The physiology of vitamin D : current concepts.

The vitamin D endocrine system, besides playing pivotal roles in calcium homeostasis & bone mineral metabolism, is now recognized to subserve a wide range of fundamental biological functions in cell differentiation, inhibition of cell growth as well as immuno modulation. Vitamin D is a prohormone which is converted into its active hormonal form 1, 25 (OH)D2 D, 1, 25 (OH)D2 D activates its cellular receptor (VDR) which activate target genes to engender its biological actions. This review provides a summary of recent understanding of the complex actions of the vitamin D hormone 1, 25 (OH)2 D which is a final product of 1alpha hydroxylation in the proximal tubular cells of kidneys. Emerging evidence also indicates both 1, 25 (OH)2 D3 independent as well as depended action of vitamin D receptor (VDR). Thus, the vitamin D system action may involve more than one single receptor and legand. The presence of 1alpha hydroxylase in many target cells other than proximal renal tubular cells indicates autocrine and paracrine functions for 1, 25 (OH)2 D3 in the control of cell proliferation and differentiation. Vitamin D and related molecules belong to a elaborate endocrine system that acts on target genomic receptors in several organ systems to control cell proliferation and differentiation.