Department of Physical Therapy, College of Health Sciences, University of Kentucky, Lexington, USA.
Center for Muscle Biology, University of Kentucky, Lexington, USA.
J Gerontol A Biol Sci Med Sci. 2021 Nov 15;76(12):2132-2140. doi: 10.1093/gerona/glab186.
The inability to fully recover lost muscle mass following periods of disuse atrophy predisposes older adults to lost independence and poor quality of life. We have previously shown that mechanotherapy at a moderate load (4.5 N) enhances muscle mass recovery following atrophy in adult, but not older adult rats. We propose that elevated transverse stiffness in aged muscle inhibits the growth response to mechanotherapy and hypothesize that a higher load (7.6 N) will overcome this resistance to mechanical stimuli. F344/BN adult and older adult male rats underwent 14 days of hindlimb suspension, followed by 7 days of recovery with (RE + M) or without (RE) mechanotherapy at 7.6 N on gastrocnemius muscle. The 7.6 N load was determined by measuring transverse passive stiffness and linearly scaling up from 4.5 N. No differences in protein turnover or mean fiber cross-sectional area were observed between RE and RE + M for older adult rats or adult rats at 7.6 N. However, there was a higher number of small muscle fibers present in older adult, but not adult rats, which was explained by a 16-fold increase in the frequency of small fibers expressing embryonic myosin heavy chain. Elevated central nucleation, satellite cell abundance, and dystrophin-/laminin+ fibers were present in older adult rats only following 7.6 N, while 4.5 N did not induce damage at either age. We conclude that age is an important variable when considering load used during mechanotherapy and age-related transverse stiffness may predispose older adults to damage during the recovery period following disuse atrophy.
在经历失用性萎缩后，肌肉无法完全恢复丢失的质量，这使得老年人容易丧失独立性和生活质量下降。我们之前的研究表明，中等负荷（4.5N）的机械疗法可促进成年大鼠萎缩后肌肉质量的恢复，但对老年大鼠无效。我们提出，老年肌肉的横向僵硬度抑制了机械疗法的生长反应，并假设更高的负荷（7.6N）将克服这种对机械刺激的阻力。F344/BN 成年和老年雄性大鼠接受 14 天的后肢悬吊，然后在 7 天的恢复期间（RE+M）或不进行机械疗法（RE），对腓肠肌施加 7.6N 的负荷。7.6N 的负荷是通过测量横向被动刚度并从 4.5N 线性扩展来确定的。在 7.6N 时，老年大鼠和成年大鼠的 RE 和 RE+M 之间，蛋白质周转率或平均纤维横截面积没有差异。然而，老年大鼠存在更多的小纤维，而成年大鼠则没有，这可以通过小纤维表达胚胎肌球蛋白重链的频率增加 16 倍来解释。只有在 7.6N 时，老年大鼠才出现较高的中央核形成、卫星细胞丰度和抗肌萎缩蛋白/层粘连蛋白+纤维，而在两个年龄组中，4.5N 均未引起损伤。我们的结论是，在考虑机械疗法中使用的负荷时，年龄是一个重要的变量，与年龄相关的横向僵硬度可能使老年人在失用性萎缩后的恢复期间容易受到损伤。
