Action of 24R, 25 dihydroxyvitamin D3 (24,25 (OH)2D3) on bone in vivo.

The administration of 24R,25(OH)2D3 for 2 years led to an increase in bone volume in the rat. Radiographs showed the striking increase in the density of the bones from animals treated with large amounts of 24R,25(OH)2D3. Total bone minerals measured by direct single photon absorptiometry showed a dose-dependent increase of up to approximately 150% that of controls in both the femur and coccyx of the treated animals. Histological observation revealed a thickened cortex with narrowed medullary area, and Villanueva's bone stain indicated the increase of mineralized bone. The result clearly demonstrates that 24,25(OH)2D3, if massive doses are given, has the biological action of increasing bone volume without disturbing the development of bone sizes.