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

Abstract

Parathyroid hormone and 1,25-dihydroxyvitamin D3 had opposite effects on calf renal 25-hydroxyvitamin D3 24-, 23- and 1.alpha.-hydroxylase activities. Parathyroid hormone administration increased renal 25-hydroxyvitamin D3-1.alpha.-hydroxylase activity 7-fold while 25-hydroxyvitamin D3-23- and 24-hydroxylase activities were essentially the same as controls. Administration of 1,25-dihydroxyvitamin D3 increased 25-hydroxyvitamin D3-23-hydroxylase and 24-hydroxylase acitivities 4-fold and decreased 25-hydroxyvitamin D3-1.alpha.-hydroxylase activity to undetectable concentrations. Vitamin D deficiency increased 25-hydroxyvitamin D3-1.alpha.-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.alpha.-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-dihydroxyvitamin 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.