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Background: A number of observations suggest that vitamin D plays an important role in maintaining normal cardiovascular function, through its receptors in cardiac muscle or in aortal smooth muscle cells (SMCs). The purpose of this study was to determine the effect of vitamin D active metabolite - calcitriol [1,25(OH)2D3/sub>] on aortal SMCs.
Material/Methods: The cells were isolated from the aortal media of newborn rats by enzymatic digestion and maintained for 6 weeks in primary culture. 1.2 µM of calcitriol was added to the medium every second day.
Results:     Light microscopic investigations revealed that the control SMCs formed multilayer with characteristic 'hills and valleys'. Uniform cellular growth patterns were observed after calcitriol treatment. Ultrastructural studies indicated more rapid modulation of SMCs from the 'contractile' to the 'synthetic' phenotype following calcitriol treatment. Elastic fibers were more abundant in treated than in control cultures. The scanning electron microscope showed an increase of regular microvilli on the surface of SMCs. Morphometrical analysis of nuclei demonstrated phenotypic heterogeneity in populations of aortal SMCs. These results suggest that 1) calcitriol can induce changes in the phenotype and in the growth pattern of aortal SMCs, which may be associated with the onset or progression of the atherosclerosis process, 2) the basis of phenotypic changes is related to the stimulation of proliferative activity in these cells.