Immobilization and its effect on bone repletion in calcium-deficient rats fed a high calcium diet

In past work we demonstrated that, in growing rats, endosteal bone loss during a low calcium diet (depletion) was replaced when the animals were fed a normal calcium diet (repletion). Because past work showed that bone mechanical stress associated with weight bearing was increased les a result of bone depletion, the present study was undertaken to determine if immobilization during the repletion period would prevent bone repletion. In rats fed a 0% calcium diet (containing normal dietary phosphorus) for 12 days, there was a 460% increase in endosteal resorbing surface and a 38% increase in medullary area. When such animals were then fed a 1.2% calcium diet (normal phosphorus) for 15 days, the amount of endosteal resorbing surface decreased below the basal level to 0. Much of the endosteal resorbing surface was actually converted to forming surface, and les a result there was a 75% increase in the rate of endosteal bone formation compared with control (bone replete) animals. Bone replete animals subjected to immobilization of the left hind limb for 15 days by nerve transection had decreases of 31% and 33% in periosteal and endosteal bone formation rates, respectively, in the immobilized tibia. However, immobilization instituted at the start of the bone repletion period did not prevent or diminish the increment in the endosteal bone formation response during bone repletion over the intervening 15 days. Thus, the immobilized tibia from bone repleting rats exhibited an endosteal bone formation rate 128% greater than that in the immobilized limb of control (bone replete) rats. This increase was not less than that seen in bone repleting rats not subjected to immobilization. Thus, calcium repletion, even without mechanical stress, was sufficient to cause a marked increase in endosteal bone formation.