INTESTINAL CA2 + TRANSPORT: VIT. D-DEPENDENT MECHANISMS

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In previous studies concerning the mechanism of vitamin D-dependent Ca2+ transport across the intestine, we prepared membrane vesicles from rat intestinal epithelial cells and observed that Golgi membrane vesicles demonstrated the highest Ca++ uptake when compared with lateral-basal and microvillus membrane vesicles. Golgi Ca++ uptake was specifically dependent on 1,25(OH)2D3, the active metabolite of vitamin D. We postulate that Golgi Ca++ uptake may represent either a Ca++ sequestration mechanism or a site where Ca++ transport related factors are prepared for plasma membrane remodeling. The specific aims of this research proposal are to determine: 1) the molecular nature of the 1,25(OH)2D3 dependent change in Ca++ uptake by Golgi, 2) the mechanism by which 1,25(OH)2D3 alters Ca++ uptake by Golgi, and 3) the relationship of Golgi Ca++ uptake to intestinal Ca++ transport. The kinetics of Ca++ binding by Golgi membrane vesicles will be analyzed. The molecules responsible for the 1,25(OH)2D3 dependent Ca++ binding by Golgi will be identified, isolated and characterized. Attempts will be made to determine if 1,25(OH)2D3 stimulates the synthesis and/or appearance of proteins in Golgi or affects Golgi lipids. The effect of 1,25(OH)2D3 on intestinal messenger RNA, particularly in relationship to Golgi Ca++ uptake, will be evaluated using in vitro cell free translation systems. Finally, the movement of factors associated with Golgi Ca++ binding, as part of a process of plasma membrane remodeling will be studied. These experiments will contribute to an understanding of the control of intestinal Ca++ transport. Regulation of intestinal calcium transport is fundamental to calcium homostasis. Therefore, these studies will help in understanding the pathophysiology of osteoporosis, kidney stones, and the effect of intestinal diseases on calcium absorption and derangements of calcium metabolism.