Establishment of rat model of sarcopenia by ovariectomy and estrogen replacement therapy

To establish a rat model of postmenopausal sarcopenia and explore the therapeutic effect of estrogen supplementary therapy on the disease.

① Forty-four 12-week-old female SD rats（220±20 g）were randomly divided into sham group and ovariectomized（OVX）group by stratified random sampling based on body weight, with 22 rats in each group. The rats of the OVX group were given bilateral ovariectomy, while those of the SHAM group underwent sham surgery. The body weight was monitored, and the grip strength of the forelimb was measured using an electronic grip strength meter. In 12 weeks after surgery, the body composition was analyzed by X-ray absorptiometry（DXA）to identify the model establishment by relative lean mass. Ten rats in each group were sacrificed to harvest the gastrocnemius, and then sarcopenia index（SI）was calculated as the ratio of gastrocnemius mass to body weight. HE staining and transmission electron microscopy were applied respectively to observe the cross-sectional area of the muscle fibers and the structure of myofibrils were determined. ② The remaining rats were further divided into SHAM group, SHAM+O group, OVX group and OVX+E₂ group, with 6 rats in each group. 17β-estradiol was injected subcutaneously in OVX+E₂ group, while control solvent（10% DMSO+90%sesame oil）was injected subcutaneously in SHAM+O group. The grip strength and body composition were measured and body composition was analyzed. ELISA was used to measure the serum levels of E₂ and FSH, HE staining was employed to observe the cross-sectional area of gastrocnemius muscle fibers, electron microscopy was conducted to observe the ultrastructure of myofibrils, and immunofluorescence staining was applied to analyze the expression of Bax and Bcl-2.

①After 12 weeks of modeling, the rats in the OVX group had significant lower relative lean mass（P<0.001）, indicating the successful establishment of sarcopenia model, and had obviously increased body weight（P<0.001）and body fat rate（P<0.001）, but decreased relative grip（P<0.001）, SI value, average cross sectional area of the muscle fibers（P<0.01）, and mitochondrial density（P=0.012）. ② Estrogen supplementary therapy resulted in similar body weight, body fat rate, grip strength, relative lean mass, SI value, average cross sectional area of the muscle fibers, and mitochondrial density in the rats in OVX+E₂ group when compared with those of SHAM group. Immunofluorescence assay revealed that OVX group had a notably lower Bcl-2/Bax ratio than the SHAM and OVX+E₂ groups（P<0.01）.

A rat model of postmenopausal sarcopenia is successfully established. Estrogen supplementary therapy can significantly improve the muscle mass loss in ovariectomized rats, which may be associated with estrogen inhibiting myocyte apoptosis.