Changes in the markers of bone metabolism following skeletal unloading

To elucidate the mechanism in disuse bone atrophy induced by skeletal unloading, we studied the indices of bone resorption and bone formation in the femur of tail-suspended rats. The duration of the suspension ranged from 1 to 14 days. Tartrate-resistant acid phosphatase mRNA, an index used to evaluate bone resorption, increased significantly more than the controls for the first 3 days of the tail-suspension experiments, compared those in controls. Osteocalcin and alkaline phosphatase, two common markers for bone formation, were also monitored. Osteocalcin mRNA started to decrease after 3 days of suspension. Five days later, alkaline phosphatase mRNA showed a decrease. Levels of both of these mRNAs remained low for the remaining suspension period. Sequential changes in the markers for bone metabolism indicate that the transient increase in bone resorption preceded the decrease in bone formation in the development of disuse bone atrophy induced by skeletal unloading.