Age-related Functional Changes of Cancellous and Cortical Bone: Histomorphometric Study of Bone in Senescent Rats Fed with a Low Calcium Diet.

Age-related functional changes of bone is not well understood. The effect of aging on bone response to a low calcium feeding was studied using senescent rats. Male Wistar rats aged 23 to 28 months were fed either a low calcium diet (0.05% Ca, 0.35% P) or a normal calcium diet (0.5% Ca, 0.35% P) by using a pair feeding technique. They were sacrificed at intervals of 6, 9, and 20 days. Bone mineral density of the femur showed no changes in both cancellous and cortical bone in the low calcium group compared with the normal calcium group. The increment of osteoclast number was observed on day 6 in the cancellous bone of the tibia, but no significant changes were found during other periods. Mineralizing surface (MS/BS) and mineral appositional rate (MAR) were increased during the experimental period. On the other hand, cortical bone showed a transient increase of MS/BS on day 6, but other parameters revealed no changes during the experimental period. The magnitude of response was more evident in the cancellous bone than in the cortical bone. These results indicated that although responsiveness of remodeling activity to low calcium feeding is decreased in senescent rats, they are obvious in both the cancellous and cortical bone.