Abstract
Tail suspension is a ground-based model for the microgravity and cephalic fluid-shift aspects of spaceflight. To evaluate osteopenia and bone metabolism induced by microgravity, 18 8-week-old female Spraque-Dawley (SD) rats completed tail suspension for 2-weeks (n=6, TS), and were compared with baseline controls (n=6, BC) and 2-week vivarium control rats (n=6, VC). The BC, VC, and TS groups were fed standard lab chow (CRF-1, Charles River, Japan) and water ad libitum while control animals (BC and VC) were pair fed with the appropriate TS group to control for caloric intake. After 2-week suspension, the rats were sacrificed and femurs were removed for analysis of bone mineral density (BMD) by dual energy X-ray absorptionmetry (DEXA, Lunar DPXL, USA) and peripheral quantitative computed tomography (pQCT, XC-960, Germany). The femoral fresh weight of the TS group was approximately 8% lower when compared with the VC group (p<0.05), but the femoral length showed no change between the VC and the TS group. On the other hand, BMD and cortical BMD of femur in the VC group were increased by approximately 16% and 4% when compared with those of the BC group (p<0.001), but those of the TS group did not increase, and were approximately 11% and 4% lower than those of the VC (p<0.01). Similarly, the cortical cross-sectional area, bone thickness, and stress-strain index (SSI), which reflect the mechanical strength of cortical bone in the VC group increased significantly (p<0.01, respectively), but those of the TS group were significantly lower than those of the VC group (p<0.01 or p<0.001). However, trabecular BMD did not show a significant difference between any groups. The serum calcium (Ca) concentration, urinary Ca, and urinary deoxypyridinoline (Dpd) excretions of the VC and the TS groups showed similar levels. However, urinary phosphate (P) excretion of the TS group dynamically decreased, being approximately 50% lower than that of the VC group (p<0.05), while serum P concentration increased (p<0.001). These results suggest that exposure to a microgravity environment induces spontaneous osteopenia in mainly cortical bone, and this osteopenia might be caused by disturbed P homeostasis.
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