Abstract
BackgroundWe previously reported early tissue injury, increased serum and tissue inflammatory cytokines and decreased grip in young rats performing a moderate demand repetitive task. The tissue cytokine response was transient, the serum response and decreased grip were still evident by 8 weeks. Thus, here, we examined their levels at 12 weeks in young rats. Since aging is known to enhance serum cytokine levels, we also examined aged rats.MethodsAged and young rats, 14 mo and 2.5 mo of age at onset, respectfully, were trained 15 min/day for 4 weeks, and then performed a high repetition, low force (HRLF) reaching and grasping task for 2 hours/day, for 12 weeks. Serum was assayed for 6 cytokines: IL-1alpha, IL-6, IFN-gamma, TNF-alpha, MIP2, IL-10. Grip strength was assayed, since we have previously shown an inverse correlation between grip strength and serum inflammatory cytokines. Results were compared to naïve (grip), and normal, food-restricted and trained-only controls.ResultsSerum cytokines were higher overall in aged than young rats, with increases in IL-1alpha, IFN-gamma and IL-6 in aged Trained and 12-week HRLF rats, compared to young Trained and HRLF rats (p < 0.05 and p < 0.001, respectively, each). IL-6 was also increased in aged 12-week HRLF versus aged normal controls (p < 0.05). Serum IFN-gamma and MIP2 levels were also increased in young 6-week HRLF rats, but no cytokines were above baseline levels in young 12-week HRLF rats. Grip strength declined in both young and aged 12-week HRLF rats, compared to naïve and normal controls (p < 0.05 each), but these declines correlated only with IL-6 levels in aged rats (r = -0.39).ConclusionAging enhanced a serum cytokine response in general, a response that was even greater with repetitive task performance. Grip strength was adversely affected by task performance in both age groups, but was apparently influenced by factors other than serum cytokine levels in young rats.
Highlights
We previously reported early tissue injury, increased serum and tissue inflammatory cytokines and decreased grip in young rats performing a moderate demand repetitive task
Serum cytokines increase with training and HRLF task performance in aged rats only We observed significant differences by age, with higher levels of IL-1a, IL-6, and IFN-g in aged rats than young rats (p < 0.001 each; Figure 2)
The correlation results in the aged rats in this study are similar to prior studies from our own lab in which we have previously found an inverse correlation between increasing serum tumor necrosis factor alpha (TNFa), macrophage inflammatory protein 2 (MIP2) and MIP3 levels and grip strength in young rats performing a HRNF task for 8 weeks [13]
Summary
We previously reported early tissue injury, increased serum and tissue inflammatory cytokines and decreased grip in young rats performing a moderate demand repetitive task. Work-related repetitive strain injuries, known as work-related musculoskeletal disorders, repetitive motion injuries and overuse injuries, are common injuries They are injuries of the musculoskeletal and/or nervous systems caused by repetitive tasks, forceful exertions, vibrations, mechanical compression (pressing against hard surfaces), or sustained or awkward positions [1,2]. All of these disorders can be worsened by the repetitive actions of daily living [3]. In the United Kingdom, 115,000 new cases of work-related repetitive strain injuries in the upper extremity were reported in 2006-2007 compared with 86,000 new cases in 2005-2006, and the number of people reporting such injuries rose from 374,000 to
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