Physical exercise during remobilization restores a normal bone trabecular network after tail suspension-induced osteopenia in young rats

Abstract

To determine how bone recovers from immobilization‐induced bone loss and to specify whether its recovering capacity is improved by physical exercise, 5‐week‐old male Wistar rats (287.07 g ± 10.65 SD) were tail 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 histomorphometric analysis, bone mass and architecture were estimated in the primary (1°sp) and secondary spongiosa (2°sp) of the proximal tibial metaphysis. Bone cellular parameters were measured in the 2°sp of the tibia. Tail suspension induced a significant decrease in BMD, 2°sp bone mass, mineral apposition rate, and bone formation rate and marked alterations of the trabecular network. In R rats, BMD was still significantly decreased, except in the distal part of the femur. Long‐bone lengthening was significantly altered. The 2°sp bone mass returned to the age‐matched control values; however, the trabeculae were still significantly thinner and bone resorption was significantly higher. R + E rats had a normal long bone lengthening and a significant increase in 2°sp bone mass and trabecular thickness when compared with R rats. Bone resorption was significantly 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 recovery helps restore a normal trabecular network, and the depressed bone resorption associated with an increased bone formation leads to an increased bone volume.