[Biological action mechanisms and effects of calcitriol]

Vitamin D is an important regulator of calcium homeostasis in the body and it plays an essential role in bone metabolism. 1,25(OH)2D (calcitriol), the active metabolite of vitamin D stimulates intestinal calcium and phosphate absorption, thereby maintaining sufficient concentrations of these ions in the extracellular fluids, necessary for normal mineralization of bone matrix. Furthermore, calcitriol exerts a number of important functions in regulating bone metabolism. Similar to other classical steroid hormones, calcitriol regulates the transcription of a large number of target genes. A large number of recent studies of the mechanisms of genomic actions of calcitriol have been published. The aim of this review is to give a summary of these recent findings. The interaction between calcitriol and the vitamin D receptor as well as receptor structure and function in relation to transcriptional regulation are discussed.