BONE TURNOVER AND CALCIUM-METABOLISM DURING 20 DAYS BED REST IN YOUNGHEALTHY-MALES AND FEMALES

Bone is a dynamic tissue that functions not only as a mechanical suppo rt, but also as a major component of the metabolic and endocrine syste ms maintaining mineral homeostasis. It has been shown that immobilizat ion induces decalcification of bone. To evaluate the effect of immobil ization on bone mineral density and calcium metabolism, we investigate d 9 young healthy males and females during 20 days bed rest. Three met hods for measuring bone mineral density were performed to quantify who le body and regional bone changes: 1) dual energy X-ray absorptiometry , 2) quantitative computed tomography, and 3) multiple scanning X-ray photodensitometry, respectively. Bone mineral density showed a rapid d ecreasing tendency, especially in both lumbar and metacarpal bones (me an+/-SE: 4.6+/-0.6% and 3.6+/-0.4%, respectively). Urinary daily excre tion of deoxypyridinoline, a sensitive marker of bone matrix resorptio n, tended to increase by day 10, and to decline by day 20 (mean+/-SE: 42.2+/-1.4, 27.6+/-2.2 nmol day-1, respectively). However, neither alk aline phosphatase nor tartrate-resistant acid phosphatase, both marker s of osteoclast and mature osteoblast function, changed. These results showed that in the early stage of immobilization, bone matrix might b e resorbed without any activation of osteoclasts, resulting in rapid d ecalcification of vertebral and cortical bones without any discernible changes in anatomical structure.