Third F. Raymond Keating, Jr., Memorial Symposium--parathyroid hormone, calcitonin and vitamin D: clinical considerations. II. Vitamin D--1973

The recent advances in our understanding of the functional metabolism of vitamin D to 25-hydroxyvitamin D (25-(OH)D) and subsequently to 1,25-dihydroxyvitamin D (1,25-(OH)₂D) are presented with a review of current views on the regulation of vitamin D metabolism at the 25-hydroxylation and 1-hydroxylation stages. It seems clear that, physiologically, the latter regulation is governed by parathyroid hormone (PTH) under conditions of hypocalcemia and by serum inorganic phosphorus levels under conditions from normal to hypercalcemia. The molecular mechanism of the regulation of the renal hydroxylations remains unknown.The metabolism of 25-(OH)D₃ to 24,25-(OH)₂D₃ and further to 1,24,25-(OH)₃D₃ has been established. The significance of these reactions is unknown, but 1,24,25-(OH)₃D₃ preferentially stimulates intestinal calcium transport.The mechanism whereby 1,25-(OH)₂D₃ stimulates intestinal calcium transport and the receptors of this metabolite in intestine are discussed. In addition, the application of the metabolites of vitamin D to clinical problems has been considered and 1α-(OH)D₃, an important analog of 1,25-(OH)₂D₃, has been introduced as a potentially important therapeutic compound.