Estrogen hormones are implicated in influencing skeletal muscle contractile function and specifically, the interactions between myosin and actin that facilitate contraction. Myosin regulatory light chain (RLC) phosphorylation (RLC-p) is highly associated with an increase in the percentage of myosin proteins strongly bound to actin during contractile activity. Myosin RLC-p is also highly associated with post-tetanic potentiation (PTP) i.e. the short-term increase in force observed following tetanic stimulation characteristic of fast-twitch skeletal muscle. The purpose of this study was to examine the influence of ovarian hormone deficiency and 17β-Estradiol (E2) replacement, on the PTP of concentric twitch force and myosin RLC-p of mouse fast twitch skeletal muscle. To this end, 4-month-old wildtype C57BL/6 mice were allocated to the ovariectomized (OVX), ovariectomized with E2 replacement (OVX+E), or sham-ovariectomized (SHAM) condition (n=8 mice). Extensor digitorum longus (EDL) muscles were surgically extracted and mounted for in vitro contractile experiments at 25° C involving the use of a brief potentiating stimulus (PS) to induce PTP. The PS, consisting of four equally spaced tetani (400Hz, 100ms) over 10.5s, significantly increased concentric twitch force and myosin RLC-p levels across condition groups. However, no significant differences in PTP, myosin RLC-p, or rates of force development and relaxation were observed between muscles of the OVX, OVX+E and SHAM conditions. A significant drop in relative tetanic force amongst the first and second tetanus within the PS was observed of muscles from OVX mice compared to that of OVX+E and SHAM mice. Specific force, calculated as the ratio of force to muscle PCSA, total work, peak work, and power production, were significantly increased following the PS across conditions, and were significantly less of muscles from OVX mice compared to OVX+E and SHAM, both prior to and following the PS. Data are contrary to the primary hypothesis that E2 influences myosin RLC-p and PTP in fast twitch skeletal muscle, though are suggestive of an influence of OVX and E2 on skeletal muscle quality
