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

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Abstract
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.