Vitamin D metabolism and calcium absorption.

Vitamin D along with parathyroid hormone (PTH) and calcitonin (CT) are the three principal effectors of calcium and phosphorus homeostasis. The secosteroid, vitamin D3, is subject to metabolic conversion to its biologically active form(s) 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and 24,25-dihydroxyvitamin D3 [24,25(OH)2D3] prior to initiation of its physiologic responses in the intestine and skeletal system. The production of 1,25(OH)2D3 is stringently regulated by a variety of endocrine signals including PTH as well as the "calcium needs" of the organism. At the target intestine, 1,25-(OH)2D3 stimulates the intestinal absorption of calcium via a mechanism analogous to that of other steroid hormones. Definitive biochemical evidence exists supporting the existence in the intestine of a highly specific protein receptor for 1,25(OH)2D3. After formation of the steroid-receptor complex, it migrates to the nucleus of the cell and stimulates messenger-RNA synthesis for proteins (including a calcium-binding protein) which are necessary for the generation of the biologic response. Current efforts to biochemically characterize vitamin D-mediated intestinal calcium transport include efforts to understand the role of calcium-binding protein in this process, as well as to identify other protein components present either in the brush border or basal lateral membranes.