Metabolism of orally administered [3H]ergocalciferol and [3H]cholecalciferol by dairy calves.

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 muCi of [3H]ergocalciferol (1.2 Ci/mmol) per calf, and the other group (three calves) was given 365 muCi 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.(ABSTRACT TRUNCATED AT 250 WORDS)