Rapid, membrane-initiated actions of 1,25 dihydroxyvitamin D: what are they and what do they mean?

Vitamin D is a conditionally required nutrient traditionally thought to influence physiology as the metabolite 1,25-dihydroxyvitamin D [1,25(OH)(2) D] by binding to the vitamin D receptor (VDR) and stimulating the transcription of genes through direct VDR-DNA interactions. However, over the past 15 y research has demonstrated that 1,25(OH)(2) D, as well as other steroid hormones, can rapidly stimulate ion fluxes and activate protein kinases by transcription-independent mechanisms. This review summarizes recent research on the rapid actions of 1,25(OH)(2) D and identifies questions that remain to be answered in this area.