Skeletal Muscle Eukaryotic Elongation Factor 2 (eEF2) Response to Acute Resistance Exercise in Young and Old Men and Women : Relationship to Muscle Glycogen Content and 5'-AMP-Activated Protein Kinase (AMPK) Activity

Abstract

Sarcopenia is associated with an age-related decrease in skeletal muscle mass, which can result in decreases in strength and physical functioning in the older population. Resistance training interventions are not completely effective in stimulating muscle protein synthesis in aged muscle and thus do not completely combat age-related atrophy. Phosphorylation (and thus theoretically activation) of the energy-sensing molecule 5'-AMP-activated protein kinase (AMPK, known to inhibit the muscle protein synthesis pathway) has been shown to be elevated for up to three hours in response to a resistance training bout in the muscles of older, but not younger individuals. Data in rats indicate that, in response to muscle contractions, elevated AMPK activity can accentuate the inhibitory phosphorylation (and thus deactivation) of its downstream intermediate, eukaryotic elongation factor 2 (eEF2, which normally stimulates protein translation and synthesis). AMPK activity is inhibited by high muscle glycogen levels. Interestingly, older individuals exhibit a lower muscle glycogen content compared to younger individuals, which may account for the greater AMPK phosphorylation response to resistance exercise in older individuals. The relationship between muscle glycogen content, AMPK activity, and eEF2 phosphorylation in response to an acute bout of resistance exercise has not yet been examined in young or old individuals. We hypothesized that inhibitory eEF2 phosphorylation would be higher in response to an acute resistance exercise bout in the skeletal muscles of older versus younger individuals. We further hypothesized that this higher eEF2 phosphorylation response would be related to a higher AMPK activation, and that higher AMPK activation would be related to lower glycogen content, in the skeletal muscles of older versus younger individuals. Seven young (21.7 ± 0.8 yrs) and 10 old (67.0 ± 2.6 yrs) untrained but physically active men and women performed 3 sets of leg extensions at a 10-repetition maximum resistance until failure after an overnight fast. Muscle biopsies were obtained from the vastus lateralis pre-exercise (PRE), immediately post exercise (0P), 1-hour post exercise (1P), and 2-hours post exercise (2P). Glycogen content was measured in muscle samples, as were the phosphorylations, by western blot, of AMPK (Thr172), acetyl-CoA carboxylase (ACC, a marker of in vivo AMPK activity) (Ser79), and eEF2 (Thr56). Muscle glycogen content was significantly lower in the old vs. young subjects at the PRE time point and decreased in response to exercise in both age groups; however, glycogen content decreased to a greater degree in young subjects such that it was equal between young and old at all post-exercise timepoints. As expected, AMPK phosphorylation was significantly increased in the old subjects immediately post exercise, but no such response was noted in the young. However, no age-related differences were observed in AMPK activity as measured by ACC phosphorylation, which was significantly elevated at 0P and 1P in both age groups. Similarly, the eEF2 phosphorylation response (elevated vs. PRE at 0P and decreased vs. PRE at 1P and 2P in both age groups) was also not affected by age. Regardless of age, higher muscle glycogen content was associated with lower AMPK activity (as assessed by phospho-ACC content) at 0P and 1P, and this lower AMPK activity was associated with lower inhibitory phosphorylation of eEF2 at those same timepoints. These findings indicate the possibility that higher muscle glycogen content may result in lower AMPK activation and consequently lower inhibitory eEF2 phosphorylation in response to a resistance training session in the muscles of both younger and older individuals, thereby potentially enabling greater translation elongation, protein synthesis, and muscle growth regardless of age.