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 s ynthesis in muscle following four days of resistance exercise in young (4-mo), middle-aged (12-mo), and old (32-mo) rats. Thirty-six male Fi scher 344/BN F1 rats (n = 12 in each group) performed an operantly con ditioned 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). Acu tely exercised animals engaged in four distinct exercise sessions with each session separated by 48 h. Rates of protein synthesis were asses sed in soleus, gastrocnemius (CAST), and extensor digitorum longus (ED L) muscles 16 h after the last exercise bout using a bilateral hindlim b 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 conce ntration (6.25 ng . m(-1)), while the other limb did not. Rates of pro tein synthesis in soleus with insulin supplementation were significant ly 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 synthesi s 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 synthes is in CAST with insulin were also significantly higher within all age groups following resistance exercise than ACUTE without insulin or NON EX with or without insulin (p < .05). Unlike soleus, rates of protein synthesis in GAST were significantly higher for old NONEX vs young NON EX (68 +/- 6 vs 45 +/- 5 nmol F incorporated . g(-1). h(-1), respectiv ely; 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 signif icantly greater in ACUTE with insulin than ACUTE without insulin or NO NEX with or without insulin (p < .05). There were no effects of age, i nsulin, or exercise on rates of protein synthesis in EDL (p > .05). Th ese 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 occurre d at the level of translation.