BONE CHANGES IN 6-MO-OLD RATS AFTER HEAD-DOWN SUSPENSION AND A REAMBULATION PERIOD

In mature rats experiencing 14-day head-down suspension or 14-day head-down suspension followed by 28-day reambulation, the hindlimb long bones, humerus, and skull were removed for the determination of morphometry and bone mineral content (BMC) and density (BMD) with dual-energy X-ray absorptiometry, dry and ash weights, and calcium content. The bones of the animals in the control groups (killed at days 0, 14, and 42) had their own maturation rate. The body weights of suspended animals were lower than those of the control animals. Suspension does not appear to impair the long-bone elongation rate. However, the tibia of suspended rats exhibited a lower calcium content, ash weight, BMC, and BMD. Similar trends were observed in the femur. In the humerus, no significant change was observed. In the skull, the values of the suspended rats were similar to those of the control rats. At the end of the reambulation period, the body weights showed no difference between the control and experimental animals. The bone alterations were not completely reversible compared with their respective controls. In the tibia, BMC and BMD were always decreased. In the femur, trends toward low values were still visible. The skull showed a decrease in BMC and ash and dry weights. This unexpected finding suggested the importance of a rapid decrease in cephalad fluid shift at the time of desuspension. Finally, we showed that dual-energy X-ray absorptiometry measurement is sufficiently accurate to detect intergroup differences.