (NS), (2) training under normoxia (NT), (3) sedentary under normobaric hypoxia (14.5% O₂) (HS), and (4) training under normobaric hypoxia (HT). The training groups in both conditions were engaged in HIIT on a treadmill five to six days per week for nine weeks. From the fourth week of the training period, the group assigned to hypoxic conditions was exposed to normobaric hypoxia. Forty-eight hours after completing the final training session, gastrocnemius muscles were surgically removed, and mitochondrial enzyme activity and mitochondrial biogenesis and dynamics regulatory protein levels were determined. Citrate synthase (CS) activity and mitochondrial oxygen phosphorylation (OXPHOS) subunits in the gastrocnemius muscle in the HT significantly exceeded those in the other three groups. Moreover, the levels of a master regulator of mitochondrial biogenesis, peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), and a mitochondrial fusion-related protein, optic atrophy 1 (OPA1), were significantly increased by HIIT under normobaric hypoxia. Our data indicates that HIIT and normobaric hypoxia increase the expression of mitochondrial biogenesis- and dynamics-related proteins in skeletal muscles.