Augmented insulin action on rates of protein synthesis after resistance exercise in rats

This study investigated whether insulin has a modulatory effect on protein synthesis rates in skeletal muscle after four sessions of resistance exercise. Male rats engaged in resistance exercise (Acute) that required full extension of the hindlimbs with weights over the scapula or performed the standing movement with no additional weight (Nonex). Two separate studies were conducted. Rates of protein synthesis for study 1 (Acute, n = 6; Nonex, n = 6) were assessed 16 h postexercise by incorporation of [3H]phenylalanine ([3H]F) into muscle protein by use of an in vivo flooding dose protocol. Rates of protein synthesis in soleus of Acute (100 +/- 9 nmol F.g-1.h-1) were significantly higher than in Nonex (72 +/- 9 nmol F.g-1.h-1, P < 0.05). Rates of protein synthesis were significantly higher in gastrocnemius of Acute vs. Nonex (48 +/- 7 vs. 25 +/- 2 nmol F.g-1.h-1) but not in extensor digitorum longus (EDL). Assessment of protein synthesis rates for study 2 was conducted 16 h after resistance exercise with use of [3H]F incorporation into muscle protein during in situ bilateral hindlimb perfusion, with each leg perfused simultaneously but separately. Perfusion medium for one leg, but not the other, contained insulin (6.25 ng/ml). Soleus and gastrocnemius of Acute had higher protein synthesis rates than Nonex only in the leg that received insulin. For gastrocnemius, rates of protein synthesis in Acute without insulin were significantly lower than in Nonex with or without insulin. Insulin had no effect on protein synthesis rates for any muscle in Nonex rats. Neither exercise nor insulin affected protein synthesis rates in EDL. We conclude that insulin is a necessary component in elevated protein synthesis rates after resistance exercise in muscles composed of primarily slow-or fast-twitch fibers, and that a physiological perturbation (resistance exercise in this study) is required to observe such modulation, because rates of protein synthesis in Nonex muscles were not influenced by insulin.