Decreased bone turnover with balanced resorption and formation prevent cortical bone loss during disuse (hibernation) in grizzly bears ( Ursus arctos horribilis )

• Published in: Bone (New York, N.Y.) Vol. 42; no. 2; pp. 396 - 404
• Main Authors: McGee, Meghan E, Maki, Aaron J, Johnson, Steven E, Nelson, O. Lynne, Robbins, Charles T, Donahue, Seth W
• Format: Journal Article
• Language: English
• Published: New York, NY Elsevier Inc 01.02.2008; Elsevier Science
• Subjects: Animals; Biological and medical sciences; Bone and Bones - metabolism; Bone Density; Bone mechanics; Bone remodeling; Bone Resorption - prevention & control; Cortical bone; Diseases of the osteoarticular system; Disuse osteoporosis; Female; Fundamental and applied biological sciences. Psychology; Grizzly bear; Hibernation - physiology; Male; Medical sciences; Orthopedics; Osteogenesis - physiology; Osteoporosis - prevention & control; Osteoporosis. Osteomalacia. Paget disease; Skeleton and joints; Ursidae - physiology; Ursus arctos horribilis; Vertebrates: osteoarticular system, musculoskeletal system; Bone mechanics; Disuse osteoporosis; Bone remodeling; Grizzly bear; Cortical bone; Prevention; Bone resorption; Osteoporosis; Diseases of the osteoarticular system; Mechanics; Osteopenia
• ISSN: 8756-3282; 1873-2763
• DOI: 10.1016/j.bone.2007.10.010

Abstract Disuse uncouples bone formation from resorption, leading to increased porosity, decreased bone geometrical properties, and decreased bone mineral content which compromises bone mechanical properties and increases fracture risk. However, black bear bone properties are not adversely affected by aging despite annual periods of disuse (i.e., hibernation), which suggests that bears either prevent bone loss during disuse or lose bone and subsequently recover it at a faster rate than other animals. Here we show decreased cortical bone turnover during hibernation with balanced formation and resorption in grizzly bear femurs. Hibernating grizzly bear femurs were less porous and more mineralized, and did not demonstrate any changes in cortical bone geometry or whole bone mechanical properties compared to active grizzly bear femurs. The activation frequency of intracortical remodeling was 75% lower during hibernation than during periods of physical activity, but the normalized mineral apposition rate was unchanged. These data indicate that bone turnover decreases during hibernation, but osteons continue to refill at normal rates. There were no changes in regional variation of porosity, geometry, or remodeling indices in femurs from hibernating bears, indicating that hibernation did not preferentially affect one region of the cortex. Thus, grizzly bears prevent bone loss during disuse by decreasing bone turnover and maintaining balanced formation and resorption, which preserves bone structure and strength. These results support the idea that bears possess a biological mechanism to prevent disuse osteoporosis.