Feedback regulation of 25-hydroxycholecalciferol metabolism by vitamin D3

1. In vitamin D-deficient chicks both vitamin D3 and 1alpha-hydroxycholecalciferol markedly decrease renal 1-hydroxylase activity and induce 24-hydroxylase activity. 2. Actinomycin D abolishes both effects. 3. These results are consistent with feedback regulation of vitamin D3 metabolism by a direct nuclear action of the vitamin or its metabolites on the kidney cells.