Effects of high-intensity exercise load on function and quality of skeletal muscle in mice under a hot and humid environment

To investigate the alterations in skeletal muscle function and mass in an experimental mouse model of high-intensity exercise in a hot and humid environment.

Twenty-four male C57BL/6J mice (7~8 weeks old, weighing 21.30±0.67 g) were randomly assigned to a control group (CON group), a normal temperature and humidity exercise group (NE group), and a high temperature and humidity exercise group (HE group), with 8 mice in each group. The HE group was subjected to a hightemperature simulation chamber, maintaining a temperature of 37~39 ℃ and humidity of 70%~80%, for a 60-minute exercise intervention at a 10° incline and 80% of maximum velocity (12 min of exercise followed by 8 min of rest, for 3 cycles). The CON group did not exercise, while the NE group exercised in the same manner in a normal temperature and humidity environment. The overall condition of the mice was evaluated by monitoring their body weight and analyzing their body composition. Their serum creatinine and urea levels were detected using an automated biochemical analyzer. After exercise, skeletal muscle function in the mice of each group was assessed by measuring their grip strength and exhaustion time. The skeletal muscle contractility and resistance to fatigue were evaluated using an in situ/in vivo/ex vivo muscle testing system. HE staining was employed to observe the morphological and structural changes in the skeletal muscles, and the average cross-sectional area and diameter of the muscle fibers were analyzed. Genes related to protein synthesis (Eif4ebp1, p70S6k) and breakdown (Foxo3, Fbxo32, Trim63) and heat stress-related genes (Hsf-1, Hspa1a, Hsp90aa) were quantified using RT-qPCR.

(1) Compared with the CON and NE groups, the HE group exhibited significant decreases in body weight (P < 0.01) and lean body mass (P < 0.05), an upward trend of creatinine level (P < 0.05), and increases in the urea content (P < 0.01). (2) The mice in the HE group had notably reduced grip strength (P < 0.001), diminished skeletal muscle contraction, and weakened resistance to fatigue (P < 0.05) than the CON and NE groups. (3) The HE group demonstrated a reduction in the average cross-sectional area of muscle fibers (P < 0.05) and a decrease in average fiber diameter (P < 0.05), with particular up-regulation of Fbxo32, Trim63 and Eif4ebp1 (P < 0.01) and downregulation of p70S6k (P < 0.05) in comparison to the NE and CON groups.(4) The expression levels of heat stress-related genes were higher in the HE group than the CON and NE groups (P < 0.05).

Highintensity exercise in a hot and humid environment can lead to a decline in skeletal muscle function and mass in mice, potentially due to the disturbance of skeletal muscle protein synthesis and degradation triggered by excessive heat stress.