Effects And Mechanism Of Aerobic Exercise Training In Combination With Leucine Supplementation On Skeletal Muscle Protein Synthesis In Asing Mice

Objective:This research aims to evaluate whether the aerobic exercise training can be a countermeasure to combat age-related muscle loss by promoting skeletal muscle protein synthesis or not, and also to investigate if exercise in combination with leucine supplementation will result in greater benefits.Methods:Thirteen-month-old ICR mice were randomly divided into the following groups:sedentary control (AlaC), exercise intervention (AlaE), leucine supplementation (LeuC), exercise combined with leucine supplementation (LeuE). Mice in LeuC and LeuE groups received daily Leucine meal that5.0%free leucine was added in, AlaC and AlaE received an Alanine meal by substituting leucine for an isonitrogenous amount of alanine (3.4%) which will not affect the protein turnover. Trained mice performed45min,2.8%-3%body weight load swimming bout daily from Monday to Friday, and the intervention period was8weeks. All the mice were sacrificed24h after the final bout (17h fasting). The blood glucose was determined with a portable blood glucose meter, serum insulin concentration was determined by ELISA, plasma free amino acids were determined with an amino acid analyzer, skeletal muscle morphology was observed by using hematoxylin and eosin staining, mRNA expression of LAT1, CD98, PAT1, SNAT2and GCN2were evaluated by Real-time quantitative RT-PCR, protein expression of PI3K III and MHC II as well as the phosphorylation rate of mTOR, p70S6K,4E-BP1and AMPK were detected by the method of Western blotting.Results:Compared to mice from sedentary control group, aerobic exercise training decreased the body weight, blood glucose concentration (p<0.05) and the phosphorylation of AMPK (p<0.01), while there was a growing trend for plasma free amino acids, gastrocnemius weight, gastrocnemius weight/body weight ratio, gastrocnemius fiber cross section area and diameter. As for mRNA expression of LAT1(p<0.01), CD98(p<0.01) and PAT1(p<0.05), protein expression of PI3K III(p<0.05), MHC II (p<0.01) as well as the phosphorylation rate of mTOR (p<0.01), p70S6K (p<0.01) and4E-BP1(p<0.01) were significantly elevated. The serum insulin concentration, mRNA expression level of SNAT2and GCN2were unchanged. Leucine supplementation also showed similar effects, but blood glucose values and mRNA expression of PAT1showed a growing trend, AMPK phosphorylation just showed a downward trend. There was a synergistic effect of aerobic exercise and leucine supplementation. The changing trend of amino acids, mRNA and protein expression as well as the phosphorylation rate was consistent with the exercise group. But, gastrocnemius weight (p<0.05), gastrocnemius fiber CSA (p<0.01) and diameter (p<0.01) were both increased significantly.Conclusions:Both aerobic exercise training and leucine supplementation can stimulate skeletal muscle protein synthesis, and exercise in combination with leucine result in better effects. As for the mechanism, leucine activates mTOR singling pathway mainly through the increasing of PI3K IE phosphorylation, but the inhibitory effect on AMPK is not significant. However, aerobic exercise training shows a more widespread role in stimulating protein synthesis by activating PI3K III and inhibiting AMPK synchronously. L-and imino amino acid transporter systems are both involved in mTOR regulation but the mechanism is unclear. It has been speculated that this could be related to their function as a dual-function regulator of intracellular signaling.