1630 Introduction Many clinical and experimental conditions that mechanically unload bone cause disuse osteoporosis and increase fracture risk. For example, disuse due to spinal cord lesion significantly decreases bone mineral density (BMD) in the tibia and femur (Dauty et al., 2000), and significantly reduces the cross-sectional moment of inertia of the femoral diaphysis (Modlesky et al., 2005). Thus, bone bending strength is reduced by spinal cord injury and fracture risk is increased (Wang et al., 2001). These disuse-induced structural changes to bone are due to unbalanced bone remodeling, which typically increases bone resorption and decreases bone formation (Rantakokko et al., 1999; Weinreb et al., 1989). Inactivity-induced changes in bone remodeling are reflected in serum and urinary markers of bone turnover. Human bedrest increases bone resorption, leading to increased urinary calcium excretion and decreased BMD (Arnaud et al., 1992; Inoue et al., 2000; LeBlanc et al., 1995; Watanabe et al., 2004; Zerwekh Mechanical unloading of bone causes an imbalance in bone formation and resorption leading to bone loss and increased fracture risk. Black bears (Ursus americanus) are inactive for up to six months during hibernation, yet bone mineral content and strength do not decrease with disuse or aging. To test whether hibernating bears have biological mechanisms to prevent disuse osteoporosis, we measured the serum concentrations of hormones and growth factors involved in bone metabolism and correlated them with the serum concentration of a bone formation marker (osteocalcin). Serum was obtained from black bears over a 7-month duration that included periods of activity and inactivity. Both resorption and formation markers increased during hibernation, suggesting high bone turnover occurred during inactivity. However, bone formation appeared to be balanced with bone resorption. The serum concentration of parathyroid hormone (PTH) was higher in the hibernation (P=0.35) and post- hibernation (P=0.006) seasons relative to pre-hibernation levels. Serum leptin was lower (P<0.004) post-hibernation relative to pre-hibernation and hibernation periods. Insulin-like growth factor I (IGF-I) decreased (P<0.0001) during hibernation relative to pre-hibernation and reached its highest value during remobilization. There was no difference (P=0.64) in 25-OH vitamin D between the three seasons. Serum osteocalcin (bone formation marker) was significantly correlated with PTH, but not with leptin, IGF-I or 25-OH vitamin D. Osteocalcin and PTH were positively correlated when samples from all seasons were pooled and when only hibernation samples were considered, raising the possibility that the anabolic actions of PTH help maintain bone formation to prevent disuse osteoporosis. Prostaglandin E 2 (PGE 2 ) release from MC3T3 osteoblastic cells was significantly affected by treatment with bear serum from different seasons (i.e. hibernation versus active periods). The seasonal changes in PGE 2 release showed trends similar to the seasonal changes in serum IGF-I. Since both PGE 2 and IGF-I are associated with collagenous bone formation, it is possible that seasonal changes in a circulating factor influence IGF-I levels in vivo in bears and PGE 2 release in osteoblastic cells in vitro. The significant decrease in serum leptin following arousal from hibernation may promote bone formation during remobilization, assuming there is a similar decrease in intracerebroventricular leptin. These findings support the idea that seasonal changes in the concentration of circulating molecules help regulate bone formation activity and may be important for preventing disuse osteoporosis in bears. Key words: black bear, Ursus americanus, bone remodeling, disuse osteoporosis, hibernation, leptin, IGF-I, PTH. Summary The Journal of Experimental Biology 209, 1630-1638 Published by The Company of Biologists 2006 doi:10.1242/jeb.02185 Parathyroid hormone may maintain bone formation in hibernating black bears (Ursus americanus) to prevent disuse osteoporosis Seth W. Donahue 1, *, Sarah A. Galley 1 , Michael R. Vaughan 2 , Patricia Patterson-Buckendahl 3 , Laurence M. Demers 4 , Josef L. Vance 1 and Meghan E. McGee 1 1 Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA, 2 Virginia Polytechnic Institute and State University, Blacksburg, VA, USA, 3 Rutgers University, Piscataway, NJ, USA and 4 The Pennsylvania State University, Hershey, PA, USA *Author for correspondence (e-mail: swdonahu@mtu.edu) Accepted 22 February 2006 THE JOURNAL OF EXPERIMENTAL BIOLOGY