The Effects of 17β-estradiol on Skeletal Muscle

The maintenance of skeletal muscle mass and function is critical for healthy aging. Muscle loss with disuse, termed muscle disuse muscle atrophy, leads to impaired functional capacity, the onset of insulin resistance, as well as a heightened risk for morbidity and mortality. With advancing age there is a chronic wasting of muscle. This is especially true in women, where rapid rates of decline in muscle mass and greater anabolic resistance are experienced around the time of menopause, despite higher protein synthesis rates. As women have a longer life expectancy, they are particularly venerable to age-related frailty and morbidity. Skeletal muscle protein turnover serves to maintain the optimal function of proteins and also provides plasticity of the tissue during altered demands such as during increased loading or unloading of the muscle. Reduced periods of physical activity also have a similar, albeit milder, impact on skeletal muscle and most, people will likely experience multiple bouts of skeletal muscle disuse during their lifetime from which some, particularly older adult women, will fail to fully recover. Thus, muscle disuse atrophy is a significant and continuing problem as reclamation of lost muscle mass, strength/function, and potentially metabolic health (particularly insulin-induced glucose disposal), following disuse is oftentimes incomplete and may be further exacerbated after menopause. Previous evidence has demonstrated that in the loss of muscle mass is less pronounced in post-menopausal women when receiving hormone replacement therapy. Skeletal muscle has estrogen-β-receptors on the cell membrane, in the cytoplasm and on the nuclear membrane, and therefore a direct mechanistic link between low estrogen levels and a decrease MPS. Interestingly, despite higher rates of protein synthesis, older women still lose muscle mass with advancing age. It has been suggested that the negative muscle protein balance is due to an enhanced rate of MPB. Insulin is