Acute insulin is not necessary for post exercise elevations of muscle protein synthesis after 5 weeks of resistance exercise training

Abstract

In rats, insulin elevates rates of muscle protein synthesis (RPS) following resistance exercise (RE), and the acute effect of insulin on RPS persists after 4 weeks of training. The purpose of this study was to determine if administration of insulin elevates RPS after acute or chronic RE in human skeletal muscle. Eight men performed unilateral RE (knee extensions; 4 sets of 7 repetitions, 2 or 3 days/week), while the other leg served as a non-exercised control. The RE leg was familiarized (3 sessions) prior to training. The initial experiment studied the effect of familiarization on RPS following the first bout of exercise. For this and the secondary experiment, both legs engaged in a single bout of RE 16 h prior to taking a muscle biopsy from the m. vastus lateralis of each leg. Individual muscle bundles were placed in specially designed clips to maintain length and assessed for RPS, with and without insulin, in vitro. Results demonstrated that the post exercise elevation of RPS after the first bout of RE only occurs in the presence of insulin, regardless of familiarization. However, after 5 weeks of RE, RPS was elevated in the trained leg, with or without insulin, but only in the presence of insulin in the untrained leg. These results suggest that insulin may be important for enhancing skeletal muscle RPS at the onset of exercise, but the importance of available insulin for post exercise elevations of RPS dissipates with training.