Fate of isotopically labeled cholecalciferol and 25-hydroxycholecalciferol in sheep

The fate of tritiated 25-hydroxycholecalciferol administered intramuscularly is compared in two groups of sheep raised either in confinement or outdoors. The uptake of radioactivity was more rapid by the indoor than the outdoor sheep. The assessment of radioactivity in plasma and tissues of outdoor sheep dosed with either tritiated 25-hydroxycholecalciferol or with equivalent amounts of tritiated cholecalciferol revealed that plasma and tissue radioactivity were substantially higher in sheep dosed with tritiated 25-hydroxycholecalciferol than in sheep dosed with tritiated cholecaliferol. Three days after the administration of tritiated cholecalciferol virtually all plasma radioactivity was associated with 25-hydroxycholecalciferol.