C - Michigan Technological University

ears must have evolved a mechanism to combat disuse-induced bone loss duringhibernation. This mechanism must conserve the balance between bone resorption andformation such that there is no net change in bone accrual. Resorption and formationmay remain in equilibrium in three ways: 1) neither the bone resorption nor the formationrate changes; 2) resorption and formation rates increase with the same acceleration;3) resorption and formation rates decrease with the same deceleration. Analyses ofserum markers of bone turnover and histomorphometric indices of cellular activities arewidely used methods to ascertain whether resorption and formation rates have changed.1.7.4 Bone response to hibernation—bears: serum formation markers andhistomorphometrySerum markers for bone formation (osteocalcin (OCN), carboxy-terminal ofpropeptide of type I procollagen (PICP)) and resorption (carboxy-terminal cross-linkedtelopeptide of type I procollagen (ICTP)) exhibit a balanced increase in the hibernatingcompared to the active bear [34, 133, 134]. Furthermore, concentrations of calcium inthe blood remain seasonally constant, although bears do not consume or excretecalcium during hibernation [35, 135]. These data suggest that bone resorption increasesduring hibernation, but that mobilized calcium is returned to the bone by increasedosteoblast activity. In contrast, histomorphometric indices of trabecular and cortical bonesuggest that although bone resorption and formation remain balanced duringhibernation, the osteoclast and osteoblast activities are both dramatically reduced duringwinter sleep [18, 20]. These paradoxical findings could be explained in a few ways. Oneexplanation is that although bone turnover decreases in the locations of biopsies, the netturnover of the entire skeleton increases during hibernation. However, the biopsies werecollected from several locations, and all of them suggest a sharp reduction in turnover[18, 20]. Furthermore, an increase in bone turnover with no net change in bone quantityor quality would be an energetically costly effort for no apparent benefit. During thisextended period of fasting, a reduction in bone metabolism is more advantageous.13