Question at position 18 The calcitriol-vitamin D receptor complex travels to the nucleus of intestinal cells where it promotes calcium absorption by increasing the synthesis of which of the following proteins?CholecalciferolCalcitoninErgocalciferolCalbindin单项选择题
A
Cholecalciferol
B
Calcitonin
C
Ergocalciferol
D
Calbindin

Question

Question at position 18 The calcitriol-vitamin D receptor complex travels to the nucleus of intestinal cells where it promotes calcium absorption by increasing the synthesis of which of the following proteins?CholecalciferolCalcitoninErgocalciferolCalbindin单项选择题

A

Cholecalciferol

B

Calcitonin

C

Ergocalciferol

D

Calbindin

BlackTom AI · Accepted Answer

Calcitriol–vitamin D receptor complex upregulates the gene encoding calbindin, a cytosolic calcium‑binding protein that facilitates intestinal transcellular calcium transport.

1. Relevant knowledge points and concepts
- Vitamin D metabolism: cholecalciferol (D3) and ergocalciferol (D2) are precursors that are hydroxylated to 25‑OH vitamin D in the liver and then to the active hormone 1,25‑dihydroxyvitamin D (calcitriol) in the kidney.
- Calcitriol binds the intracellular vitamin D receptor (VDR); the calcitriol–VDR forms a heterodimer with retinoid X receptor (RXR) and binds vitamin D response elements on DNA to regulate transcription.
- Key targets in intestinal enterocytes include calbindin (especially calbindin‑D9k), the apical Ca2+ channel TRPV6, and the basolateral Ca2+ pump (PMCA1b), all promoting net Ca2+ absorption.

2. Step‑by‑step reasoning process
- Calcitriol produced in kidney enters intestinal epithelial cells.
- Calcitriol binds VDR; the complex heterodimerizes with RXR and binds specific DNA response elements.
- This binding increases transcription of genes coding for proteins that mediate transcellular Ca2+ movement.
- The primary cytosolic protein induced is calbindin, which binds Ca2+ intracellularly and shuttles it to the basolateral membrane where it is extruded, increasing net absorption.

3. Intermediate steps (molecular sequence)
- D3/D2 → 25‑OH D (liver) → 1,25‑(OH)2D (kidney = calcitriol).
- Calcitriol + VDR → VDR–RXR → binds DNA → ↑ mRNA for calbindin (and TRPV6, PMCA1b) → ↑ calbindin protein → ↑ Ca2+ transport.

4. Why the correct answer is correct
- Calbindin is the canonical protein whose synthesis is increased by calcitriol–VDR in enterocytes to promote Ca2+ absorption.

5. Why other options are incorrect
- Cholecalciferol and ergocalciferol are vitamin D precursors, not proteins whose synthesis is induced by calcitriol in the intestine.
- Calcitonin is a peptide hormone from thyroid C cells that lowers plasma Ca2+ (opposes PTH/calcitriol effects) and is not induced by calcitriol to increase intestinal Ca2+ absorption.

6. Conclusion
The correct choice is Calbindin, matching the provided answer.

Question at position 18 The calcitriol-vitamin D receptor complex travels to the nucleus of intestinal cells where it promotes calcium absorption by increasing the synthesis of which of the following proteins?CholecalciferolCalcitoninErgocalciferolCalbindin单项选择题

类似问题

10. Which of the following decreases calcium absorption in the small intestine?

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Calcium absorption is likely to be highest in

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What is the primary function of Vitamin D?

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Question at position 18 The calcitriol-vitamin D receptor complex travels to the nucleus of intestinal cells where it promotes calcium absorption by increasing the synthesis of which of the following proteins?CholecalciferolCalcitoninErgocalciferolCalbindin单项选择题

类似问题

10. Which of the following decreases calcium absorption in the small intestine?

18. Calcitriol increases blood calcium levels by _____________.

Calcium absorption is likely to be highest in

Vitamin D is used to enhance absorption of

​Comprehension-Level Multiple-Choice Items A person’s absorption of calcium varies based on:​

What is the primary function of Vitamin D?

For which of the following functions is Vitamin D is important?

Oxalic acid, a component of fiber, can inhibit the absorption of

更多留学生实用工具

希望你的学习变得更简单

Comprehension-Level Multiple-Choice Items A person’s absorption of calcium varies based on: