title: Prioritization of skeletal muscle growth for emergence from hibernation.
abstract: Mammalian hibernators provide an extreme example of naturally occurring challenges to muscle homeostasis. The annual hibernation cycle is characterized by shifts between summer euthermy with tissue anabolism and accumulation of body fat reserves, and winter heterothermy with fasting and tissue catabolism. The circannual patterns of skeletal muscle remodelling must accommodate extended inactivity during winter torpor, the motor requirements of transient winter active periods, and sustained activity following spring emergence. Muscle volume in thirteen-lined ground squirrels (Ictidomys tridecemlineatus) calculated from MRI upper hindlimb images (n=6 squirrels, n=10 serial scans) declined from hibernation onset, reaching a nadir in early February. Paradoxically, mean muscle volume rose sharply after February despite ongoing hibernation, and continued total body mass decline until April. Correspondingly, the ratio of muscle volume to body mass was steady during winter atrophy (October-February) but increased (+70%) from February to May, which significantly outpaced changes in liver or kidney examined by the same method. Generally stable myocyte cross-sectional area and density indicated that muscle remodelling is well regulated in this hibernator, despite vastly altered seasonal fuel and activity levels. Body composition analysis by echo MRI showed lean tissue preservation throughout hibernation amid declining fat mass by the end of winter. Muscle protein synthesis was 66% depressed in early but not late winter compared with a summer fasted baseline, while no significant changes were observed in the heart, liver or intestine, providing evidence that could support a transition in skeletal muscle regulation between early and late winter, prior to spring emergence and re-feeding.
authors.affiliation: Cell & Developmental Biology, University of Colorado School of Medicine, Mail Stop 8108, 12801 East 17th Avenue, Aurora, CO 80045, USA ahindle@mgh.harvard.edu.
authors.forename: Allyson G
authors.initials: AG
authors.lastname: Hindle
authors.affiliation: Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706, USA.
authors.forename: Jessica P
authors.initials: JP
authors.lastname: Otis
authors.affiliation: Cell & Developmental Biology, University of Colorado School of Medicine, Mail Stop 8108, 12801 East 17th Avenue, Aurora, CO 80045, USA.
authors.forename: L Elaine
authors.initials: LE
authors.lastname: Epperson
authors.affiliation: Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706, USA.
authors.forename: Troy A
authors.initials: TA
authors.lastname: Hornberger
authors.affiliation: Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706, USA.
authors.forename: Craig A
authors.initials: CA
authors.lastname: Goodman
authors.affiliation: Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706, USA.
authors.forename: Hannah V
authors.initials: HV
authors.lastname: Carey
authors.affiliation: Cell & Developmental Biology, University of Colorado School of Medicine, Mail Stop 8108, 12801 East 17th Avenue, Aurora, CO 80045, USA.
authors.forename: Sandra L
authors.initials: SL
authors.lastname: Martin
mesh_terms: D000818:Animals; D001835:Body Weight; D005260:Female; D006605:Hibernation / Q000502:physiology; D006614:Hindlimb; D008297:Male; D024510:Muscle Development / Q000502:physiology*; D009124:Muscle Proteins / Q000032:analysis; D018482:Muscle, Skeletal / Q000254:growth & development / Q000378:metabolism*; D009133:Muscular Atrophy; D014176:Protein Biosynthesis; D012589:Sciuridae / Q000254:growth & development / Q000502:physiology*; D012621:Seasons
chemical_list: D009124:Muscle Proteins
keywords: Disuse atrophy; Echo magnetic resonance imaging; Muscle regeneration; SUnSET; Spermophilus