Fig. 4. Proposed differences in bone remodeling and calcium metabolism between human bedrest (Shackelford et al., 2004), spinal cord injury (SCI) (Maimoun et al., 2005; Modlesky et al., 2005), hyperparathyroidism (HPT) (Brockstedt et al., 1995; Monchik and Gorgun, 2004) and hibernating bears. There is increased remodeling activity in all four conditions; however, bone resorption (Resorb) and formation (Form) appear to be balanced in hibernating bears whereas they are unbalanced in the other three conditions. This imbalance leads to increased cortical porosity and decreased bone mineral density (BMD) and cortical thickness in humans. However, cortical porosity decreases and cortical cross-sectional area and mineral content are unchanged in hibernating bears (McGee et al., 2006). As in human inactivity, resorption increases and formation decreases in hibernating bats and hamsters, leading to increased cortical porosity and decreased cortical thickness (Kwiecinski et al., 1987; Steinberg et al., 1979; Steinberg et al., 1981; Steinberg et al., 1986). In the human conditions, calcium liberated by bone resorption is excreted in urine and increases total (tCa) and ionized (iCa) serum calcium concentration. Bears, however, do not urinate during hibernation and thus it is likely that the calcium liberated by bone resorption is recycled back into bone, which is possibly facilitated by PTH increasing renal reabsorption of calcium.