Vitamin D - new action mechanisms and effects
The classic arena of vitamin D (cholecalciferol) action is the maintenance of calcium homeostasis. Intestinal, bone and kidney tissue interact in a tightly regulated manner to achieve this end. Since the identification of 1,25(OH)2D3 (1,25dihydroxyvitamin D3) as the active metabolite of vitamin D, its endocrine properties have been well documented. Calcitriol, as this metabolite is also called, acts on gene expression via a receptor in the cell nucleus. During the last decade, however, additional actions of 1,25(OH)2D3 on the cell membrane that trigger rapid signal transduction mechanisms have been reported. Recently many other tissues have also joined the classic target organs of calcitriol, for example the pancreas, the immune system, the skin and the parathyroid gland, as well as an array of tumour"tissues. In these instances calcitriol has intriguing non-calcaemic actions on cell differentiation and function, which opens exciting new therapeutic possibilities for the use of synthetic analogues of vitamin D.
This review gives a brief presentation of the classic hypercalcaemic and more recent non-calcaemic effects of calcitriol. It also surrunarises its classic slow genomic and new rapid non-genomic action mechanisms and gives a brief overview of possible clinical applications.