Tishkoff Daniel X, Nibbelink Karl A, Holmberg Kristina H, Dandu Loredana, Simpson Robert U
Department of Pharmacology, University of Michigan Medical School, Ann Arbor, MI 48109, USA.
Endocrinology. 2008 Feb;149(2):558-64. doi: 10.1210/en.2007-0805. Epub 2007 Nov 1.
We have previously shown that the active form of vitamin D, 1,25 dihydroxyvitamin D3 [1,25(OH)(2)D(3)], has both genomic and rapid nongenomic effects in heart cells; however, the subcellular localization of the vitamin D receptor (VDR) in heart has not been studied. Here we show that in adult rat cardiac myocytes the VDR is primarily localized to the t-tubule. Using immunofluorescence and Western blot analysis, we show that the VDR is closely associated with known t-tubule proteins. Radioligand binding assays using (3)H-labeled 1,25(OH)(2)D(3) demonstrate that a t-tubule membrane fraction isolated from homogenized rat ventricles contains a 1,25(OH)(2)D(3)-binding activity similar to the classic VDR. For the first time, we show that cardiac myocytes isolated from VDR knockout mice show accelerated rates of contraction and relaxation as compared with wild type and that 1,25(OH)(2)D(3) directly affects contractility in the wild-type but not the knockout cardiac myocyte. Moreover, we observed that acute (5 min) exposure to 1,25(OH)(2)D(3) altered the rate of relaxation. A receptor localized to t-tubules in the heart is ideally positioned to exert an immediate effect on signal transduction mediators and ion channels. This novel discovery is fundamentally important in understanding 1,25(OH)(2)D(3) signal transduction in heart cells and provides further evidence that the VDR plays a role in heart structure and function.
我们之前已经表明，维生素D的活性形式，1,25-二羟基维生素D3 [1,25(OH)₂D₃]，在心脏细胞中具有基因组和快速的非基因组效应；然而，心脏中维生素D受体（VDR）的亚细胞定位尚未得到研究。在此我们表明，在成年大鼠心肌细胞中，VDR主要定位于横管。通过免疫荧光和蛋白质印迹分析，我们表明VDR与已知的横管蛋白紧密相关。使用³H标记的1,25(OH)₂D₃进行的放射性配体结合试验表明，从匀浆大鼠心室分离的横管膜部分含有与经典VDR相似的1,25(OH)₂D₃结合活性。我们首次表明，与野生型相比，从VDR基因敲除小鼠分离的心肌细胞显示出收缩和舒张速率加快，并且1,25(OH)₂D₃直接影响野生型心肌细胞的收缩性，但不影响基因敲除心肌细胞的收缩性。此外，我们观察到急性（5分钟）暴露于1,25(OH)₂D₃会改变舒张速率。定位于心脏横管的受体处于理想位置，可对信号转导介质和离子通道立即产生影响。这一新颖发现对于理解心脏细胞中的1,25(OH)₂D₃信号转导至关重要，并进一步证明VDR在心脏结构和功能中发挥作用。
