24,25(OH)2D3, bone formation,and bone resorption in vitamin D-deficient,azotemic rats

Summary Bone formation, mineralization, and resorption were measured in vitamin D-deficient, azotemic rats given two different dosages of 24,25(OH)₂D₃ daily and in vehicle-treated controls (C). The intraperitoneal administration of 65 pmol over a 10 day period corrected the hypocalcemia observed in C, whereas 130 pmol produced mild hypercalcemia. Both dosages reduced osteoid width, osteoid area, and mineralization front width form control values. The rates of bone and matrix formation were unaffected by treatment. In C, matrix formation exceeded bone formation and resulted in osteoid accumulation; both dosages of 24,25(OH)₂D₃ reversed this relationship such that bone formation exceeded matrix formation in each treatment group. The rates of osteoid maturation and initial mineralization increased during repletion with 24,25(OH)₂D₃ at both dosage levels. However, the serum calcium concentration was correlated with both osteoid maturation rate (r=0.68,P<0.01) and initial mineralization rate (r=0.63,P<0.01) when all three experimental groups were considered. Bone resorption was unchanged from control values during treatment with 24,25(OH)₂D₃. The results suggest that 24,25(OH)₂D₃ promotes the maturation and mineralization of osteoid, and that this metabolite differs in its effects on bone formation and resorption. It is not clear, however, that the changes in bone dynamics observed are independent of the calcemic response induced by metabolite repletion under the conditions of this experiment.