Different response of amino acids on muscle protein synthesis and degradation under nutritional stresses

1.低营养条件下亮氨酸的作用饲喂一周无蛋白饲料时，骨骼肌重量下降，但补充1.5%亮氨酸可抑制体重减轻。通过3-甲基组氨酸浓度的A-V差评估的肌肉蛋白质降解也被添加亮氨酸的饮食抑制。肌肉蛋白质合成速率没有变化。虽然泛素-蛋白酶体表达和钙蛋白酶活性没有减少喂养亮氨酸补充饮食，LC-3 II的表达下降。这些结果表明，在低营养条件下，亮氨酸促进肌肉蛋白质的降解而非合成，其降解可能受自噬的调控.重复摄入亮氨酸对肌肉蛋白质合成和降解的影响对大鼠间隔6小时给予亮氨酸，通过从分离的肌肉释放3-甲基组氨酸来评价，降低了肌原纤维蛋白质降解。然而，通过磷酸化S6 K1评估的合成没有，表明重复补充亮氨酸对肌肉萎缩有益。亮氨酸对废用性骨骼肌萎缩的影响后肢制动使骨骼肌蛋白质降解增加，导致骨骼肌萎缩。在该模型中，补充亮氨酸的饮食减轻了由于肌肉蛋白降解特别是泛素-蛋白酶体引起的肌肉萎缩。

1. The function of leucine under low nutritional conditions Skeletal muscle weight decreased in feeding of one week protein free diet, however the weight loss was suppressed in 1.5% leucine supplementation. Muscle protein degradation evaluated by A-V difference of 3-methylhistidine concentration was also suppressed by leucine supplemented diet. The rate of muscle protein synthesis was not changed. Although ubiquitin-proteasome expression and calpain activity were not reduced in feeding of leucine supplemented diet, LC-3II expression was decreased. These results suggested that leucine enhances muscle protein degradation rather synthesis and the degradation might be regulated by autophagy in low nutritional conditions.2. Effects of repeated leucine consumption on muscle protein synthesis and degrdation Six hours interval leucine administration to rats decreased myofibrillar protein degradation evaluated by 3-methylhistidine release from the isolated muscle. However, synthesis evaluated by phosphrylation of S6K1 did not, suggesting that repeated leucine supplementation is beneficial for muscle atrophy.3. Effects of leucine on muscle atrophy in disuse model Immobilization of hind limb increased muscle protein degradation and then resulted muscle atrophy. In this model, leucine supplemented diet attenuated muscle atrophy due to muscle protein degradation especially ubiquitin-proteasome.