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Mechanisms of intestinal calcium absorption


  • Felix Bronner

    Corresponding author
    1. Department of Biostructure and Function, University of Connecticut Health Center, Farmington, Connecticut 06030-6125
    2. Department of Pharmacology, University of Connecticut Health Center, Farmington, Connecticut 06030-6125
    • University of Connecticut Health Center, BLD 26 Rm 1007, 263 Farmington Ave., Farmington, CT 06030-6125.

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Calcium is absorbed in the mammalian small intestine by two general mechanisms: a transcellular active transport process, located largely in the duodenum and upper jejunum; and a paracellular, passive process that functions throughout the length of the intestine. The transcellular process involves three major steps: entry across the brush border, mediated by a molecular structure termed CaT1, intracellular diffusion, mediated largely by the cytosolic calcium-binding protein (calbindinD9k or CaBP); and extrusion, mediated largely by the CaATPase. Chyme travels down the intestinal lumen in ∼3 h, spending only minutes in the duodenum, but over 2 h in the distal half of the small intestine. When calcium intake is low, transcellular calcium transport accounts for a substantial fraction of the absorbed calcium. When calcium intake is high, transcellular transport accounts for only a minor portion of the absorbed calcium, because of the short sojourn time and because CaT1 and CaBP, both rate-limiting, are downregulated when calcium intake is high. Biosynthesis of CaBP is fully and CaT1 function is approximately 90% vitamin D-dependent. At high calcium intakes CaT1 and CaBP are downregulated because 1,25(OH)2D3, the active vitamin D metabolite, is downregulated. J. Cell. Biochem. 88: 387–393, 2003. © 2002 Wiley-Liss, Inc.