3-FOLD INDUCTION OF RENAL 25-HYDROXYVITAMIN-D-3-24-HYDROXYLASE ACTIVITY AND INCREASED SERUM 24,25-DIHYDROXYVITAMIN-D-3 LEVELS ARE CORRELATED WITH THE HEALING-PROCESS AFTER CHICK TIBIAL FRACTURE

To investigate the possible biological actions of 24,25-dihydroxyvitam in D-3 (24,25(OH)(2)D-3), a tibial fracture-healing model was establis hed in White Leghorn chicks. Three-week-old White Leghorn chicks fed a vitamin D-3-replete diet were divided into four groups (control, anes thetized, sham, and fractured). On varying days after tibial fracture (F) or sham manipulation (S), renal 25(OH)D-3-1 alpha-hydroylase and 2 5(OH)D-3-24-hydroxylase (24-hydroxylase) activities and serum Ca2+ con centrations were measured. Metofane anesthesia was found to have no ef fect on the activity of either of the hydroxylases; the activities of the hydroxylases in the control, anesthetized, and sham-operated birds were similar. By 10 days after tibial fracture, the renal 24-hydroxyl ase activity increased more than 3-fold in F (1.33 +/- 0.07 pmol/mg of protein) as compared with S (0.42 +/- 0.03 pmol/mg of protein) (p < 0 .0001). A time-dependent study of the renal 24-hydroxylase activity du ring the fracture repair process revealed a slow increase from the fir st day after fracture, a higher activity at 8 days, which peaked at 10 -11 days, which is consistent with the formation of the callus. The 24 -hydroxylase activity then returned to the same level as the sham grou p 14 days after fracture. There was no significant difference in serum Ca2+ levels between the F and S groups over the 3-week postfracture p eriod. Serum levels of vitamin D-3 metabolites were also measured duri ng the fracture healing process: a 3.4x increase of the 24,25(OH)(2)D- 3 level in the fractured group (3.64 +/- 1.16 nM) was observed as comp ared with the control groups (1.08 +/- 0.49 nM) at 10 days after fract ure (p = 0.068). No significant differences were observed in the plasm a levels of 25(OH)D-3 or 1 alpha,25(OH)(2)D-3 between the group with a fracture and the controls. Exposure of primary chick kidney cells in culture to serum obtained from chicks with a tibial fracture for 20 h resulted in an similar to 40% increase in the activity of the 24-hydro xylase as compared with cells exposed to serum from control birds. The se results suggest that 24,25(OH)(2)D-3 is involved in the early proce ss of fracture repair and that there is some form of physiological com munication between the fractured bone and the kidney so as to increase the renal 24-hydroxylase and the circulating concentration of this me tabolite.