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Received May 8, 2006
Revised June 13, 2006
Accepted after revision July 24, 2006
1 University of Texas Medical Branch
* To whom correspondence should be addressed. E-mail: blrasmus{at}utmb.edu.
Resistance exercise is a potent stimulator of muscle protein synthesis and muscle cell growth with the increase in protein synthesis being detected within 2-3 hours post-exercise and remaining elevated for up to 48 hours. However, during exercise muscle protein synthesis is inhibited. An increase in AMP-activated protein kinase (AMPK) activity has recently been shown to decrease mammalian target of rapamycin (mTOR) signalling to key regulators of translation initiation. We hypothesized that the cellular mechanism for the inhibition of muscle protein synthesis during an acute bout of resistance exercise in humans would be associated with an activation of AMPK and an inhibition of downstream components of the mTOR pathway (4E-BP1 and S6K1). We studied 11 subjects (7 men, 4 women) before, during, and for two hours following a bout of resistance exercise. Muscle biopsies were collected at each time point from the vastus lateralis. We utilized immunoprecipitation and immunoblotting methods to measure muscle AMPKá2 activity, mTOR associated upstream and downstream signalling proteins, and stable isotope techniques to measure muscle fractional protein synthetic rate (FSR). AMPKá2 activity (pmol•min-1•mg of protein-1) at baseline was 1.7 ± 0.3, increased immediately post-exercise (3.0 ± 0.6), and remained elevated at 1h post-exercise (P< 0.05). Muscle FSR decreased during exercise and was significantly increased at 1h and 2h post-exercise (P<0.05). 4E-BP1 phosphorylation at Thr37/46 was significantly reduced immediately post-exercise (P<0.05). We conclude that AMPK activation and a reduced phosphorylation of 4E-BP1 may contribute to the inhibition of muscle protein synthesis during resistance exercise. However, by 1-2h post-exercise muscle protein synthesis increased in association with an activation of PKB, mTOR, S6K1, and eEF2.
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