MOLECULAR BIOLOGY OF THE INTERRELATIONSHIP BETWEEN BONE CELLS IN THE LOW CALCIUM ENVIRONMENT

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Gene expressions of bone matrix proteins during low calcium environment was studied using a rat fed with a low calcium diet. Thirty days old male Wistar rats, mean body weight 70-85 g, were fed either normal calcium diet (0.5% Ca, 0.35% P, controls) or low Ca diet (0.05% Ca, 0.35% P, low Ca group). Rats were sacrificed under ether anesthesia at day 6. Bone tissues were removed and further processed for in situ hybridization and northern blot analysis. Contact microradiography showed a remarkable bone resorption in the metaphysis of trabecular bone. Many osteoclasts appeared on the bone surface. In addition, multi layrs of osteoblasts, which have the prominent Golgi-area, occupied the larger area of bone surface. In situ hybridization showed the high levels of osteopontin and collagen type I genes in the osteoblast in multi layrs. This observation indicated that bone formation activity was elevated during the low calcium state. Northern blot, however, failed to show the difference in gene expression between the control and low calcium rats. This discrepancy may be due to the different response to the low calcium stress between the cancellous and the cortical bone. The present result demonstrates that despite the severe bone resorption due to the low calcium environment, the bone formation was stimulated in the cancellous bone. This is a reflection of high rate of bone turn over, which was initiated by the response of calcium homeostasis during the calcium deficiency.