Prior studies by our laboratory have shown that vitamin D-deprivation for 11 to 18 weeks resulted in alterations in the lipid fluidity and fatty acid composition of phosphatidylcholine (PC) and phosphatidylethanolamine (PR) of rat proximal small intestinal brush- border membranes. Moreover, administration of 1,25(OH)2D3 to D-deprived animals rapidly (1-2 h) corrected these membrane alterations, temporally preceding the earliest detectable stimulation of transmucosal Ca2+ transport by this hormone )(5 h). These observations led us to hypothesize that these vitamin D-induced membrane changes were responsible for this hormone's effects and Ca2+ transport. Based on these prior findings, the overall goal of the present proposal is to determine whether these vitamin D-induced brush-border membrane alterations are indeed responsible for its Ca2+ transport effects. To achieve this goal, tow sets of experiments are planned. First, weanling male-Wistar rats will be deprived of dietary and light sources of vitamin D for 14 weeks (group A). Age-matched dietary D-replete rats will serve as controls (group B). Rats from each group will be treated with analogs of vitamin D or vehicle, respectively. After 2-5 h, the rats will be sacrificed and brush-border membrane vesicles isolated. These preparations will be examined and compared with respect to their: 1) lipid fluidity; 2) lipid composition including individual molecular species of PC nd PE; 3) Ca2+ transport; and 4) various enzymatic activities. Once these studies are accomplished, membranes will be prepared from animals i groups A and B and treated in vitro with nonspecific lipid transfer protein together with synthetic liposomes in order to enrich these membranes with specific molecular species of PC and/or PE found to be altered in the earlier studies (see above). After treatment, membrane vesicles from both groups will be analyzed and compared with respect to their lipid composition, Ca2+ transport and fluidity. It is anticipated that these experiments will increase our knowledge of the mechanism(s) involved i the action of vitamin D on intestinal calcium transport.
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