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.