Vitamin D and endothelial function in chronic kidney disease
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Vitamin D deficiency in patients with chronic kidney disease, measured by reduced serum concentrations of 25 hydroxy vitamin D, is highly prevalent and associated with both endothelial dysfunction and an increased risk of cardiovascular disease. Observational studies in chronic kidney disease have demonstrated that vitamin D therapy reduces the risk of cardiovascular disease. In patients with chronic kidney disease and concomitant vitamin D deficiency, the effect of vitamin D therapy on endothelial function, which is associated with cardiovascular disease, is poorly understood. The mechanism by which vitamin D affects endothelial function is unclear. Methods Presented in this thesis, two studies have addressed these issues: 1. A double blind, randomized controlled trial evaluating the effect of ergocalciferol compared to placebo on microcirculatory endothelial function in patients with non-dialysis chronic kidney disease and concomitant vitamin D deficiency 2. In vitro and in vivo experiments to determine the mechanistic effect of ergocalciferol on endothelial function in an experimental model of uraemia. Results In the clinical study, ergocalciferol increased vitamin D serum concentrations and improved microcirculatory endothelial function measured by laser Doppler flowmetry after iontophoresis of acetylcholine. Oxidative stress measured by skin autofluorescence for advanced glycation end products did not change in the ergocalciferol group but increased significantly in the placebo group. Ergocalciferol increased endothelial nitric oxide synthase expression and activity in cultured human endothelial cells and improved endothelial function in an in vivo model of mild uraemia. The findings from the in vivo and clinical studies occurred independently of changes in blood pressure, conduit artery function, serum calcium, phosphate and parathyroid hormone supporting in vitro findings that ergocalciferol acts directly on the endothelium. Conclusion Ergocalciferol improved endothelial function in both rodent and human subjects with chronic kidney disease. Experimental evidence suggests this effect occurs through an endothelium dependent mechanism involving changes in the upregulation and function of endothelial nitric oxide synthase.
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