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Biological Activity of the Vitamin D Metabolite 1,25-Dihydroxycholecalciferol in Chickens and Rats^{1, 2,}

Department of Biochemistry, University of California, Riverside, California 92502

The biological activity of the vitamin D₂ metabolite 1,25-dihydroxycholecalciferol was assessed in three experiments in the chick and two experiments in the rachitic rat. In the official rat line test 1,25-dihydroxycholecalciferol was found to be equally as active as an equivalent dose of cholecalciferol. Doses of 20 to 250 pmoles (0.01 to 0.10 µg) of 1,25-dihydroxy-[26,27-²H] cholecalciferol or standard amounts of cholecalciferol were fed daily to White Leghorn cockerels for 3 weeks after hatching. The following vitamin D-related responses were measured: a) the rate of growth; b) intestinal transport of test doses of ⁴⁵Ca²⁺ in vivo and c) in vitro; d) percentage bone ash; e) serum calcium elevation. For each of these assays the 1,25-dihydroxycholecalciferol was judged to be "much more active" a) "more active" (b,d), and "as active" (c,e), as the parent cholecalciferol, where "much more active" is a response 2 to 4 times and "more active" is a response 1 to 2 times that obtained with an equivalent dose of cholecalciferol. In some instances (d,e), the relative increment of response mediated by 1,25-dihydroxycholecalciferol was greater at the low dose level (20 pmoles/day) than at a higher dose level (100 pmoles/day). The relative activity determination of 1,25-dihydroxycholecalciferol, obtained after daily administration, in comparison with cholecalciferol was markedly less than that previously obtained when the activity of single doses of these steroids were analyzed. A single dose of 1,25-dihydroxycholecalciferol was 4.4 ± 1.1 (SE) [Science 171: 79 (1971)] times as effective as cholecalciferol in stimulating the intestinal absorption of Ca²⁺, and 5.5 ± 1.4 (SE) [J. Biol. Chem. 247: 5728 (1972)] times as effective as cholecalciferol in stimulating serum Ca²⁺ elevation. These results collectively support the concept that 1,25-dihydroxycholecalciferol is the most highly potent form of Vitamin D₃ yet known.

KEY WORDS: • 1,25-dihydroxycholecalciferol • vitamin D metabolite • biological activity • calcium • calciferol

¹ Supported in part by PHS grants AM-09012 and AM 14,750. Anthony W. Norman is the recipient of a PHS Career Development Award 1-KD-AM-13,654.

² A preliminary report of these results was presented at the 62nd Federation of American Societies for Experimental Biology meeting in Atlantic City, New Jersey, April 11, 1972. Federation Proc. 31: 685 (1972, abstr.).

Manuscript received 31 March 1972.

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