Aging-related effects of bed rest followed by eccentric exercise rehabilitation on skeletal muscle macrophages and insulin sensitivity

Abstract

The pro- and anti-inflammatory macrophages are associated with insulin sensitivity and skeletal muscle regeneration. Infiltrating macrophages in skeletal muscle during a period of physical inactivity and subsequent reloading/rehabilitation in older adults is unknown, but may provide insight into mechanisms related to the development of metabolic disease and changes in muscle cell size. The purpose of this study was to determine if skeletal muscle macrophage infiltration is modulated differently between young and older adults after bed rest and exercise rehabilitation and if these responses are related to muscle and insulin sensitivity changes. 14 young and 9 older adults underwent 5-days of bed rest followed by 8-weeks of lower limb eccentric exercise rehabilitation (REHAB). Dual-energy X-ray absorptiometry, magnetic resonance imaging and myofiber analysis were used to identify muscle morphology and CLIX-IR and CLIX-β were used to assess insulin sensitivity. Skeletal muscle macrophages, CD68 (pan), CD11b (M1), CD163 (M2), CD206 (M2), were characterized using immunohistochemistry and gene expression. Insulin sensitivity, independent of age, decreased ~38% following bed rest and was restored following REHAB. We found robust age-related differences in muscle atrophy during bed rest, yet older and younger adults equally hypertrophied during REHAB. Interestingly, there were age-related differences in macrophage content (CD68+CD11b+ and CD68+CD11b- cells) but both young and old similarly increased macrophages with REHAB. Satellite cell changes during rehab corresponded to macrophage content changes. Muscle tissue resident macrophages and gene expression, were not associated with changes in insulin sensitivity following bed rest and REHAB. These data suggest that muscle macrophages are modulated as a result of exercise rehabilitation following bed rest and may more associated with muscle regrowth/hypertrophy rather than insulin sensitivity in young or older adults. This trial was registered at clinicaltrials.gov as NCT01669590.

Keywords: Aging; Atrophy; Eccentric exercise; Immune cells; M1; M2; Muscle-resident; Recovery; Regrowth; Rehabilitation; Training.

Conflict of interest statement

Conflicts of interest

The authors have no conflicts of interest.

Copyright © 2017 Elsevier Inc. All rights reserved.

Figures

Figure 1

Changes (PRE TO BED REST, BED REST TO REHAB or PRE TO REHAB) in quadriceps muscle CSA (assessed by MRI) (A,B), leg lean mass (assessed by DXA) (C,D), vastus lateralis mean myofiber CSA (E,F), knee extension power (G,H) and trunk lean mass (assessed by DXA) (I,J)) as mean ± SEM bar graphs between older and younger adults after BED REST and following rehabilitation (REHAB). A,C,E,G and I depict mean responses while B,D,F,H, J depicts the individual responses.

Figure 2

Clamp-like index as CLIX-IR (A) and CLIX-β (B) as a proxy of insulin sensitivity in older and younger adults before (PRE) and following BED REST and rehabilitation (REHAB). Data are mean ± SEM. * vs PRE, # vs BED REST as p

Figure 3

Vastus lateralis myofiber-type specific satellite cell content in older and younger adults at PRE and following BED REST and rehabilitation (REHAB). Data are mean ± SEM. ˆ for age difference, * vs PRE, # vs BED REST and & vs MHC II as p<0.05 unless indicated otherwise.

Figure 4

Vastus lateralis macrophage marker (CD68, CD11b, CD206, and CD163) immunofluorescence in older and younger adults at PRE and following BED REST and rehabilitation (REHAB). Data are mean ± SEM. ˆ for age difference, * vs PRE, # vs BED REST as p<0.05 unless indicated otherwise.

Figure 5

Vastus lateralis mRNA expression of macrophage markers CD68 (A), IL-10 (B), TNFα (C), IL-4Rα (D), NOS3 (E), TGFβ1 (F), COX-2 (G) and IL-1β (H) in older and younger adults at PRE and following BED REST and rehabilitation (REHAB). Data are mean ± SEM. * vs PRE, # vs BED REST as p<0.05 unless indicated otherwise.