Assessment of biomarkers of protein anabolism in skeletal muscle during the life span of the rat: sarcopenia despite elevated protein synthesis

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Abstract
Loss of muscle strength is a principal factor in the development of physical frailty, a condition clinically associated with increased risk of bone fractures, impairments in the activities of daily living, and loss of independence in older humans. A primary determinant in the decline in muscle strength that occurs during aging is a loss of muscle mass, which could occur through a reduction in the rate of protein synthesis, an elevation in protein degradation, or a combination of both. In the present study, rates of protein synthesis and the relative expression and function of various biomarkers involved in the initiation of mRNA translation in skeletal muscle were examined at different times throughout the life span of the rat. It was found that between 1 and 6 mo of age, body weight increased fourfold. However, by 6 mo, gastrocnemius protein synthesis and RNA content per gram of muscle were lower than values observed in 1-mo-old rats. Moreover, the relative expression of two proteins involved in the binding of initiator methionyl-tRNA to the 40S ribosomal subunit, eukaryotic initiation factors (eIF)2 and eIF2B, as well as the 70-kDa ribosomal protein S6 kinase, S6K1, was lower at 6 mo compared with 1 mo of age. Muscle mass, protein synthesis, and the aforementioned biomarkers remained unchanged until ∼21 mo. Between 21 and 24 mo of age, muscle mass decreased precipitously. Surprisingly, during this period protein synthesis, relative RNA content, eIF2B activity, relative eIF2 expression, and S6K1 phosphorylation all increased. The results are consistent with a model wherein protein synthesis is enhanced during aging in a futile attempt to maintain muscle mass.