TREATMENT WITH GROWTH-HORMONE AND IGF-I IN GROWING RATS INCREASES BONE-MINERAL CONTENT BUT NOT BONE-MINERAL DENSITY

Human growth hormone (hGH) and insulin-like growth factor I (IGF-I) bo th stimulate bone formation and have been proposed as therapeutic agen ts for osteoporosis. We examined the effect of hGH and IGF-I alone and in combination on bone size, bone mineral content (BMC), and bone min eral density (BMD) in 10- to 12-week-old growing female Sprague-Dawley rats. Sixty rats were assigned to treatment with either placebo, hGH, IGF-I or both for 4 weeks. After 4 weeks, the right femurs and tibias were excised, and ex vivo BMC and the area of the tibia and femur wer e measured by dual-energy X-ray absorptiometry (DXA); volume of these bones was measured by Archinedes' principle. In addition, proximal tib ial bone density was measured directly by peripheral quantitative comp uterized tomography (pQCT), Bone length, area, and volume in all treat ed groups was greater than controls, Areal bone density by DXA (BMC/ar ea) was higher in IGF-treated rats and lower in GH-treated rats than i n controls, Volumetric bone density (BMC/volume) was lower in treated groups than in controls, Measurements by pQCT confirmed that true bone density was lower in all treated groups than in controls, We conclude that treatment with hGH or IGF-I increased bone size and mineral cont ent but decreased bone density in growing rats, Because areal correcti on of BMC did not adequately correct for the increased bone volume in IGF-treated rats, results of areal bone density by DXA should be inter preted with caution when treatment causes a disparity in bone size bet ween groups.