Disuse-Induced Skeletal Muscle Atrophy Is Associated with Reduced Serum Klotho Levels in a Rat Model

Background: Klotho is an anti-aging protein involved in phosphate homeostasis, oxidative stress regulation, and tissue regeneration. Although circulating Klotho levels decline with chronological aging, its response to acute disuse-induced skeletal muscle atrophy remains poorly understood. Methods: Sixteen male Wistar rats (3 months old) were randomly assigned to a control group (n = 8) or a disuse group subjected to a tail suspension–based unloading protocol (3 h/day for 15 days) (n = 8). Skeletal muscle atrophy was evaluated histologically in the extensor digitorum longus (EDL) and soleus muscles using hematoxylin–eosin staining, and muscle fiber cross-sectional area (CSA) was quantified with ImageJ. Interstitial connective tissue changes were assessed qualitatively using Masson’s trichrome staining. Serum Klotho concentrations were measured by enzyme-linked immunosorbent assay (ELISA). Results: Rats exposed to disuse demonstrated histological findings consistent with early disuse-related skeletal muscle alterations in both EDL and soleus muscles, accompanied by a significant reduction in muscle fiber CSA compared with controls. Serum Klotho levels were significantly lower in the disuse group (p < 0.05). Correlation analysis revealed a moderate positive association between circulating Klotho concentrations and mean muscle fiber CSA (r = 0.58, p = 0.02). Mild interstitial connective tissue expansion was qualitatively observed in the soleus muscle following disuse. Conclusions: Short-term disuse-induced skeletal muscle atrophy is associated with a reduction in circulating Klotho levels, independent of chronological ageing. These findings suggest that serum Klotho may serve as a candidate biomarker reflecting early disuse-related muscle degeneration. Further mechanistic studies are warranted to clarify the role of Klotho in skeletal muscle atrophy and regeneration.