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1周无蛋白饲粮使骨骼肌体重下降，而添加1.5%亮氨酸可抑制骨骼肌体重下降。以3-甲基组氨酸浓度A-V差评价的肌肉蛋白质降解也被亮氨酸添加的饲料所抑制。肌肉蛋白质合成速率没有变化。虽然添加亮氨酸的饲粮没有降低泛素蛋白酶体的表达和钙蛋白酶活性，但LC-3II的表达降低。这些结果表明，亮氨酸促进肌肉蛋白质的降解而非合成，在低营养条件下，这种降解可能受到自噬的调节。重复亮氨酸消耗对肌肉蛋白质合成和降解的影响间隔6小时给鼠亮氨酸可降低肌原纤维蛋白的降解，通过离体肌肉中3-甲基组氨酸的释放来评估。然而，通过磷酸化评价S6K1的合成没有，这表明反复补充亮氨酸对肌肉萎缩有益。亮氨酸对废用模型肌肉萎缩的影响后肢固定化增加肌肉蛋白降解，导致肌肉萎缩。在该模型中，添加亮氨酸的日粮减轻了肌肉蛋白质降解，尤其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.