title: 24R,25-dihydroxyvitamin D3: an essential vitamin D3 metabolite for both normal bone integrity and healing of tibial fracture in chicks.
abstract: We tested the hypothesis that 24R,25-dihydroxyvitamin D3 [24R,25-(OH)2D3] is an essential vitamin D metabolite for the development of normal bone integrity and the healing of fractures. The natural 24R,25-(OH)2D3 and its synthetic epimer 24S,25-dihydroxyvitamin D3 [24S,25-(OH)2D3] were tested alone or in combination with 1alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3], on normal bone development and other related variables of the Ca2+ homeostasis system [serum Ca2+, 25-hydroxyvitamin D3 (25OHD3), 24,25-(OH)2D3, and 1alpha,25-(OH)2D3 levels] in chicks. Mechanical testing of torsional strength was carried out on the femur. 24R,25-(OH)2D3 (80 nmol/kg diet) alone was sufficient for normal bone growth and integrity similar to that achieved by the vitamin D3-replete controls. Next, chicks were fed a 25OHD3-replete diet (75 nmol/kg diet) for 8 days after hatching, and then 25OHD3 was withdrawn to minimize any residual circulating metabolites before the imposition of standardized tibial fractures 14 days later. Vitamin D metabolites were administered for 2 weeks to determine their effects on the mechanical properties of healed tibia. 24S,25-(OH)2D3 combined with 1alpha,25-(OH)2D3 or 1alpha,25-(OH)2D3 alone resulted in poor healing [strength values of 0.158 +/- 0.011 and 0.123 +/- 0.009 Nm (Newton x meter), respectively] compared with that in the 25OHD3-treated control group (0.374 +/- 0.029 Nm). In contrast, the fractured tibia of the birds fed 24R,25-(OH)2D3 in combination with 1alpha,25-(OH)2D3 showed healing equivalent to that in the control group, with strength values of 0.296 +/- 0.043 Nm. These results suggest that when 24R,25-(OH)2D3 is present at normal physiological concentrations, it is an essential vitamin D3 metabolite for both normal bone integrity and healing of fracture in chicks.
authors.affiliation: Department of Biochemistry, University of California, Riverside 92521, USA.
authors.forename: E G
authors.initials: EG
authors.lastname: Seo
authors.forename: T A
authors.initials: TA
authors.lastname: Einhorn
authors.forename: A W
authors.initials: AW
authors.lastname: Norman
mesh_terms: D015650:24,25-Dihydroxyvitamin D 3 / Q000008:administration & dosage / Q000097:blood / Q000494:pharmacology*; D000818:Animals; D001696:Biomechanical Phenomena; D001846:Bone Development / Q000187:drug effects; D001842:Bone and Bones / Q000503:physiopathology*; D002118:Calcium / Q000097:blood; D002645:Chickens; D017102:Fracture Healing / Q000187:drug effects*; D013718:Tensile Strength; D013978:Tibial Fractures / Q000503:physiopathology*
chemical_list: D015650:24,25-Dihydroxyvitamin D 3; D002118:Calcium