The purpose of this study was to determine the extent to which hindlimb muscles of mdx mice adapt to a voluntary endurance type of exercise. mdx and C57BL mice engaged in 8 weeks of wheel running or maintained normal cage activities. Beneficial adaptations that occurred in mdx mice included changes in muscle mass, fiber size, and fiber types based on myosin heavy chain (MHC) isoform expression. These adaptations occurred without increases in fiber central nuclei and embryonic MHC expression. An undesirable outcome, however, was that muscle mitochondrial enzyme activities did not improve with exercise in mdx mice as they did in C57BL mice. Cellular remodeling of dystrophic muscle following exercise has not been studied adequately. In this study we found that some, but not all, of the expected adaptations occurred in mdx mouse muscle. We must better understand these (non)adaptations in order to inform individuals with DMD about the benefits of exercise.

Estradiol (E₂) deficiency decreases muscle strength and wheel running in female mice. It is not known if the muscle weakness results directly from the loss of E₂ or indirectly from mice becoming relatively inactive with presumably diminished muscle activity. The first aim of this study was to determine if cage activities of ovariectomized mice with and without E₂ treatment differ. Ovariectomized mice were 19-46% less active than E₂-replaced mice in terms of ambulation, jumping, and time spent being active (P ≤ 0.033). After E₂-deficient mice were found to have low cage activities, the second aim was to determine if E₂ is beneficial to muscle contractility, independent of physical activities by the mouse or its hindlimb muscles. Adult, female mice were ovariectomized or sham-operated and randomized to receive E₂ or placebo and then subjected to conditions that should maintain physical and muscle activity at a constant low level. After 2 wk of hindlimb suspension or unilateral tibial nerve transection, muscle contractile function was assessed. Soleus muscles of hindlimb-suspended ovariectomized mice generated 31% lower normalized (relative to muscle contractile protein content) maximal isometric force than suspended mice with intact ovaries (P ≤ 0.049). Irrespective of whether the soleus muscle was innervated, muscles from ovariectomized mice generated ∼20% lower absolute and normalized maximal isometric forces, as well as power, than E₂-replaced mice (P ≤ 0.004). In conclusion, E₂ affects muscle force generation, even when muscle activity is equalized.

Estradiol (E2) treatment in young adult, ovariectomized mice increases physical activity and reverses deleterious effects on skeletal muscle. Here we test the hypothesis that E2 treatment improves muscle function and physical activity in aged, ovarian-senescent mice. Plasma E2 levels and vaginal cytology confirmed ovarian senescence in 20-month-old C57BL/6 mice. Mice were then randomly divided into activity groups, having access to a running wheel or not, and further into those receiving E2 or placebo. Placebo-treated mice wheel ran more than E2-treated mice (P=0.03), with no difference between treatment groups in cage activities such as time spent being active and ambulation distance (P≥0.55). Soleus muscles from aged mice that wheel ran adapted by getting larger and stronger, irrespective of E2 status (P≤0.02). Soleus muscle fatigue resistance was greater in mice treated with E2 (P=0.02), but maximal isometric tetanic force was not affected (P≥0.79). Because E2 treatment did not improve physical activity or overall muscle function in the aged, ovarian-senescent mice as predicted, a second study was initiated to examine E2 treatment of young adult mice prematurely ovarian senescent from exposure to the chemical, 4-vinylcyclohexene diepoxide (VCD). 4-month-old C57BL/6 female mice were dosed with oil (control) or VCD. Vaginal cytology confirmed ovarian senescence in all mice treated with VCD 63 days after the onset of dosing, and then a subset of the VCD mice received E2 (VCD+E2). Wheel running distance did not differ among control, VCD, and VCD+E2 mice (P≥0.34). Soleus muscle concentric, isometric, and eccentric in vitro forces were greater in VCD+E2 than VCD mice (P<0.04), indicating beneficial estrogenic effects on muscle function. In general, aged and young mice with senescent ovaries were less responsive to E2 treatment, in terms of physical activities and muscle function, than what has previously been shown for young, ovariectomized mice. These results bring forth the possibility that some component of the residual, follicle-depleted ovarian tissue influences physical activity in mice or that aging diminishes the responsiveness of skeletal muscle and related tissues to E2 treatment.

Purpose The primary objective of this study was to determine if strength loss and recovery following eccentric contractions is impaired in healthy and dystrophic female mice with low levels of ovarian hormones. Methods Female C57BL/6 (wildtype) or mdx mice were randomly assigned to ovarian-intact (Sham) and ovariectomized (Ovx) groups. Anterior crural muscles were tested for susceptibility to injury from 150 or 50 eccentric contractions in wildtype and mdx mice, respectively. An additional experiment challenged mdx mice with a 2-wk treadmill running protocol followed by an eccentric contraction injury to posterior crural muscles. Functional recovery from injury was evaluated in wildtype mice by measuring isometric torque 3, 7, 14, or 21 days following injury. Results Ovarian hormone deficiency in wildtype mice did not impact susceptibility to injury as the ~50% isometric torque loss following eccentric contractions did not differ between Sham and Ovx mice (p=0.121). Similarly in mdx mice, hormone deficiency did not affect percent of pre injury isometric torque lost by anterior crural muscles following eccentric contractions (p=0.952), but the percent of pre injury torque in posterior crural muscles was lower in Ovx compared to Sham mice (p=0.014). Recovery from injury in wildtype mice was affected by hormone deficiency. Sham mice recovered pre injury isometric strength by 14 days (96 ± 2%) while Ovx mice maintained deficits at 14 and 21 days post injury (80 ± 3% and 84 ± 2%; p<0.001) Conclusion Ovarian hormone status did not impact the vulnerability of skeletal muscle to strength loss following eccentric contractions. However, ovarian hormone deficiency did impair the recovery of muscle strength in female mice.