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 bo nes, humerus, and skull were removed for the determination of morphome try and bone mineral content (BMC) and density (BMD) with dual-energy X-ray absorptiometry, dry and ash weights, and calcium content. The bo nes 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 app ear to impair the long-bone elongation rate. However, the tibia of sus pended rats exhibited a lower calcium content, ash weight, BMC, and BM D. Similar trends were observed in the femur. In the humerus, no signi ficant 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 ream bulation period, the body weights showed no difference between the con trol and experimental animals. The bone alterations were not completel y reversible compared with their respective controls. In the tibia, BM C and BMD were always decreased. In the femur, trends toward low value s were still visible. The skull showed a decrease in BMC and ash and d ry weights. This unexpected finding suggested the importance of a rapi d decrease in cephalad fluid shift at the time of desuspension. Finall y, we showed that dual-energy X-ray absorptiometry measurement is suff iciently accurate to detect intergroup differences.