Changes in vitamin D metabolites and bone histology in rats during recovery from rickets

The relative roles of 25-hydroxyvitamin D (25-OHD), 1,25-dihydroxyvitamin D (1,25-(OH)2D) and 24,25-dihydroxyvitamin D (24,25-(OH)2D) in bone mineralization are largely unknown. Young vitamin D depleted rats were fed increasing amounts of vitamin D and grouped radiologically in accordance with the rat line test. They ranged from severely rachitic to normal. Radiology was correlated with serum levels of 25-OHD, 1,25-(OH)2D, 24,25-(OH)2D, ionized calcium, magnesium, and phosphate, with bone histology, and with the total mineral content of the animals. Serum 1,25-(OH)2D rose in a linear fashion to supranormal values during bone healing and correlated with the radiological degree of rickets. Serum 25-OHD was below detection limit in the most rachitic and low in the radiologically normal rats, whereas 24,25-(OH)2D was low in all groups. These two metabolites showed no correlation with the radiologic, histologic or biochemical parameters. In rachitic rats, 1,25-(OH)2D appears to play a major role in bone healing and possibly exerts a direct effect on bone cells. It cannot be ruled out, however, that the effect is mediated through a rise in serum levels of calcium and phosphorus, although signs of bone healing were seen in the presence of a subnormal calcium X phosphorus product. Initiation of mineralization can take place with unmeasurable 25-OHD, and 24,25-(OH)2D seems to be without importance.