Mechanisms of intestinal calcium absorption

✍ Scribed by Felix Bronner

• Publisher: John Wiley and Sons
• Year: 2002
• Tongue: English
• Weight: 166 KB
• Volume: 88
• Category: Article
• ISSN: 0730-2312
• DOI: 10.1002/jcb.10330

✦ Synopsis

Abstract

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 ∼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. J. Cell. Biochem. 88: 387–393, 2003. © 2002 Wiley‐Liss, Inc.