A link between calcitriol and bone resorption.

Calcitriol exposure stimulated a human osteosarcoma cell line, U2-OS, to produce a factor(s) which stimulated bone degradation in human monocyte cultures and osteoclastic bone resorption in fetal rat long bone cultures. The factor(s) was elicited by as little as 10(-10) M calcitriol. The factor is effective in stimulating peripheral blood monocytes to degrade bone, suggesting a direct effect on cellular bone breakdown. The fetal long bone assays suggest that the osteoblast-like cells produce a factor(s) which stimulates osteoclasts. This is confirmed by the fact that human calcitonin added to the long bone cultures blocks the stimulation. The effect of PTH appears to be through the production of factors which stimulate osteoblasts. The present study suggests that a similar factor(s) may be responsible for the effect of calcitriol on bone resorption.