Estradiol treatment, physical activity, and muscle function in ovarian-senescent mice

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Estradiol (E 2) treatment in young adult, ovariectomized mice increases physical activity and reverses deleterious effects on skeletal muscle. Here we test the hypothesis that E 2 treatment improves muscle function and physical activity in aged, ovarian-senescent mice. Plasma E 2 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 E 2 or placebo. Placebo-treated mice wheel ran more than E 2-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 E 2 status ( P ≤ 0.02). Soleus muscle fatigue resistance was greater in mice treated with E 2 ( P = 0.02), but maximal isometric tetanic force was not affected ( P ≥ 0.79). Because E 2 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 E 2 treatment of young adult mice prematurely ovarian senescent from exposure to the chemical, 4-vinylcyclohexene diepoxide (VCD). Four-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 E 2 (VCD + E 2). Wheel running distance did not differ among control, VCD, and VCD + E 2 mice ( P ≥ 0.34). Soleus muscle concentric, isometric, and eccentric in vitro forces were greater in VCD + E 2 than in 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 E 2 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 E 2 treatment.