Studies on the Vitamin D Endocrine System in the Avian

This report summarizes the current understanding of the mode of action of vitamin D and emphasizes the contributions to this system that have been based on results obtained in avian species. A complex endocrine system coordinates the metabolism of vitamin D into 1,25-dihydroxycholecalciferol[1,25(OH)2D3] and 24R,25-dihydroxycholecalciferol[24R,25)OH)2D3] and 28 other metabolites; of these 16 were originally isolated and chemically characterized from avian systems. Key advances in understanding the mode of action of the seco-steroid 1,25(OH)2D3 and the scope of its action have been made by studying the tissue distribution of both its receptor and its gene-induced product, a 28,000-dalton calcium-binding protein termed calbindin-D28K. To date no less than 23 tissues have been found to have specific 1,25(OH)2D3 receptors; of these 10 were identified in avian studies. Similarly, nine tissues express the vitamin D-induced calbindin; seven have been reported in avian tissues. The second dihydroxylated metabolite, 24R,25(OH)2D3, has been reported to be capable of inducing a variety of specific biological effects, some when the steroid is administered alone and some in the presence of its companion dihydroxylated metabolite 1,25(OH)2D3. There is emerging evidence for the existence of specific receptors for 24R,25(OH)2D3, particularly in chondrocytes and parathyroid glands (both studied in avian systems). These observations collectively demonstrate the broad scope of the vitamin D endocrine system.