Effects of exercise and immobilization on bone formation and resorption in young rats.

The influence of physical activity on bone formation and resorption was studied in the following three groups of 6-wk-old female rats: 30 controls, 24 sciatic denervation immobilized, and 28 treadmill exercised. Bone formation and resorption were determined from 45Ca retention, 45Ca excretion, bone calcium content, bone volume, and resorbing surface and bone formation rate assessed by tetracycline labeling. 45Ca (30 microCi) was administered intravenously to each animal before study, and the excretion of isotope in the urine and feces was then determined during the 6-wk experimental period. Exercise resulted in an initial increase in total excretion of 45Ca (P < 0.01) followed by a drop to below control levels (P < 0.001). The femoral 45Ca retention and calcium content of the exercised group were higher than that of the control group at week 6. Periosteal bone formation rate in the tibia was enhanced during days 32-41 (P < 0.01). With immobilization, the weekly excretion of 45Ca was persistently higher (P < 0.01), and the femoral 45Ca retention (P < 0.05) and calcium content (P < 0.01) were lower than the control group. Periosteal and endosteal bone formation rates were lower than in the controls over the first 31 days. In conclusion, exercise in young growing rats is associated with an initial increase and then a decrease in bone resorption while active bone formation is sustained. Immobilization for 6 wk results in an increase in bone resorption and a rapid fall in bone formation.