Question

Parathyroid hormone enhances Ca absorption by
A. Increasing the pH of the gut to alkaline side
B. Liberating lysine and arginine amino acids from dietary proteins
C. Decreasing the synthesis of phosphorus
D. Stimulating the synthesis of calcitriol
Choose the correct answer from the options given below:

• A. A and C only
• B. A and D only
• C. C and D only
• D. B and D only

Hint:

PTH does not act directly on the gut to increase calcium absorption.
Instead, it acts indirectly by stimulating the kidneys to synthesize active Vitamin D (calcitriol), which then increases gut calcium absorption.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Calcium homeostasis in the body is regulated by parathyroid hormone (PTH), calcitonin, and vitamin D.
When blood calcium levels fall, the parathyroid glands release PTH to restore calcium balance via effects on the bones, kidneys, and intestines.

Step 2: Detailed Explanation:
Parathyroid hormone (PTH) acts on multiple organs to increase plasma calcium levels:
In the kidneys, PTH stimulates the enzyme 1-$\alpha$-hydroxylase. This enzyme converts 25-hydroxyvitamin D into its active form, 1,25-dihydroxyvitamin D (calcitriol). This corresponds to statement D.
Calcitriol then travels to the intestine, where it upregulates calcium-binding proteins (like calbindin) to increase calcium absorption from dietary sources.
In the gut, calcium absorption is also favored by a slightly alkaline pH, which prevents calcium from precipitating as insoluble salts. This corresponds to statement A.
Statements B and C are incorrect: PTH does not regulate calcium absorption by breaking down specific amino acids like lysine or arginine, nor does it decrease the body's synthesis of phosphorus.
Therefore, PTH enhances intestinal calcium absorption through both alkaline gut pH and the stimulation of calcitriol synthesis.

Step 3: Final Answer:
The correct statements are A and D, corresponding to Option (B).