Intramuscular mitochondrial and lipid metabolic changes of rats after regular high-intensity interval training (HIIT) of different training periods

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 as well as mitochondrial and lipid metabolic changes after HIIT training (2, 4, 6, 8, and 10 weeks). We selected 72 h 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-alpha and COX IV decreased after 2 and 4 weeks of training and raised after 6 and 8 weeks of training. The expression level of citrate synthase (CS) decreased after 2, 4, 8, and 10 weeks of training, and showed an upward trend after 6 weeks of training. While the activity of CS decreased after 2 and 8 weeks of training and showed an upward trend after 6 weeks of HIIT.Conclusion Given the abovementioned changing trends, we propose two speculations: (A) the damaged mitochondria oxidation capacity might be one of the causes of IMTG accumulation observed after 2 and 4 weeks of HIIT. This phase might be similar to the condition of type 2 diabetes. (B) after 6-week HIIT, mitochondria function and biogenesis might be improved and the IMTG contents declined to baseline. This might be explained as: mitochondrial enhancement increased the capacity of lipid oxidation and then offset the increase in IMTG achieved during the first 4 weeks. For HIIT Rat Modelling, if the aim is to observe HIIT-induced positive effects, caution should be exercised when considering 2 and 4 weeks of training under our HIIT frame. Also, implementing six-week training is at least effective for mitochondrial enhancement when using similar HIIT frame of this study.

Automated summary

This study investigated the effects of high-intensity interval training (HIIT) of different durations on intramuscular triglyceride abundance, mitochondrial function, and lipid metabolism in SD rats. The results showed that HIIT for 2, 4, and 10 weeks significantly increased intramuscular triglyceride abundance, while lipid metabolism-related proteins exhibited inconsistent changes. Mitochondrial function decreased after 2 and 4 weeks of training but improved after 6 weeks, and intramuscular triglyceride contents declined. Caution should be exercised when considering 2 and 4 weeks of training under similar HIIT protocols, and at least 6 weeks of training is effective for mitochondrial enhancement.