PHYSICAL EXERCISE DURING REMOBILIZATION RESTORES A NORMAL BONE TRABECULAR NETWORK AFTER TAIL SUSPENSION-INDUCED OSTEOPENIA IN YOUNG-RATS

To determine how bone recovers from immobilization-induced bone loss a nd to specify whether its recovering capacity is improved by physical exercise, 5-week-old male Wistar rats (287.07 g +/- 10.65 SD) were tai l suspended for 14 days, then returned to either normal weight-bearing (R) or controlled physical exercise for 28 days (R + E). Bone mineral density (BMD) was measured in three parts of the femur. Using histomo rphometric analysis, bone mass and architecture were estimated in the primary (1 degrees sp) and secondary spongiosa (2 degrees sp) of the p roximal tibial metaphysis. Bone cellular parameters were measured in t he 2 degrees sp of the tibia. Tail suspension induced a significant de crease in BMD, 2 degrees sp bone mass, mineral apposition rate, and bo ne formation rate and marked alterations of the trabecular network In R rats, BMD was still significantly decreased, except in the distal pa rt of the femur. Long-bone lengthening was significantly altered. The 2 degrees sp bone mass returned to the age-matched control values; how ever, the trabeculae were still significantly thinner and bone resorpt ion was significantly higher. R + E rats had a normal long bone length ening and a significant increase in 2 degrees sp bone mass and trabecu lar thickness when compared with R rats. Bone resorption was significa ntly depressed, and osteoid surfaces and thickness were significantly increased. Thus, although bone mass returns to normal values in the R group, trabecular alterations persist. Physical exercise during recove ry helps restore a normal trabecular network, and the depressed bone r esorption associated with an increased bone formation leads to an incr eased bone volume.