Mechanisms of intestinal calcium absorption

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 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.

Synthesis and characterization of nano-sized CaCO3 in purified diet

“…Calcium containing substances are required by children, pregnant and lactating woman and also osteoporosis people for bones and teeth development. The value of calcium is close to the recommended daily allowance of 15,000 ppm per day for both adults and children [14]. While for osteoporosis people, the calcium intake is higher than the recommended daily allowance.…”

“…CaCO3 is the most prevalent form of a calcium supplement [9][10][11][12]. The low efficacy of CaCO3 may be overcome by applying nano-sized materials, which can induce cellular uptake by endocytosis [13][14] In this work, nano-sized CaCO3 was obtained by milling process and this CaCO3 nano particle was added to purified diet with the various concentrations. CaCO3, is the most commonly used mineral as a source of inorganic salt.…”

The epithelial calcium channels TRPV5 and TRPV6: regulation and implications for disease

“…1), consisting of passive entry of Ca 2+ across the luminal or apical membrane, cytosolic diffusion of Ca 2+ bound to vitamin D 3 -sensitive Ca 2+ -binding proteins (calbindin-D 28K and/or calbindin-D 9K ) and active extrusion of Ca 2+ across the opposite basolateral membrane by the Na + -Ca 2+ -exchanger (NCX1) and/ or Ca 2+ -ATPase (PMCA1b; Bronner 2003;Hoenderop et al 2002b). The molecular identity of the apical Ca 2+ entry pathway remained elusive until the epithelial Ca 2+ channels TRPV5 (previously named ECaC1; Hoenderop et al 1999) and TRPV6 (previously named Ca 2+ transporter 1) were identified (Montell et al 2002b;Peng et al 1999).…”

Calbindin Independence of Calcium Transport in Developing Teeth Contradicts the Calcium Ferry Dogma

“…The 30-year-old paradigm that calcium is ferried through cytosol by mobile calcium-binding proteins (calbindins) remains widely accepted (3)(4)(5)(6)(7)(8)(9). Calbindins are thought to facilitate the naturally poor diffusion of calcium in cytosol and simultaneously buffer calcium at safe concentrations.…”