Absence of regulatory effects of 1α,25-dihydroxyvitamin D 3 on 25-hydroxyvitamin D 3 metabolism in rats constantly infused with parathyroid hormone

Abstract

The regulatory role of 1α,25-dihydroxyvitamin D 3 [1α,25-(OH) 2-D 3] in metabolism of 25-hydroxyvitamin D 3 was studied in sham-operated (sham) or thyroparathyroidectomized (TPTX) vitamin D-deficient rats into which calcium and parathyroid hormone (PTH) were constantly infused. A single dose of 325 or 650 pmol of 1α,25-(OH) 2-D 3 caused significant inhibition of 1α,25-(OH) 2-D 3 synthesis in D-deficient sham rats. This inhibition by 1α,25-(OH) 2-D 3, however, was not observed in D-deficient TPTX rats into which PTH was constantly infused. These results can be explained by supposing that the major regulatory effect of 1α,25-(OH) 2-D 3 on 1α,25-(OH) 2-D 3 synthesis is realized mostly, if not all, by suppressing endogenous secretion of PTH.