Alterations to longitudinal muscle morphology and mechanical function following immobilization and recovery in female ovariectomized and intact rats

Serial sarcomere number (SSN) is reduced following immobilization of a muscle in a shortened position in male rodents thereby impairing muscle contractile function. However, in female rodents the effects of oestrogen on SSN and mechanical changes following immobilization and re‐ambulation are unknown. Here we investigated the effects of oestrogen deficiency (i.e., ovariectomized; OVX model) on SSN loss, recovery and contractile function following hindlimb immobilization and 4 weeks of voluntary ambulation. In female Sprague–Dawley rats [(Intact; high oestrogen; n = 24; mass = 290.6 g ± 25.1 g), (OVX; low oestrogen; n = 24; mass = 317.8 g ± 27.3 g)], each left hind‐limb was immobilized in maximal plantar flexion (shortened muscle position) for 14 days. In vivo plantar flexor torque and power were assessed pre‐cast and at 0, 1, 2 and 4 weeks post‐cast. Six rats per group were euthanized at each post‐cast time point for measurements of soleus and medial gastrocnemius SSN. Immediately post‐cast, soleus and medial gastrocnemius SSN and plantar flexor peak torque were reduced similarly in both groups (∼16% and ∼45%, respectively; P < 0.05). After 4 weeks of re‐ambulation, intact rats demonstrated full recovery of SSN and torque, whereas OVX rats exhibited persistent deficits (−10% and −30%, respectively; P < 0.05). These findings indicate that oestrogen facilitates serial sarcomerogenesis and functional recovery following immobilization in female skeletal muscle.