Burini R, Santidrian S, Moreyra M, Brown P, Munro H N, Young V R
Metabolism. 1981 Jul;30(7):679-87. doi: 10.1016/0026-0495(81)90083-4.
The effects of thyroid status and of dietary protein and energy content on muscle protein breakdown have been compared in young rats, using urinary N tau-methylhistidine (3-methylhistidine; 3-Mehis) output as a measure of myofibrillar protein breakdown. Recently, thyroidectomized young rats receiving an adequate diet grew slowly and showed a reduction in 3-Mehis output, relative to the weight of their leg muscles. When the thyroidectomized rats were given 2-5 microgram thyroxine (T4) per 100 g body weight, output of 3-Mehis increased, even when growth was prevented by feeding a diet low in protein. This demonstrates that thyroid-status affects muscle protein breakdown through a mechanism other than via a change in growth rate. In confirmation of previous studies, administration of a diet low in protein to young rats prevented growth and reduced output of 3-Mehis relative to leg muscle weight. The reduction in protein breakdown is not likely to be due to reduced thyroid function, since the diet low in protein increased plasma total triiodothyronine (T3) levels. Furthermore, thyroidectomized rats fed the low-protein diet also underwent a decrease in output of 3-Mehis. In contrast to protein deficiency, a moderate dietary energy deficiency allowed some growth to occur, and output of 3-Mehis, relative to muscle mass, was not depressed. Nevertheless, total T3 concentration in the plasma of these animals was considerably reduced, indicating once more that total circulating thyroid hormone levels are not the only determinants of muscle protein breakdown. Finally, reduced plasma total T3 levels occurred in energy-deficient diets even in thyroidectomized rats receiving exogenous T4. This suggests that diet affects metabolism of the hormone independently of changes in secretory output from the thyroid gland.
在幼鼠中，已比较了甲状腺状态以及饮食中蛋白质和能量含量对肌肉蛋白质分解的影响，采用尿N-τ-甲基组氨酸（3-甲基组氨酸；3-Mehis）排出量作为肌原纤维蛋白分解的指标。最近，接受充足饮食的甲状腺切除幼鼠生长缓慢，且相对于腿部肌肉重量，其3-Mehis排出量减少。当给甲状腺切除的大鼠按每100克体重给予2 - 5微克甲状腺素（T4）时，即使通过喂食低蛋白饮食来阻止生长，3-Mehis的排出量仍会增加。这表明甲状腺状态通过一种不同于生长速率变化的机制影响肌肉蛋白质分解。正如先前研究所证实的，给幼鼠喂食低蛋白饮食会阻止其生长，并相对于腿部肌肉重量降低3-Mehis的排出量。蛋白质分解的减少不太可能是由于甲状腺功能降低，因为低蛋白饮食会增加血浆总三碘甲状腺原氨酸（T3）水平。此外，喂食低蛋白饮食的甲状腺切除大鼠3-Mehis的排出量也会减少。与蛋白质缺乏相反，适度的饮食能量缺乏允许一些生长发生，并且相对于肌肉质量，3-Mehis的排出量并未降低。然而，这些动物血浆中的总T3浓度显著降低，再次表明循环甲状腺激素的总水平并非肌肉蛋白质分解的唯一决定因素。最后，即使在接受外源性T4的甲状腺切除大鼠中，能量缺乏饮食也会导致血浆总T3水平降低。这表明饮食对激素代谢的影响独立于甲状腺分泌量的变化。
