Extraction of Vitamin D Metabolites by Bones of Normal Adult Dogs

K. Olgaard; J. Schwartz; D. Finco; M. Arbelaez; J. Haddad; L. Avioli; S. Klahr; E. Slatopolsky

doi:10.1172/JCI110496

Extraction of Vitamin D Metabolites by Bones of Normal Adult Dogs

K. Olgaard, J. Schwartz, D. Finco, M. Arbelaez, J. Haddad, L. Avioli, S. Klahr, and E. Slatopolsky

Published March 1, 1982 - More info

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Abstract

Using the isolated perfused canine tibia we examined the extraction of [³H]25(OH)D₃, [³H]1,25(OH)₂D₃ and [³H]24,25(OH)₂D₃ by bone of normal adult dogs. The studies were performed with and without vitamin D binding protein (DBP) in the perfusate to examine the effect of protein binding on the extraction of the vitamin D metabolites. An average of 48±2% of [³H]25(OH)D₃ was extracted by bone, when no DBP was present. However, addition of only a small amount of DBP (∼720 ng/ml of perfusate) nearly completely abolished the extraction of [³H]25(OH)D₃ by bone. No degradation and/or transformation of the labeled 25(OH)D₃ could be demonstrated during passage through the isolated perfused bone. The extraction of [³H]24,25(OH)₂D₃ in a DBP-free medium averaged 33±5%. Addition of 720 ng of DBP/ml of perfusate completely inhibited the extraction of this metabolite. The extraction of [³H]1,25(OH)₂D₃ averaged 30±3% in a DBP free medium and no inhibition of the extraction was demonstrated after addition of DBP (720 ng/ml of perfusate). However, addition of DBP in a concentration of 14.4 μg/ml of perfusate reduced the extraction of 1,25(OH)₂D₃ to 8±2%, a value still significantly higher than that seen after addition of 20 times less DBP to perfusions with 25(OH)D₃ and 24,25(OH)₂D₃. It is concluded that the isolated perfused bone of normal dogs can extract significant amounts of 25(OH)D₃, 1,25(OH)₂D₃, and 24,25(OH)₂D₃. Small concentrations of DBP (720 ng/ml) in the perfusate significantly inhibited the extraction of 25(OH)D₃ and 24,25(OH)₂D₃. A carrier role for DBP is suggested and it is proposed that the levels of free vitamin D are important for extraction of the metabolites by bone. Therefore, due to the different affinities of DBP for the various metabolites of vitamin D, only 1,25(OH)₂D₃ is extracted in vitro in significant amounts by bone of normal adult dogs, in the presence of DBP.