Question

The increased intestinal absorption of calcium is stimulated directly by a. parathyroid hormone. b. 1,25 -dihydroxyvitamin \(\mathrm{D}_{3}\). c. calcitonin. d. all of these.

Short answer

The increased intestinal absorption of calcium is stimulated directly by 1,25-dihydroxyvitamin D3 (calcitriol). This hormone promotes calcium movement across the intestinal cells by binding to specific receptors in the intestine. The other options, parathyroid hormone and calcitonin, do not have a direct effect on calcium absorption in the intestine. Therefore, the correct answer is b. 1,25-dihydroxyvitamin D3.

Step-by-step solution

Option A: Parathyroid Hormone

Parathyroid hormone (PTH) is produced by the parathyroid glands. It plays a crucial role in calcium and bone metabolism. PTH stimulates the release of calcium from the bones and kidneys and increases calcium absorption in the intestines indirectly by stimulating the production of 1,25-dihydroxyvitamin D3. Therefore, it does not directly stimulate the increased intestinal absorption of calcium.

Option B: 1,25-dihydroxyvitamin D3

1,25-dihydroxyvitamin D3, also known as calcitriol, is the active form of vitamin D. It is produced in the kidneys and plays a major role in calcium absorption from the intestines. 1,25-dihydroxyvitamin D3 directly stimulates the increased intestinal absorption of calcium by binding to specific receptors in the intestine and promoting the movement of calcium across the intestinal cells.

Option C: Calcitonin

Calcitonin is a hormone produced by the thyroid gland. It plays a role in maintaining calcium levels in the body by inhibiting bone resorption (the process of breaking down bone to release calcium). Calcitonin's primary function is to lower blood calcium levels when they are too high. It does not have a direct effect on calcium absorption in the intestine.

Option D: All of these

As we have analyzed in the previous steps, parathyroid hormone and calcitonin do not directly stimulate the increased intestinal absorption of calcium. Only 1,25-dihydroxyvitamin D3 does. Therefore, the correct answer is not "all of these." Based on the analysis of each option, the correct answer is: b. 1,25-dihydroxyvitamin D3

Key concepts

Parathyroid Hormone

The parathyroid hormone (PTH) is a crucial player in the body's regulation of calcium levels. It is secreted by the parathyroid glands located behind the thyroid in the neck. One of PTH's primary functions is to increase the concentration of calcium in the blood. When calcium levels are low, PTH is released, which prompts the conversion of vitamin D into its active form, 1,25-dihydroxyvitamin D3 (calcitriol). Although PTH does contribute to the increased intestinal absorption of calcium, it does so indirectly.
It stimulates the bones to release calcium and signals the kidneys to reduce the amount of calcium excreted in urine. Furthermore, it increases the production of 1,25-dihydroxyvitamin D3, which then acts directly on the intestines to increase calcium absorption. Understanding this cascade of events helps to clarify why PTH itself is not the direct stimulant of increased calcium absorption in the intestines.

1,25-dihydroxyvitamin D3

The compound 1,25-dihydroxyvitamin D3, also known as calcitriol, is the active form of vitamin D in the body. This hormone is synthesized in the kidneys and its production is stimulated by PTH. Its central role is to regulate calcium absorption in the intestines, where it binds to vitamin D receptors on the cells lining the intestinal tract.
Calcitriol facilitates the transport of calcium across the intestinal cell membranes, thus increasing the amount of calcium that enters the bloodstream. It is the direct stimulator of calcium absorption reflected in the textbook exercise, making it an essential component for understanding calcium and bone metabolism. Importantly, knowing that 1,25-dihydroxyvitamin D3 is the direct agent can help clarify study points related to nutrient absorption and effects of vitamin D on bone health.

Calcitonin

Calcitonin, a hormone secreted by the C-cells of the thyroid gland, serves as an important regulator in maintaining a balanced level of calcium in the blood. Contrary to the actions of PTH, calcitonin is released when calcium levels are high, and it works to lower them by inhibiting the activity of osteoclasts—the cells responsible for bone resorption.
This action prevents the release of calcium from bones, which contrasts with PTH's role in releasing calcium from bones to increase blood calcium levels. In terms of direct impact on intestinal calcium absorption, calcitonin does not play a significant role. Consequently, through understanding the function of calcitonin, students can differentiate how various hormones contribute distinctly to calcium homeostasis.

Calcium and Bone Metabolism

The interplay of hormones in calcium and bone metabolism is a complex but fascinating subject crucial for maintaining skeletal strength and proper biological functions. Calcium is a vital mineral, not only for bone health but also for muscle function, nerve transmission, and blood clotting. Bone serves as the primary reservoir for calcium; it is continuously remodeled, with old bone resorbed and new bone formed.
PTH, 1,25-dihydroxyvitamin D3, and calcitonin are key hormones orchestrating this process. PTH increases blood calcium levels by stimulating bone resorption, while calcitonin does the opposite. Meanwhile, 1,25-dihydroxyvitamin D3 ensures that enough calcium is absorbed from our diet to meet the body's needs. Together, these hormones maintain calcium balance, impacting not only bone density and strength but also the overall mineral homeostasis in the body.