Proceedings of The Physiological Society
King's College London (2009) Proc Physiol Soc 14, PC34
Acute resistance exercise results in increased leucine transport and amino acid transporter expression
M. G. MacKenzie1, D. Hamilton1, P. Taylor1, K. Baar1
1. Molecular Physiology, University of Dundee, Dundee, United Kingdom.
Following an acute bout of resistance exercise in either humans or rodents intramuscular levels of amino acids ([aa]i), for example leucine, lycine and glutamine, increase. This increase is associated with both accelerated protein synthesis and degradation and we hypothesize that the increased [aa]i levels are a direct consequence of high force muscle contractions. The mechanism as to how this increase in [aa]i is induced is currently unknown; conceivably, it could be the result of increased protein breakdown, increased amino acid transporter level/activity, or a combination of both. Since the Na2+-independent (LAT1) and Na2+-dependent (SNAT2) amino acid transporters carry leucine and glutamine respectively, we hypothesized that they would be responsive to resistance exercise. To investigate this hypothesis, rats underwent a unilateral bout of resistance exercise where the right sciatic nerve was electrically stimulated (under isoflurane anaesthesia) using a Grass stimulator at a frequency of 100 Hz, 6-12 V, 1 ms duration, 9ms delay for 10 sets of 6 repetitions. Each repetition lasted 2s, a 10s recovery was permitted between repetitions, and a 1min recovery was allowed between sets resulting in a 20 min stimulation protocol. 0.5, 1.5, 3, 6, 18 and 48 hours following the acute bout of contractions, stimulated and contralateral control muscles were rapidly removed and snap frozen in liquid nitrogen. mRNA and protein levels of LAT1 and SNAT2 as well as 5 other amino acid transporters were determined by RT-PCR and western blotting throughout the time course. LAT1 transcription increases 5.15±0.84 fold 3h after exercise and stays high for up to 48h (2.56±0.19 fold). SNAT2 expression increases at 30mins (1.73±0.11 fold) and peaks at 6h (2.56±0.19 fold). The other transporters did not change. LAT1 protein increased 32±12% in the exercised muscle 90mins after contraction while SNAT2 protein levels were unchanged. In addition, c-Myc, a key transcription factor involved in control of cell growth and connected to the nutrient-sensing signalling network, precedes LAT1, increasing immediately after exercise (2.05±0.2 fold), peaking at 3h (7.47±0.43 fold) and staying high for 48h (1.87±0.43 fold). Interestingly we have identified a potential c-myc binding site in the LAT1 promoter. As c-myc activity is known to be increased by mTOR, through the degradation of MAD1, this may provide a mechanism for increased LAT1 expression and the positive effects of amino acids on protein synthesis following resistance exercise.
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