Abstract
Anabolic resistance to feeding in aged muscle is well-characterized; however, whether old skeletal muscle is intrinsically resistant to acute mechanical loading is less clear. The aim of this study was to determine the impact of aging on muscle protein synthesis (MPS), ribosome biogenesis, and protein breakdown in skeletal muscle following a single bout of resistance exercise. Adult male F344/BN rats aged 10 (Adult) and 30 (Old) months underwent unilateral maximal eccentric contractions of the hindlimb. Precursor rRNA increased early post-exercise (6–18 h), preceding elevations in ribosomal mass at 48 h in Adult and Old; there were no age-related differences in these responses. MPS increased early post-exercise in both Adult and Old; however, at 48 h of recovery, MPS returned to baseline in Old but not Adult. This abbreviated protein synthesis response in Old was associated with decreased levels of IRS1 protein and increased BiP, CHOP and eIF2α levels. Other than these responses, anabolic signaling was similar in Adult and Old muscle in the acute recovery phase. Basal proteasome activity was lower in Old, and resistance exercise did not increase the activity of either the ATP-dependent or independent proteasome, or autophagy (Cathepsin L activity) in either Adult or Old muscle. We conclude that MPS and ribosome biogenesis in response to maximal resistance exercise in old skeletal muscle are initially intact; however, the MPS response is abbreviated in Old, which may be the result of ER stress and/or blunted exercise-induced potentiation of the MPS response to feeding.
Highlights
Skeletal muscle anabolic resistance is a term that describes a reduced anabolic response to a given stimulus, and is often used in an age-related context – i.e., old individuals exhibit anabolic resistance compared with young (Cuthbertson et al, 2005)
Muscle protein synthesis was moderately elevated (18–33%) after resistance exercise, and this response was more sustained in adult animals (Adult > Old at 48 h, P = 0.046; Figure 1A)
Increases in muscle protein synthesis (MPS) at 6 and 18 h after exercise were similar between Adult and Old; MPS was significantly greater in Adult than Old late in recovery (48 h). mechanistic/mammalian target of rapamycin complex 1 (mTORC1) signaling was similar in Adult and Old after resistance exercise; levels of IRS-1 were significantly lower in Old animals
Summary
Skeletal muscle anabolic resistance is a term that describes a reduced anabolic response to a given stimulus (for example: feeding, resistance exercise, or chronic loading), and is often used in an age-related context – i.e., old individuals exhibit anabolic resistance compared with young (Cuthbertson et al, 2005). Old individuals are reported to exhibit a reduced ability to drive their motor units at high muscle contraction intensities (Kamen et al, 1995). It is unclear from human studies the degree to which sarcopenia is the result of intrinsic and/or extrinsic factors – i.e., the contribution of the aging process per se vs age-related reductions in habitual physical activity or motor unit activation (Clarke, 2004)
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