![]() This suggests that bears have unique mechanisms to prevent disuse-induced bone loss and reduce muscle atrophy during the inactivity of hibernation. An important adaptive consequence is that bears maintain skeleton function and preserve mobility during and after winter hibernation. Although bears are largely inactive during hibernation, they show no loss in bone mass and less loss in muscle mass and strength than would be anticipated over such a prolonged period of physical inactivity ( Harlow et al. This unbalance in resorption/formation leads to increased serum and urinary calcium concentrations ( Watanabe et al. Disuse-induced bone loss occurs by increases in osteoclastic bone resorption and/or decreases in osteoblastic bone formation ( Zerwekh et al. Physical inactivity decreases mechanical load on skeleton, which when prolonged leads to losses of muscle and bone mass and strength in non-hibernating mammalian species ( Kaneps et al. Black bears hibernate for up to 6 months each year, and during hibernation, bears remain largely immobile and do not eat, drink, urinate, defecate, and they reduce metabolic rate by 20–50%, yet maintain core body temperatures above 30☌ ( Nelson 1980 Tøien et al. Mammalian hibernation is an adaptation involving metabolic suppression to conserve energy during periods of low food availability in highly seasonal or unpredictable environments. Elevated expression of multiple anabolic genes without induction of bone resorption genes, and the down regulation of apoptosis-related genes, likely contribute to the adaptive mechanism that preserves bone mass and structure through prolonged periods of immobility during hibernation. No coordinated directional changes were detected for genes involved in bone resorption, although some genes responsible for osteoclast formation and differentiation ( Ostf1, Rab9a, and c-Fos) were significantly underexpressed in bone of hibernating bears. Apoptosis genes demonstrated a tendency for downregulation during hibernation. The Gene Ontology and Gene Set Enrichment Analysis showed an elevated proportion in hibernating bears of overexpressed genes in six functional sets of genes involved in anabolic processes of tissue morphogenesis and development including skeletal development, cartilage development, and bone biosynthesis. A total of 241 genes were differentially expressed ( P1.4) in the ilium bone of bears between winter and summer. To obtain insight into molecular mechanisms preventing disuse bone loss, we conducted a large-scale screen of transcriptional changes in trabecular bone comparing winter hibernating and summer non-hibernating black bears using a custom 12,800 probe cDNA microarray. Although bears are largely inactive during hibernation, they show no loss in bone mass and strength. Physical inactivity reduces mechanical load on the skeleton, which leads to losses of bone mass and strength in non-hibernating mammalian species.
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