Differential Responses To Heat Stress On The Regeneration Of Injured Rat Soleus And Plantaris Muscles

PURPOSE: Growing evidence has suggested that heat stress (HEAT) can promote regeneration and induce mitochondrial biogenesis in skeletal muscle. However, the information about the impact of HEAT following skeletal muscle injury remains limited. Therefore, this study was to investigate the impact of HEAT under different conditions after skeletal muscle injury on muscle regeneration, fibrosis, and mitochondrial content in the regenerating muscle. METHODS: Male Wistar rats (10 weeks of age) were randomly assigned to injured and injured with HEAT groups. To induce muscle injury, bupivacaine was injected into slow soleus (SOL) and fast plantaris (PL) muscles bilaterally in all rats. The HEAT group was further divided into three groups: a single bout of HEAT for 30 min at 42 °C (HS42) or 40 °C (HS40) with anesthesia immediately following the injury, and intermittent HEAT (HI42: 42 °C for 30 min/day on alternating days) without anesthesia during 2-14 days after the injury. At 28 days after the injury, SOL and PL muscles were dissected, and histochemical and western blot analyses were performed. RESULTS: Although SOL mass relative to body weight and mitochondrial-related protein (VDAC1) content did not differ between injured and HS42 groups, the fibrotic area in regenerating SOL muscle was significantly lower in HS42 than in injured groups (P < 0.05). However, the relative muscle weight and fibrotic area of regenerating PL in the HS42 group were significantly lower and higher than those in the injured group, respectively (P < 0.05). The VDAC 1 content in the PL muscle did not change when HEAT at 42 °C was applied immediately after the injury. A similar trend was noted in the HS40 group, but the anti-fibrotic effect of HEAT on regenerating SOL was reduced. In contrast, PL mass relative to body weight tended to be greater in HI42 than in injured groups. The fibrotic area in both SOL and PL muscles in the HI42 group also tended to be less than that in the injured group. CONCLUSIONS: These results indicated that HEAT following skeletal muscle injury would have different impact on regenerative responses of the injured muscle depending on the heating conditions and/or muscle types. Our data also suggested that the intermittent HEAT started from early phase after skeletal muscle injury may promote muscle regeneration regardless of muscle types. JSPS KAKENHI (23 K10584), Yamaha Motor Foundation for Sports