Vitamin D receptors: not just in the nucleus anymore

Calcitriol, the active metabolite of vitamin D, is a steroid hormone that regulates calcium metabolism and cell differentiation by interacting with its nuclear receptor--the vitamin D receptor (VDR)--and by stimulating gene transcription. During the last decade, calcitriol also has been shown to stimulate rapid signal transduction pathways. This observation supports the hypothesis that a membrane-bound receptor similar to those that mediate peptide hormone biology exists. Recent research provides evidence for such a unique membrane VDR. Future research challenges are to integrate the membrane VDR into existing knowledge of vitamin D biology.