| Research Abstract |
A low pH microclimate layer of about 500-700um thickness is known to exit on the mucosal surface of the small intestine. It is also shown that the low pH of this layer is maintained by the activity of the Na^+/H^+ antiport in the luminal membrane of the epithelial cells and the formation of mucin layer which has a property of anion exchanger. Physiological importance of this low pH layer in Ca^<2+> and Mg^<2+> absorption is still unknown. To clarify this problem, our first series of experiments intended to know basal values of the lowest pH in this layer along the small intestine and regional difference in activity of Ca^<2+> and Mg^<2+> trans-port. It was found that, in guinea pig, the lowest value of pH was uniform along the intestine (about pH 6.0) and active Ca^<2+> and Mg^<2+> fluxes were seen along the entire length of the intestine. Active Mg^<2+> flux was not significantly inhibited by the presence of equimolar Ca^<2+>, but Ca^<2+> flux was significantly depressed by Mg^<2+>, N
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a^+-free conditions and ouabain inhibited Mg^<2+> flux more strongly than Ca^<2+> flux. pH-dependence of various hardly-soluble Ca salts, such as CaHPO_4, CaCO_3, Ca-oxalate, phytine, was examined by changing pH of their suspention fluids in the presence or absence of 150 mM NaCl. It was found that, in the normal pH range of the microclimate layer, i.e. 7.4-5.5, CaHPO_4 and CaCO_3 revealed clear pH-dependent dissociation, and the dissociation was markedly enhanced by NaCl. In the presence of 150 mM NaCl, their dissocation reached 100% at pH 5.5. Phytine(6Ca-phytate) also showed increased dissocation with lowering pH in this pH range. Ca-oxalate did not show the pH-dependent increase in dissociation. Using everted ileal preparations, net Ca^<2+> flux from suspentions of these hardly-dissociable Ca salts was examined at pH 7.4 and 6.0 in the presence of 1 mM DTT.Marked increased in net Ca flux were observed in the suspentions of CaHPO_4, CaCO_3 and 6Ca-phytate, indicating that the low pH microclimate layer is playing an important role in Ca^<2+> absorption from food. Less
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