Lack of influence of estrogen on myosin phosphorylation and post-tetanic potentiation in muscles from young adult C57BL mice.

Estrogen influences myosin phosphorylation and post-tetanic potentiation in murine fast muscle. We tested the hypothesis that this influence is mediated by estrogen effects on skeletal myosin light chain kinase (skMLCK) activity. To this end, extensor digitorum longus muscles from female wildtype and skMLCK-absent (skMLCK-/-) mice were grouped as follows: ovariectomized with estrogen (E+), ovariectomized without estrogen (E-), sham surgery, and intact baseline. At 8 weeks of age, the ovariectomized groups were ovariectomized followed by implantation of either a 0.1 mg 17ß-estradiol (E+) or placebo pellet (E-). Two weeks later, muscles were isolated and suspended in vitro (25° C) for determination of regulatory light chain phosphorylation and post-tetanic potentiation. Regulatory light chain phosphorylation was not different across conditions within either genotype although wildtype values were significantly greater than skMLCK-/- values. Consistent with this, the potentiation of concentric twitch force was similar between E+ and E- groups within each genotype but wildtype values were greater than skMLCK-/- values. However, unaltered estradiol levels following ovariectomy, likely due to previously underappreciated confounds of mouse age, development, and growth during estrogen supplementation, prevented direct testing of the hypothesis. Future studies should note the importance of estrous cycles and continuing physiological developments of young adult mice when working with ovarian hormones.