MATURE RAT SKELETAL CHANGES IN RESPONSE TO SIMULATED MICROGRAVITY: A GENDER COMPARISON : Medicine & Science in Sports & Exercise

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MATURE RAT SKELETAL CHANGES IN RESPONSE TO SIMULATED MICROGRAVITY

A GENDER COMPARISON

Allen, M R.1; Bloomfield, S A. FACSM1

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Medicine & Science in Sports & Exercise 33(5):p S299, May 2001.
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Previous studies have documented decrements in tibial bone density and bone formation rate with long-term hindlimb unloading (HU) in mature adult male rats, yet have not examined the response of adult female rats to HU. Therefore, the purpose of this study was to compare the bone response to 28-d HU in male and female Sprague-Dawley rats (6 mo at sacrifice). Bone mineral density (BMD) and area were determined by peripheral quantitative computed tomography (pQCT; Stratec XCT-M, Norland Corp) at the proximal tibia, a site composed of both cortical and cancellous bone. Percent labeled periosteal surface (%LABEL), a measure of bone formation, was assessed near the tibiafibula junction (TFJ; a pure cortical bone site) using fluorochrome labeling. Two-way analyses of variance were performed within gender between 28-d HU (HuM, n = 8; HuF, n = 7) and 28-d cage activity controls (CnM, n = 8; CnF, n = 8) using a significance level of p < 0.05. Both male and female HU groups had significantly reduced soleus weight (−52% and −53%, respectively, vs. controls) confirming effective HU. Over 28-d, CnM body weight significantly increased while HuM weight was unchanged. Neither HuF nor CnF changed body weight over 28-d. Proximal tibia pQCT showed that HuM had significantly lower cancellous BMD (−19%), cortical BMD (−3%), total area (−11%), and cortical area (−9%), compared to CnM. Interestingly, no changes in BMD nor area at the proximal tibia were found in the HuF group vs. CnF. A significantly lower %LABEL was observed in both HuM (−55%) and HuF (−68%) compared to their controls, suggesting declines in bone mineralization rate after 28-d HU. These data suggest that there are gender-specific responses to HU in adult rats at sites composed of both cortical and cancellous bone, with female rats not exhibiting the decrements in BMD nor bone area observed in males. However, both genders experience similar declines in mineralization at the purely cortical site we studied. Further studies are necessary to determine if adult female rats continue to show resistance to change at cortical/cancellous sites beyond 28-d of HU. Supported by NSBRI #NCC9-58H, #2.

©2001The American College of Sports Medicine