Bone response to normal weight bearing after a period of skeletal unloading

Skeletal unloading in the growing rat induces a temporary inhibition of bone formation and thereby a deficit in bone calcium compared with age-matched, normally loaded animals. To determine whether this deficit can be restored by skeletal reloading we measured bone formation rate at the tibiofibular junction and total bone calcium in the tibia and lumbar vertebra in rats whose hindlimbs were unloaded for 2 wk and then reloaded by return to normal weight bearing. Continuously loaded or unloaded animals were also studied. Skeletal unloading reduced bone formation by 34% and tibial and vertebral calcium by 12 and 22%, respectively. Reloading significantly increased the rates of bone formation and calcium accretion 30-34% above normally loaded animals, and by 2 wk had decreased the deficit in tibial and vertebral calcium by 36 and 23%, respectively. These data indicate that the deficit in bone calcium induced by skeletal unloading in the growing rat can be restored in part by return to normal weight bearing. However, the time required to restore bone calcium exceeds the time required to produce the original calcium deficit.