Heat‐Stress effects on the myosin heavy chain phenotype of rat soleus fibers during the early stages of regeneration

INTRODUCTION: We investigated heat-stress effects on the adult myosin heavy chain (MyHC) profile of soleus muscle fibers at an early stage of regeneration. METHODS: Regenerating fibers in adult rats were analyzed 2, 4, or 6 days after bupivacaine injection. Rats were heat stressed by immersion in water (42 ± 1°C) for 30 minutes 24 hours after bupivacaine injection and every other day thereafter. RESULTS: No adult MyHC isoforms were observed after 2 days, whereas some fibers expressed only fast MyHC after 4 days. Heat stress increased fast and slow MyHC in regenerating fibers after 6 days. Regenerating fibers expressing only slow MyHC were observed only in heat-stressed muscles. Bupivacaine injection increased the number of Pax7(+) and MyoD(+) satellite cells in regenerating fibers, more so in heat-stressed rats. CONCLUSION: The results indicate that heat stress accelerates fast-to-slow MyHC phenotype conversion in regenerating fibers via activation of satellite cells.