The effects of hibernation and forced disuse (neurectomy) on bone properties in arctic ground squirrels

Abstract

Bone loss is a well-known medical consequence of disuse such as in long-term space flight. Immobilization in many animals mimics the effects of space flight on bone mineral density. Decreases in metabolism are also thought to contribute to a loss of skeletal mass. Hibernating mammals provide a natural model of disuse and metabolic suppression. Hibernating ground squirrels have been shown to maintain bone strength despite long periods of disuse and decreased metabolism during torpor. This study examined if the lack of bone loss during torpor was a result of the decrease in metabolic rate during torpor or an evolutionary change in these animals affording protection against disuse. We delineated changes in bone density during natural disuse (torpor) and forced disuse (sciatic neurectomy) in the hind limbs of the arctic ground squirrel (AGS) over an entire year. We hypothesized that the animals would be resistant to bone loss due to immobilization and disuse during the winter hibernation season when metabolism is depressed but not the summer active season. This hypothesis was not supported. The animals maintained bone density (dual-energy X-ray absorptiometry) and most bone structural and mechanical properties in both seasons. This was observed in both natural and forced disuse, regardless of the known metabolic rate increase during the summer. However, trabecular bone volume fraction (microcomputed tomography) in the distal femur was lower in neurectomized AGS at the study endpoint. These results demonstrate a need to better understand the relationship between skeletal load (use) and bone density that may lead to therapeutics or strategies to maintain bone density in disuse conditions.

Keywords: Bone density; disuse; hibernation; muscle atrophy; torpor.

Figure 1

Dual‐energy X‐ray absorptiometry (DXA) bone density and mineral content are maintained during the hibernation and increase during active seasons. (A) Femur region of interest (ROI). (B) Hind limb ROI. (C) Timeline of capture, surgery, DXA measurements, μ CT, and mechanical testing. Animals were captured from the wild in July, surgeries and baseline DXA measurement were obtained in early November. DXA scans though September were conducted the first week of each month. In October, the final DXA measurements were obtained and animals were euthanized. Bones were removed, cleaned of nonosseous tissue between October and December. μ CT and mechanical testing were conducted in December. Femurs from all test limbs showed similar trends of maintaining bone density (D) and mineral content (E) during the hibernation season and increasing at the start of the active season. Total hind limb bone density (F) also was maintained during hibernation and increased in the spring. Control indicates left hind limb that did not undergo any procedure. Test is the right hind limb that had sciatic neurectomy (NEUR, n = 5) or sham surgery (SHAM, n = 4). All data are mean ± SEM. Gray area denotes hibernation season during winter 2013. Statistical analysis via ANOVA followed by Tukey's post hoc test. Significance was determined at P ≤ 0.05.

Figure 2

Body mass remained unchanged with or without treatment over the course of the experiments while dual‐energy X‐ray absorptiometry hind limb fat and lean tissue fluctuate between the hibernation and active seasons. Body mass did not differ between test groups (A). Both hind limbs from each test group showed similar trends in dynamics over the hibernation timeline in fat tissue percent (B) and lean tissue mass (C). All data are mean ± SEM. Shaded area indicates the hibernation season during winter 2013. Statistical analysis via ANOVA followed by Tukey's post hoc test. Significance was determined at P ≤ 0.05. NEUR n = 5 and SHAM n = 4.