Possible link between changes in plasma 24,25-dihydroxyvitamin D and healing of bone resorption in dialysis osteodystrophy

Presystemic 24-hydroxylation of oral 25-hydroxyvitamin D3 in rats

The metabolism of 25-hydroxyvitamin D3 (25-OHD3) was compared following its intracardial or gastric administration. The rats were deprived of calcium and vitamin D. A mixture of radiolabeled (0.3 microCi) and stable (2 micrograms) 25-OHD3 was given as a single dose. After 24 h the rats given the dose by gastric tube had significantly lower serum concentrations of 25-OHD3 and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] than those injected intracardially. In contrast, serum 24,25-dihydroxyvitamin D3 [24,25-(OH)2D3] was much higher in the rats given the 25-OHD3 dose by gastric tube (6.2 nmol/liter +/- 1.3 SD, n = 7) compared to the intracardial group (0.9 nmol/liter +/- 0.5, p less than 0.001). The preceding results were based on specific radioactivity of metabolites. The same findings were obtained by reanalyzing the samples using conventional competitive binding assays for 25-OHD3, 1,25-(OH)2D3, and 24,25-(OH)2D3. The results show that orally administered 25-OHD3 is partly metabolized to 24,25-(OH)2D3 presystemically.

Abnormal vitamin D metabolism in Paget's disease of bone

We have studied several biochemical indices of bone turnover and vitamin D metabolism in 32 untreated patients with Paget's disease and in 32 age-matched control subjects. patients with Paget's disease, as expected, were characterized by high bone turnover, as judged by alkaline phosphatase and urinary excretion of hydorxyproline. Serum values of 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) and the ratio of 24,25(OH)2D3 to 25-OHD were significantly lower in patients than in control subjects. Serum concentrations of 1,25(OH)2D3 were normal in Paget's disease. The distribution of values for 24,25(OH)2D3 was log normal. On the basis of the normal range computed from control subjects, patients were divided into those with low or normal values for 24,25(OH)2D3. Disease activity, as judged by biochemical indices was significantly higher in the patients with the lower values of 24,25(OH)2D3. We conclude that Paget's disease is characterized by low circulating concentrations of 24,25(OH)2D3, particularly in patients with more extensive or severe disease.