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Regulation of Calf Renal 25-Hydroxyvitamin D-Hydroxylase Activities by Calcium-Regulating Hormones

National Animal Disease Center, Agricultural Research Service, US Department of Agriculture, PO Box 70, Ames, IA 50010

ABSTRACT

Parathyroid hormone and 1,25-dihydroxyvitamin D₃ had opposite effects on calf renal 25-hydroxyvitamin D₃ 24-, 23-, and 1-hydroxylase activities. Parathyroid hormone administration increased renal 25-hydroxyvitamin D₃-1-hydroxylase activity 7-fold while 25-hydroxyvitamin D₃-23- and 24-hydroxylase activities were essentially the same as controls. Administration of 1,2 5-dihydroxyvitamin D₃ increased 25-hydroxyvitamin D₃-23-hydroxylase and 24-hydroxylase activities 4-fold and decreased 25-hydroxyvitamin D3-1-hydroxylase activity to undetectable concentrations. Vitamin D deficiency increased 25-hydroxyvitamin D₃-1-hydroxylase activity 13-fold, and 25-hydroxyvitamin D₃-23-hydroxylase and 24-hydroxylase activities were undetectable. These results confirm previous reports with regard to control of renal 25-hydroxyvitamin D₃-24-hydroxylase and 1-hydroxylase in other species and represent new findings relative to the control of 25-hydroxyvitamin D₃-23-hydroxylase. Plasma P was lower and 1,25-dihydroxy vitamin D₃ higher in calves treated with parathyroid hormone, and Ca and 1,25-dihydroxyvitamin D₃ were lower in the vitamin D-deficient calves. 1,25-Dihydroxyvitamin D₃-treated calves had higher plasma P and lower Mg than controls. Further studies using this calf model should lead to better understanding of Ca-regulating hormones control of vitamin D metabolism.