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J Physiol. 2015 Sep 15;593(18):4259-73. doi: 10.1113/JP270699. Epub 2015 Jul 31.

Age-related differences in lean mass, protein synthesis and skeletal muscle markers of proteolysis after bed rest and exercise rehabilitation.

Author information

1
Division of Nutrition, Salt Lake City, UT, USA.
2
Department of Physical Therapy, University of Utah, Salt Lake City, UT, USA.
3
Institute of Sports Medicine, Department of Orthopedic Surgery M, Bispebjerg Hospital and Centre for Healthy Aging, Faculty of Health Sciences, University of Copenhagen, Denmark.
4
Exercise and Sport Science, University of Utah, Salt Lake City, UT, USA.
5
Cardiovascular Medicine, University of Utah, Salt Lake City, UT, USA.
6
Department of Internal Medicine, University of Texas Medical Branch, Galveston, TX, USA.

Abstract

Bed rest-induced muscle loss and impaired muscle recovery may contribute to age-related sarcopenia. It is unknown if there are age-related differences in muscle mass and muscle anabolic and catabolic responses to bed rest. A secondary objective was to determine if rehabilitation could reverse bed rest responses. Nine older and fourteen young adults participated in a 5-day bed rest challenge (BED REST). This was followed by 8 weeks of high intensity resistance exercise (REHAB). Leg lean mass (via dual-energy X-ray absorptiometry; DXA) and strength were determined. Muscle biopsies were collected during a constant stable isotope infusion in the postabsorptive state and after essential amino acid (EAA) ingestion on three occasions: before (PRE), after bed rest and after rehabilitation. Samples were assessed for protein synthesis, mTORC1 signalling, REDD1/2 expression and molecular markers related to muscle proteolysis (MURF1, MAFBX, AMPKα, LC3II/I, Beclin1). We found that leg lean mass and strength decreased in older but not younger adults after bedrest (P < 0.05) and was restored after rehabilitation. EAA-induced mTORC1 signalling and protein synthesis increased before bed rest in both age groups (P < 0.05). Although both groups had blunted mTORC1 signalling, increased REDD2 and MURF1 mRNA after bedrest, only older adults had reduced EAA-induced protein synthesis rates and increased MAFBX mRNA, p-AMPKα and the LC3II/I ratio (P < 0.05). We conclude that older adults are more susceptible than young persons to muscle loss after short-term bed rest. This may be partially explained by a combined suppression of protein synthesis and a marginal increase in proteolytic markers. Finally, rehabilitation restored bed rest-induced deficits in lean mass and strength in older adults.

PMID:
26173027
PMCID:
PMC4594296
DOI:
10.1113/JP270699
[Indexed for MEDLINE]
Free PMC Article

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