Heat treatment combined with hybrid exercises retraining mitigates cellular markers of protein turnover after hindlimb suspension in male mice: A pilot study

Recent evidence suggests that heat treatment (HT) and resistance training can limit skeletal muscle mass loss during immobilization. However, the effects of repeated HT sessions combined with hybrid exercises (EX), which promote both endurance and resistance responses, on muscle protein turnover following hindlimb unloading (HU) remain unexplored. This study investigated the effects of reloading strategies using HT and/or EX on protein synthesis, ribosome biogenesis and autophagic markers. Eight-week-old male C57BL/6J mice underwent HU (3 weeks), HU+HT (1 week, 5 sessions, 30 min, 40°C), HU+EX (5 sessions of high-intensity inclined treadmill running), HU+EX+HT or HU+REL (passive reloading). HU+HT did not induce additional effects compared with the HU+REL group concerning protein synthesis rates, although one marker of protein synthesis (phosphorylated ribosomal protein S6) tended to increase. HU+HT did not lead to an increase in rRNA content. HU+EX induced a concomitant decrease in the mitochondrial fission markers phosphorylated dynamin-related protein 1 and phosphorylated AMP-activated protein kinase, suggesting improved mitochondrial efficiency. However, no additional effects were observed for rRNA or heat shock proteins compared with the HU+REL group. Finally, exercise followed by heat treatment post-HU (HU+EX+HT) appears to be detrimental, as protein synthesis rates did not increase, despite an increase in rRNA content. Thus, at this sampling time point (14 days post-HU), neither heat therapy nor hybrid retraining produced additional effects on the studied markers compared with a normal resumption of activity. In contrast, the sequential application of the two stressors may inhibit post-HU muscle mass recovery, as evidenced by the absence of increased protein synthesis rates.