Black bears (Ursus americanus) are able to hibernate for up to seven months with no negative effects on their bone mass, whereas in humans, prolonged inactivity causes weakening of bones. Scientists are studying the physiology of hibernating bears with the hope of discovering how their specialized physiological processes might lead to new therapies for treating osteoporosis.
Disuse Osteoporosis is Bone Loss Caused by Inactivity
Prolonged inactivity such as extended bed rest or weightlessness experienced by astronauts during spaceflights causes bones to become weak and brittle, a condition known as disuse osteoporosis. Bone loss occurs through the process of resorption, in which cells called osteoclasts attach to bone and dissolve it, releasing the calcium into the bloodstream.
To counteract bone loss, cells called osteoblasts form new bone from collagen and calcium and phosphate mineral salts. Bones are continuously being dissolved and remade by these two processes. If the rate of resorption exceeds the rate of bone formation, as happens with aging and inactivity, bones lose overall mass, resulting in osteoporosis.
Bone Formation and Resorption are Controlled by Hormones
A complex interaction of hormones and receptors is involved in the regulation of bone turnover. The rates of resorption and bone formation are primarily controlled by three calcium-regulating hormones: calcitonin, calcitriol and parathyroid hormone. Calcitonin fosters bone formation, while parathyroid hormone acts indirectly through cell surface receptors to stimulate osteoclast activity. Calcitriol works with parathyroid hormone to bring about resorption.
Other regulatory factors include norepinephrine, a hormone that mediates osteoclast formation; osteocalcin, a protein found in bone that binds calcium; and leptin, a hormone that affects the function of osteoblasts.
Bone Loss Does Not Occur in Hibernating Bears
Researchers at Michigan Technological University report that unlike the bones of humans, bear bones maintain mass and strength during long periods of inactivity, such as experienced during hibernation when they remain inactive for five to seven months. In contrast to some other types of hibernating animals such as bats and ground squirrels that awaken periodically to eat and excrete waste, bears remain asleep throughout the entire winter.
Research is underway to determine what aspects of the metabolism of hibernating bears allow them to maintain bone mass during such long periods of inactivity. While disuse normally results in decreased rates of bone formation, in hibernating bears the rate of resorption is also decreased, balancing the two and reducing overall bone turnover.
Studies have shown that the levels of various hormones differ between hibernating and active bears. Serum concentrations of norepinephrine are decreased in hibernating bears, while osteocalcin levels are elevated. Osteocalcin is normally excreted in the urine and accumulates in the blood in cases of impaired kidney function. Since bears do not urinate during hibernation, osteocalcin levels increase. Parathyroid hormone levels correlate with osteocalcin, and may contribute to the reabsorption of calcium by the kidneys.
Another possible contributing factor to the maintenance of bone mass in hibernating bears is the sequence of bear parathyroid hormone, which differs from that of human parathyroid hormone by nine amino acids. Research has indicated that different forms of parathyroid hormone differ in their effect on bone metabolism.
Further research using hibernating bears as a model to shed light on the mechanisms of bone metabolism may lead to improved therapies for the treatment of osteoporosis or other bone diseases.
Sources:
Young, Stacey. "Bear Bones." Accessed March 16, 2010.
Meghan McGee-Lawrence, Hannah Carey and Seth Donahue. "Mammalian hibernation as a model of disuse osteoporosis: the effects of physical inactivity on bone metabolism, structure and strength." Am J Physio Regulatory Integrative Comp Physiol 295: 1999-2014, 2008. Accessed March 16, 2010.
Terrie Schultz - Terrie Schultz holds a BS in Biochemistry and an MA in Cell and Molecular Biology. She worked for many years in the biomedical field in ...
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