Introduction: Given low muscle mass conditions such as sarcopenia increase the likelihood of adverse events and are associated with higher morbidity and mortality, investigating ways to limit muscle loss is vital. Vitamin D (vitD) is well known for its role in the regulation of calcium and bone mineralisation and given the presence of the vitD receptor in skeletal muscle, it is presumed that vitD also regulates muscle mass. Whether vitD directly effects muscle mass and function, particularly in a setting of acute, rapid muscle loss, is unknown. Thus, this project manipulated levels of vitD and investigated the loss of muscle mass and function in a pre-clinical mouse model of bed rest/inactivity.
Methods: 8-week-old male C57Bl/10 mice (n=12) were fed for 6 weeks on diets containing normal (1000; NORM), low (0; LVD), or high (20,000; HVD) IU/Kg cholecalciferol content. After 4 weeks of feeding, animals were anaesthetised, and casts were applied to the right hindlimb of each mouse to induce atrophy. Following 2 weeks of casting, animals were again anaesthetised, casts were removed, and the extensor digitorum longus (EDL), soleus (SOL), and tibialis anterior (TA) muscles extracted for analysis. Animal experimentation was approved by Victoria University Animal Ethics Committee (AEETH 22/005).
Results: Immobilisation (IM) induced atrophy in EDL, SOL, and TA muscles, demonstrated by reduced muscle mass (EDL & SOL) (p<0.0001) and cross-sectional area (TA) (p<0.0001) compared to uncast muscles. LVD-IM EDLs were smaller (p<0.01) and exhibited reduced force production (p<0.05) compared to NORM-IM EDLs. EDL force-frequency relationships were impacted by diet with LVD-IM muscles shifting left compared to NORM-IM muscles (diet effect p<0.0001). Haematoxylin and Eosin stain analysis revealed a significant increase in percentage of unhealthy tissue and an increased percentage of fibres with centralised nuclei in LVD-IM EDLs (p<0.01, p<0.01).
Discussion: Immobilisation and LVD diet profoundly affected morphometric and functional characteristics of fast twitch muscles. Greater atrophy in a low vitamin D environment has implications for muscle loss with acute hospitalisation and bed rest and is particularly relevant to the context of changes in ageing muscle (sarcopenia). VitD depletion may act as a catabolic catalyst under conditions of skeletal muscle immobilisation, and further investigation is warranted to uncover mechanistic changes driving these results.