Effects of hypoxia and hyperoxia on exercise-induced metabolomic and transcriptomic profiles in equine skeletal muscle

To explore the molecular mechanisms underlying oxygen-dependent regulation of skeletal muscle adaptations, eight Thoroughbred horses performed 2 min of exercise at a velocity corresponding to 95% maximal O2 uptake under a normoxic condition, while using inspired O2 levels of 0.21 (normoxia), 0.26 (hyperoxia) or 0.16 (hypoxia). At the end of the exercise, arterial O2 saturation was significantly higher with hyperoxia and lower with hypoxia than with normoxia. However, no significant difference in plasma lactate or muscle glycogen concentrations was observed across the O2 conditions. A metabolomic analysis showed that muscle metabolite concentrations involved in glycolysis and the tricarboxylic acid cycle significantly changed in response to exercise but did not significantly differ across the O2 conditions. RNA-sequencing data showed that fewer genes were significantly altered by acute exercise in hyperoxia (upregulated: 523; downregulated: 116) and hypoxia (upregulated: 857; downregulated: 320) compared with normoxia (upregulated: 1628, downregulated: 924). Among them, numerous genes, including well-known exercise-responsive genes, such as NR4A3, PPARGC1A, PDK4 and VEGFA, were altered following exercise, irrespective of the O2 environment. Hyperoxic exercise induced responses of genes related to lysosomal activity, such as M6PR and CTNS, whereas hypoxic exercise triggered hypoxia-responsive gene expression, including PIK3R1, THPO and AKAP1. These findings suggest that arterial O2 availability does not necessarily alter global metabolic or transcriptomic response following a single exercise bout in horses. However, inspired O2 fraction-specific gene responses may play roles in long-term skeletal muscle adaptations and could contribute to the development of optimized training strategies for improved well-being and performance.