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Metabolism of Orally Administered [³H]Ergocalciferol and [³H]Cholecalciferol by Dairy Calves

Joel L. Sommerfeldt^1,*, Joseph L. Napoli, E. Travis Littledike, Donald C. Beitz^* and Ronald L. Horst^2,

^* Department of Animal Science, Iowa State University Ames, IA 50011 Department of Biochemistry, The University of Texas Health Science Center Dallas, TX 75235 National Animal Disease Center, Agricultural Research Service, U.S. Department of Agriculture, Ames, IA 50010

Concentrations of ergocalciferol, cholecalciferol, and their metabolites in plasma were determined after a single oral dose of [³H]ergocalciferol or [³H]cholecalciferol was given to 95- to 105-kg Jersey bull calves. One group (three calves) was given 365 µCi of [³H]ergocalciferol (1.2 Ci/mmol) per calf, and the other group (three calves) was given 365 µCi of [³H]cholecalciferol (1.2 Ci/mmol) per calf. Fourteen blood samples were taken from each calf during the 3 weeks after administration. Total plasma radioactivity was highest at 80 hours in both groups (8400 dpm/ml and 4600 dpm/ml in the [³H]cholecalciferol- and [³H]ergocalciferol-treated calves, respectively). For determination of the time-dependent appearance and disappearance of plasma vitamin D and vitamin D metabolites, the plasma ³H-labeled steroids were extracted and separated by high-performance liquid chromatography. In both groups, [³H]vitamin D peaked at 24–48 hours and was the predominant radioactive form in plasma 10–15 hours after dosing. After 15 hours, 25-[³H]hydroxyvitamin D became the predominant labeled metabolite, reaching its maximal concentration between 48 and 96 hours. Concentrations of 25-[³H]hydroxycholecalciferol were about twice those of 25-[³H]hydroxyergocalciferol. The appearance/disappearance profile of 25,26-[³H]dihydroxycholecalciferol and 1,25[³H]hydroxycholecalciferol resembled that of 25-[³H]hydroxycholecalciferol. The amount of 1,25-[³H]dihydroxyvitamin D in the plasma of [³H]ergocalciferol-treated calves was one-half to one-fourth the amount of those metabolites in the plasma of [³H]cholecalciferol-treated calves. The amount of 24,25-[³H]dihydroxyvitamin D steadily increased in the plasma and was highest at 200–400 hours. In addition, four unidentified ³H-labeled vitamin D metabolites appeared in the plasma. These data suggest that ruminants, like other species, discriminate in their metabolism of ergocalciferol and cholecalciferol.

KEY WORDS: • ergocalciferol • cholecalciferol • metabolism • calves • discrimination

¹ Dr. Sommerfeldt's present address is Department of Agriculture, Murray State University, Murray, KY 42071.

² Address all correspondence to: Dr. Ronald L. Horst, National Animal Disease Center, ARS, USDA, P.O. Box 70, Ames, IA 50010.

Manuscript received 10 June 1983.

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