Insulin stimulation of protein synthesis in rat skeletal muscle following resistance exercise is maintained with advancing age

This study examined whether or not insulin elevates rates of protein synthesis in muscle following four days of resistance exercise in young (4-mo), middle-aged (12-mo), and old (32-mo) rats. Thirty-six male Fischer 344/BN F1 rats (n = 12 in each group) performed an operantly conditioned activity which required full extension of the hindlimbs with weights over the scapula (ACUTE; n = 6 for each age group) or with no additional weight (nonexercised; NONEX; n = 6 for each age group). Acutely exercised animals engaged in four distinct exercise sessions with each session separated by 48 h. Rates of protein synthesis were assessed in soleus, gastrocnemius (CAST), and extensor digitorum longus (EDL) muscles 16 h after the last exercise bout using a bilateral hindlimb perfusion to measure the incorporation of tritiated phenylalanine (F) into muscle protein. One limb of the bilateral hind limb preparation received a medium that contained rat insulin at a physiological concentration (6.25 ng . m(-1)), while the other limb did not. Rates of protein synthesis in soleus with insulin supplementation were significantly higher within all age groups following resistance exercise vs ACUTE without insulin and NONEX with or without insulin (p < .05). Rates of protein synthesis in soleus were not different within age groups for NONEX with or without insulin (p < .05), but rates of protein synthesis for young NONEX were significantly higher (p < .05) than middle-aged or old NONEX (204 +/- 9 vs 149 +/- 6 or 141 +/- 9 nmol F incorporated . g(-1). h(-1), respectively; means +/- SE). Rates of protein synthesis in CAST with insulin were also significantly higher within all age groups following resistance exercise than ACUTE without insulin or NONEX with or without insulin (p < .05). Unlike soleus, rates of protein synthesis in GAST were significantly higher for old NONEX vs young NONEX (68 +/- 6 vs 45 +/- 5 nmol F incorporated . g(-1). h(-1), respectively; p < .05), but not middle-aged NONEX (51 +/- 3 nmol F incorporated . g(-1). h(-1)). Translational efficiency (rates of protein synthesis. unit of RNA(-1) h(-1)) for CAST supplemented with insulin was significantly greater in ACUTE with insulin than ACUTE without insulin or NONEX with or without insulin (p < .05). There were no effects of age, insulin, or exercise on rates of protein synthesis in EDL (p > .05). These data suggest that following resistance exercise, insulin increased rates of protein synthesis in both soleus and CAST regardless of age, and it appeared that this insulin-mediated elevation may have occurred at the level of translation.