Amplification of the Anabolic Response of Bone to Intermittent PTH after Disuse

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Our previous work demonstrated that daily administration of parathyroid hormone (PTH) effectively accelerates the recovery of bone parameters during recovery from hindlimb unloading (HU). We have now tested PTH administration in normally weightbearing (WB) rats of the same age, gender and strain in order to test the hypothesis that a previous period of disuse will amplify the anabolic response. Mature male Sprague Dawley rats were randomized into 2 groups, WB+ VEH and WB+PTH, and given daily IP injections of saline or PTH (1–34, 80μg/kg/day), respectively, for 28 d following 28 d of single housed cage activity. In vivo peripheral quantitative computed tomography (pQCT) scans of the proximal tibia were taken at baseline, after 28 days of WB and after 28 recovery days. Static and dynamic histomorphometry were done using standard staining and fluorochrome labeling techniques. Our findings suggest that a previous period of disuse enhances the responsiveness to intermittent PTH, as indicated by a higher bone formation and a more robust increase in BMD vs. WB rats.