Department of Health and Kinesiology, Texas A&M University , College Station, Texas.
Department of Mechanical Engineering, Texas A&M University , College Station, Texas.
J Appl Physiol (1985). 2018 Nov 1;125(5):1456-1467. doi: 10.1152/japplphysiol.00736.2017. Epub 2018 Aug 9.
Mechanical unloading has long been understood to contribute to rapid and substantial adaptations within skeletal muscle, most notably, muscle atrophy. Studies have often demonstrated that many of the alterations resulting from disuse are reversed with a reintroduction of load and have supported the concept of muscle plasticity. We hypothesized that adaptations during disuse and recovery were a repeatable/reproducible phenomenon, which we tested with repeated changes in mechanical load. Rats were assigned to one of the following five groups: animals undergoing one or two bouts of hindlimb unloading (28 days), with or without recovery (56 day), or control. Following the completion of their final time point, posterior crural muscles were studied. Muscle sizes were lower following 28 days of disuse but fully recovered with a 56-day reloading period, regardless of the number of disuse/recovery cycles. Mixed protein fractional synthesis rates consistently reflected mass and loading conditions (supported by anabolic signaling), whereas the myofibrillar protein synthesis response varied among muscles. Amino acid concentrations were assessed in the gastrocnemius free pool and did not correlate with muscle atrophy associated with mechanical unloading. Muscle collagen concentrations were higher following the second unloading period and remained elevated following 56 days of recovery. Anabolic responses to alterations in load are preserved throughout multiple perturbations, but repeated periods of unloading may cause additive strain to muscle structure (collagen). This study suggests that whereas mass and anabolism are reproducibly reflective of the loading environment, repeated exposure to unloading and/or reloading may impact the overall structural integrity of muscle. NEW & NOTEWORTHY Repeatability should be considered a component of skeletal muscle plasticity during atrophy and recovery. Muscle anabolism is equally affected during a first or second disuse bout and returns equally with adequate recovery. Elevated muscle collagen concentrations observed after the second unloading period suggest altered structural integrity with repeated disuse.
机械卸载长期以来被认为有助于骨骼肌肉的快速和实质性适应，尤其是肌肉萎缩。研究经常表明，许多由于停用而导致的改变在用负荷重新引入时会得到逆转，并支持肌肉可塑性的概念。我们假设停用和恢复期间的适应是可重复的/可复制的现象，我们用机械负荷的反复变化来检验这一假设。将大鼠分配到以下五个组之一：接受一次或两次后肢卸载（28 天）的动物，有或没有恢复（56 天），或对照。在完成最后一个时间点后，研究了后肢肌肉。在 28 天的停用后，肌肉大小降低，但在 56 天的再加载期间完全恢复，无论停用/恢复周期的数量如何。混合蛋白分数合成率始终反映肌肉大小和加载条件（由合成代谢信号支持），而肌原纤维蛋白合成反应在不同肌肉之间存在差异。在比目鱼肌游离池中评估了氨基酸浓度，与机械卸载相关的肌肉萎缩无关。在第二次卸载后，肌肉胶原浓度升高，在 56 天的恢复后仍保持升高。对负荷变化的合成代谢反应在多次干扰中得以保留，但反复的卸载期可能会对肌肉结构（胶原）造成附加的压力。这项研究表明，尽管质量和合成代谢对负荷环境具有可重复性，但反复暴露于卸载和/或再加载可能会影响肌肉的整体结构完整性。新的和值得注意的是，在萎缩和恢复期间，可重复性应被视为骨骼肌肉可塑性的一个组成部分。在第一次或第二次停用期间，肌肉合成代谢同样受到影响，并且在适当恢复时同样恢复。在第二次卸载后观察到的肌肉胶原浓度升高表明，反复停用会导致结构完整性改变。
