Heat therapy preserves myofibre size and <scp>SERCA</scp>‐mediated Ca²⁺ uptake in the mouse soleus after tenotomy surgery

Heat therapy (HT) has been shown to induce physiological adaptations in muscle, including a reduction in the severity of muscle atrophy resulting from unloading. The muscle atrophy caused by unloading can be partially attributed to the dysregulation of Ca²⁺ in the muscle cell, which can activate calpain-mediated proteolysis. The sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) is a primary regulator of Ca²⁺ in muscle, and SERCA dysfunction has been repeatedly demonstrated in various models of muscle unloading. Heat shock protein 70 (HSP70) is a heat-inducible chaperone protein that binds to SERCA and protects against its dysfunction. While previous research has shown HT to upregulate HSP70 in rodent muscle, even in the unloaded state, the effects of HT on SERCA function in rodent skeletal muscle under these conditions remain unknown. Here, we characterized the effects of 4 weeks of HT on soleus muscle size, HSP70 expression, SERCA function, and maximal calpain activity in male C57BL/6J mice subjected to muscle unloading through tenotomy. Four weeks of HT preserved the cross-sectional area of soleus myofibres following tenotomy, while also upregulating HSP70, maintaining SERCA-mediated Ca²⁺ uptake, and reducing maximal calpain activity. Therefore, our research offers new insights into the advantages of HT for muscle health and physiology.