- An elderly person suffering from calcium deficiency was advised to take calcium rich food and to supplement the diet with vitamin D. The absorption of calcium in the intestine was increased with the supplementation of vitamin-D. Following explanations were offered for this
increased calcium absorption by vitamin D:
A. The synthesis of Calbindin-D9K and Calbindin-D28K in enterocytes was stimulated.
B. The number of Ca2+ -ATPase molecules in enterocytes was increased.
C. The synthesis of divalent metal transporters 1 (DMT1) in the enterocytes was stimulated.
D. The number of hephaestin in the enterocytes was increased.
Which of the above combinations is correct?
(1) A and B (2) B and C
(3) C and D (4) A and DCalcium deficiency is a common issue, especially in the elderly, leading to problems like osteoporosis and impaired bone health. To improve calcium status, dietary calcium intake is advised along with vitamin D supplementation, which significantly enhances intestinal calcium absorption. Vitamin D’s active form, 1,25-dihydroxyvitamin D3, regulates key proteins and mechanisms in the intestine that facilitate calcium uptake.
How Vitamin D Increases Calcium Absorption
Vitamin D stimulates the synthesis and function of proteins involved in the transcellular active transport of calcium in enterocytes of the small intestine. The process involves three key stages:
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Calcium entry at the apical membrane: Calcium enters the enterocytes through specialized calcium channels, primarily TRPV6 (transient receptor potential vanilloid type 6).
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Intracellular calcium transport: Inside the cell, calcium binds to calcium-binding proteins such as Calbindin-D9K and Calbindin-D28K (the latter predominantly in kidneys), which facilitate safe and efficient transport of calcium through the cytoplasm.
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Basolateral calcium extrusion: Calcium is actively pumped out of the cell into the bloodstream via Ca2+-ATPase pumps, particularly PMCA1b (plasma membrane Ca2+ ATPase).
Explanation of the Statements
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Statement A: Correct. Vitamin D stimulates the synthesis of Calbindin-D9K in enterocytes, facilitating intracellular calcium transport.
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Statement B: Correct. Vitamin D increases the number and activity of Ca2+-ATPase pumps, enhancing calcium extrusion into circulation.
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Statement C: Incorrect. Divalent metal transporter 1 (DMT1) primarily transports iron, not calcium.
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Statement D: Incorrect. Hephaestin is involved in iron metabolism, particularly in ferroxidase activity during iron absorption, and is not directly related to calcium transport.
Correct Combination
The right combination explaining vitamin D enhanced calcium absorption is:
(1) A and B
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