Effects of repeated High-Intensity Interval Training interventions on oxidative metabolism adaptations in human skeletal muscle

Skeletal muscle is a highly plastic tissue. Exercise is a powerful stimulus that can remodel skeletal muscle by activating signalling pathways that change its metabolism and contractile properties. Resistance training is an effective intervention strategy to improve strength and muscle hypertrophy. It has been shown that an initial exposure to a hypertrophic stimulus leads to faster and greater growth of skeletal muscle if repeated subsequently. The permanence of acquired myonuclei and conserved epigenetic modifications are parallel processes that appear to be involved in the phenomenon known as 'muscle memory'. Endurance exercise is a strong tool for modulating mitochondrial biogenesis. Acute high-intensity endurance exercise has been shown to promote epigenetic modifications and gene expression of mitochondrial biomarkers compared to low-intensity exercise. This thesis aimed to understand whether oxidative metabolism adapts to repeated aerobic exercise training interventions retaining adaptations over time. To achieve this goal twenty healthy young subjects (25 ± 5 years) underwent two repeated high-intensity interval training (HIIT) interventions (training and retraining), separated by 12 weeks of detraining. The training intervention consisted of 8 weeks of combined high-intensity cycling and interval sprint exercise performed 3 days per week. In-vivo peak oxygen consumption (V̇O2peak) and peak power output (Wpeak) significantly improved during both training and retraining, but increment changes were not different between the two interventions. No change in ex-vivo mitochondrial respiration was found both after training and retraining. Muscle capillarization (capillary density) and endothelial function (CD31 protein expression) were improved after training and retraining intervention. Interestingly, the increase in CD31 content was retained during detraining. Mitochondrial biogenesis markers (CS, PGC1α and ANT1 protein content) showed a tendency to increase after training, which became significantly higher after retraining. Mitochondrial dynamic markers showed a pro-fusion adaptation both after training and retraining based on the significant increase of OPA1, MFN2 and MFN1 protein content. MFN1 showed a tendency to increase after training, which became significantly higher after retraining. At the molecular level, these data show that retraining effects appear to be modulated by the first training, suggesting a potential muscle memory mechanism triggered by endurance exercise. Considering that single fibres cross-sectional area has not been affected by the repeated HIIT interventions, thereby excluding the presence of hypertrophy and the addition of new myonuclei, we investigated the potential presence of an epigenetic memory mechanism. Genome-wide DNA methylation analysis identified differentially methylated positions (DMPs) induced by training. Two epigenetic memory profiles were revealed, characterized by the maintenance of hypomethylation even during prolonged detraining, and involving differentially methylated regions related to genes involved in skeletal muscle metabolic pathways. Six genes were identified as epigenetic memory genes with increased transcript expression after training, which was maintained after detraining and retraining. Since the retention of transcript expression was also observed in genes involved in lactate transport (SLC16A3), biomarkers of lactate transport (MCT1, MCT4, LDH) were studied. After retraining, MCT1 protein content increased compared to baseline, showing a memory profile at the level of protein expression. These data provided evidence for a skeletal muscle memory mechanism, elicited by high-intensity aerobic training.