Intramuscular Mitochondrial Adaptation and Lipid Metabolic Alteration in Rats after Chronic High-intensity Interval Training (HIIT) of Different Training Periods

Abstract Background : High-intensity Interval Training (HIIT) is a time-efficient form of exercise and has gained popularity in recent years. However, at molecular level, the understanding about the effects of HIIT is not comprehensive, and even less is elucidated about HIIT of different training duration cycles, although different durations always lead to different post-training consequences. Method: In this study, by training SD rats using HIIT protocols lasting for different training duration cycles, we investigated the adaptive response of intramuscular triglyceride abundance and mitochondria enhancement after HIIT training (2, 4, 6, 8, and 10 weeks). We selected 72h after the last session of training as the time point of sacrifice. Results: The suppressed activation of the cAMP-PKA pathway indicates that skeletal muscle was in the recovery phase at this time point. Intramuscular triglyceride abundance was significantly elevated after 2, 4, and 10 weeks of HIIT. However, the lipid metabolism-related proteins inconsistently changed in a chaotic trend (see Table. 1). The expression levels of PGC1-α and COX IV decreased after 2 and 4 weeks of training and began to rise when the training duration reached 6 weeks. Interestingly, the variation tendency of PGC1-α and COX IV is very similar to that of CPT-1B. Conclusion: Given the fact that CPT-1B is responsible for the transfer of free fatty acids into the mitochondria to facilitate muscular lipid oxidation, there is a great possibility that: A) the IMTG accumulation observed within 0-4 weeks [defined as phase 1 here] of HIIT might be primarily attributed to damaged mitochondria oxidation capacity. B) when the training duration reached 6 weeks [defined as phase 2], mitochondria function and biogenesis began to be improved by training stimulus and might result in the disappearance of IMTG increase. We believe the phase 2 is rather similar to the condition of athletes` paradox, which should be a healthy adaptation induced by exercise training. Also, for HIIT Rat Modelling, intervention duration cycles longer than 4 weeks are recommended if similar HIIT frame of this study is used.