Trabecular bone turnover, bone marrow cell development, and gene expression of bone matrix proteins after low calcium feeding in rats

Low-calcium-fed animals have been accepted as one of the experimental model s showing a reduction in bone mass. However, the effects of short-term low- calcium feeding on bone turnover, the development of osteoprogenitor cells, and gene expression of bone matrix proteins have not been reported. In thi s study, we examined the effect of a low-calcium diet on rat tibia and anal yzed the changes in the bone by histomorphometry, bone marrow cell culture, and in situ and Northern hybridization of the bone matrix proteins. Rats w ere fed either a low-calcium diet (0.05% Ca) or a normal calcium diet (0.5% Ca) using the pair feeding technique. They were killed at day 0, 12 h, and days 1, 2, and 3, In the low-calcium group, the serum parathyroid hormone (PTR) level was temporarily increased in 12 h after feeding the low-calcium diet. Bone mineral density in the trabecular bone was significantly decrea sed from 1 day after the low-calcium feeding, but cortical bone did not sho w any changes during the experimental period. The bone volume per tissue vo lume in the proximal tibia also decreased from day 1 in the low-calcium gro up. The number of osteoclasts and osteoblasts on the trabecular bone surfac e was increased in the low-calcium group compared with the normal-calcium g roup. An ex vivo study showed that the number of progenitors of osteoclasts and osteoblasts in bone marrow was also increased in the low-calcium group of rats. The localization of type I collagen mRNA was observed in osteobla sts in the low-calcium group. The Northern hybridization study showed that the gene expression of type I collagen, osteopontin, and osteocalcin was in creased at day 3 in the low-calcium group. These results indicated that the trabecular bone surface quickly responded to the low-calcium feeding and t hat bone remodeling activity was activated probably by PTH, The changes in bone marrow cell populations and the gene expression of bone matrix protein s are closely associated with increased bone turnover induced by the low-ca lcium diet, resulting in rapid bone loss of the trabecular bone. (C) 1999 b y Elsevier Science Inc. All rights reserved.