Mechanisms of intestinal calcium absorption.
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 (calbindinD(9k) or CaBP); and extrusion, mediated largely by the CaATPase. Chyme travels down the intestinal lumen in approximately 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)(2)D(3), the active vitamin D metabolite, is downregulated.
Bronner
Felix
F
Department of Biostructure and Function, University of Connecticut Health Center, Farmington, Connecticut 06030-6125, USA. bronner@neuron.uchc.edu
D000818:Animals; D001693:Biological Transport, Active / Q000502:physiology; D064026:Calbindins; D002118:Calcium / Q000378:metabolism* / Q000493:pharmacokinetics; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D004058:Diffusion; D006801:Humans; D007408:Intestinal Absorption / Q000502:physiology*; D064030:S100 Calcium Binding Protein G / Q000378:metabolism; D014807:Vitamin D / Q000378:metabolism
D064026:Calbindins; D064030:S100 Calcium Binding Protein G; D014807:Vitamin D; D000252:Calcium-Transporting ATPases; D002118:Calcium