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Insulin-facilitated increase of muscle protein synthesis after resistance exercise involves a MAP kinase pathway

¹Department of Health and Kinesiology, Texas A&M University, College Station, Texas; Departments of ²Geriatrics and ³Physiology and Biophysics, University of Arkansas for Medical Sciences, and ⁴Central Arkansas Veterans Health Care System, Little Rock, Arkansas; and ⁵Department of Physiology and Pharmacology, Karolinska Institute, Stockholm, Sweden

Submitted 29 November 2005 ; accepted in final form 11 January 2006

Recent studies have implicated the mTOR-signaling pathway as a primary component for muscle growth in mammals. The purpose of this investigation was to examine signaling pathways for muscle protein synthesis after resistance exercise. Sprague-Dawley rats (male, 6 mo old) were assigned to either resistance exercise or control groups. Resistance exercise was accomplished in operantly conditioned animals using a specially designed flywheel apparatus. Rats performed two sessions of resistance exercise, separated by 48 h, each consisting of 2 sets of 25 repetitions. Sixteen hours after the second session, animals were killed, and soleus muscles were examined for rates of protein synthesis with and without insulin and/or rapamycin (mTOR inhibitor) and/or PD-098059 (PD; MEK kinase inhibitor). Results of this study demonstrated that rates of synthesis were higher (P < 0.05) with insulin after exercise compared with without insulin, or to control muscles, regardless of insulin. Rapamycin lowered (P < 0.05) rates of synthesis in controls, with or without insulin, and after exercise without insulin. However, insulin was able to overcome the inhibition of rapamycin after exercise (P < 0.05). PD had no effect on protein synthesis in control rats, but the addition of PD to exercised muscle resulted in lower (P < 0.05) rates of synthesis, and this inhibition was not rescued by insulin. Western blot analyses demonstrated that the inhibitors used in the present study were selective and effective for preventing activation of specific signaling proteins. Together, these results suggest that the insulin-facilitated increase of muscle protein synthesis after resistance exercise requires multiple signaling pathways.

mammalian target of rapamycin; rapamycin; extracellular signal-related kinases; PD-098059

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