Effects of 1,25(OH)2D3 on bone tissue in the rabbit: studies on fracture healing, disuse osteoporosis, and prednisone osteoporosis.

A closed tibial fracture, which was controlled by an intramedullary stainless steel pin, was created in 16 rabbits. Eight rabbits were treated with 75 ng of 1,25(OH)2D3 daily as subcutaneous (s.c.) injections. After three weeks, the fractured tibia resisted a force of 101.7 +/- 21.0 Newtons in the control group and 57.3 +/- 8.0 Newtons in animals given 1,25(OH)2D3 (m +/- SE, P less than 0.05). In another group of eight rabbits, the left hindleg was immobilized in a plastic splint. Four rabbits were given 75 ng of 1,25(OH)2D3/day s.c. and the effect of immobilization was studied on the calcaneus. Bone ash/cm3 of the calcaneus on the immobilized side was decreased by 11 +/- 2% in control rabbits and by 20 +/- 2% in the group treated with 1,25(OH)2D3 indicating a more advanced immobilization osteoporosis (m +/- SE, P less than 0.05), which was also demonstrated by studies of bone density. Eighteen rabbits were used in a study of the effects of 1,25(OH)2D3 on the development of prednisolone osteoporosis. The dose of prednisolone was 2.5 mg per day, given by the oral route. After four months, the density of the femur was 1.53 +/- 0.02 g/cm2 in control rabbits and 1.42 +/- 0.01 in prednisolone-treated animals (P less than 0.01). In rabbits additionally given 1,25(OH)2D3, the mean value for bone density was further lowered (n.s.). It appears that 1,25(OH)2D3 exaggerates disuse osteoporosis and prednisolone osteoporosis and impairs fracture healing in rabbits. These results differ from what has been shown earlier with 1,25(OH)2D3 treatment in the rat.