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Evidence that Discrimination against Ergocalciferol by the Chick is the Result of Enhanced Metabolic Clearance Rates for its Mono- and Dihydroxylated Metabolites

Derrel A. Hoy^*, Charles F. Ramberg, Jr. and Ronald L. Horst^*,1

^* National Animal Disease Center, Agricultural Research Service, U.S. Department of Agriculture, Ames, IA 50010 University of Pennsylvania, School of Veterinary Medicine, Kennett Square, PA 19348

Studies on the discrimination between ergocalciferol (vitamin D₂) and cholecalciferol (vitamin D₃) by chickens have led to conflicting conclusions. To investigate this problem in more detail, radiolabeled vitamin D and vitamin D metabolites were administered intravenously, which allowed determination of their relative plasma clearance rates. The study involved three groups of adult male chickens (five per group). Birds in group 1 were codosed with [³H]vitamin D₂ and [³H]vitamin D₃. Birds in group 2 were co-dosed with [³H]25-dihydroxyvitamin D₂ and [³H]25-dihydroxyvitamin D₃ and birds in group 3 were co-dosed with [³H]1,25-dihydroxyvitamin D₃ and [³H]1,25-dihydroxyvitamin D₂. The results indicated that the plasma turnover rate of [³H]vitamin D₂ is 1.5 times faster than that of [³H]vitamin D₃. Plasma turnover of the 25-hydroxylated metabolites differed, with [³H]25-dihydroxyvitamin D₂ clearing faster (11 times) than [³H]25-dihydroxyvitamin D₃. The largest difference appeared in the 1,25-dihydroxyvitamin D₂ clearing approximately 33 times faster then [³H]1,25-dihydroxyvitamin D₃. These data, therefore, indicate that discrimination against vitamin D₂ sterols in the chick occurs primarily between steps in the metabolism of vitamin D and not at the point of metabolism or excretion of the parent vitamin.

KEY WORDS: • vitamin D₂ • kinetics • chickens • vitamin D₃ • metabolism

¹ To whom reprint requests should be sent; National Animal Disease Center, ARS, USDA, P.O. Box 70, Ames, IA 50010.

Manuscript received 9 June 1987. Revision accepted 4 January 1988.

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