Osmosis from Elsevier - Calcitonin
Calcitonin is a hormone produced by the C cells of the thyroid gland, which plays a role in regulating blood calcium levels. It is released when blood calcium levels rise above normal. Calcitonin lowers calcium levels primarily by acting on bones and kidneys. In bones, it binds to receptors on osteoclasts, reducing their activity and thus decreasing bone resorption, which in turn lowers blood calcium levels. In the kidneys, calcitonin reduces calcium and phosphate reabsorption, leading to increased excretion in urine. Despite its role, the exact necessity of calcitonin in calcium homeostasis is unclear, as the body can maintain calcium levels without it. However, it is sometimes used in high doses to treat hypercalcemia.
Key Points:
- Calcitonin is produced by C cells in the thyroid gland and helps regulate blood calcium levels.
- It lowers blood calcium by inhibiting osteoclast activity in bones, reducing bone resorption.
- In the kidneys, calcitonin decreases calcium and phosphate reabsorption, increasing their excretion.
- The necessity of calcitonin in calcium regulation is uncertain, as the body can maintain levels without it.
- Calcitonin is used in high doses to treat conditions like hypercalcemia.
Details:
1. ð Hormonal Regulation of Calcium Levels
- The body's calcium level is tightly regulated by three key hormones: parathyroid hormone (PTH), vitamin D, and calcitonin.
- Parathyroid hormone (PTH) is secreted by the parathyroid glands and increases blood calcium levels by stimulating calcium release from bones, increasing calcium absorption in the intestines, and conserving calcium in the kidneys.
- Vitamin D enhances intestinal absorption of calcium and also supports the bone resorption actions of PTH, further increasing calcium levels.
- Calcitonin, produced by the thyroid gland, counteracts the effects of PTH and vitamin D by lowering blood calcium levels, primarily through inhibiting bone resorption and increasing calcium excretion in the kidneys.
- These hormones act in concert to maintain blood calcium levels within a narrow range, crucial for normal physiological functions such as nerve conduction, muscle contraction, and blood clotting.
- For example, in conditions of hypocalcemia, the body increases PTH and vitamin D levels to restore calcium balance, illustrating the feedback mechanisms in place.
2. 𧎠Types of Calcium: Diffusible vs. Non-Diffusible
- Extracellular calcium is divided almost equally into diffusible and non-diffusible types.
- Diffusible calcium includes ionized calcium and calcium bound to small anions, which can easily cross cell membranes and participate in various physiological processes.
- Non-diffusible calcium is primarily bound to proteins and cannot cross cell membranes, playing a critical role in maintaining calcium balance and serving as a reservoir.
3. ð The Role and Origin of Calcitonin
- Calcitonin is a polypeptide hormone responsible for regulating blood calcium levels.
- It originates from the C cells of the thyroid gland, located in the neck.
- C cells are found in the connective tissue between follicles of the thyroid gland.
- The synthesis process begins with prepr calcitonin, consisting of 141 amino acids, indicating the initial stage of its production.
- Proteolytic cleavage reduces prepr calcitonin to procalcitonin with 116 amino acids, marking a significant step in its maturation.
- Further cleavage produces immature calcitonin with 33 amino acids, illustrating a near-final stage before full maturation.
- Finally, mature calcitonin with 32 amino acids is formed, ready for biological function.
- Mature calcitonin is stored in secretory granules in C cells until it is released, highlighting its role in immediate calcium regulation when needed.
4. ð§ Calcitonin's Enigmatic Role in Calcium Homeostasis
- Calcitonin is not involved in the minute-to-minute regulation of blood calcium concentration, indicating it does not play a central role in immediate calcium homeostasis.
- The absence of calcitonin, such as after the surgical removal of the thyroid gland, does not appear to significantly impact blood calcium regulation, suggesting other mechanisms compensate for its loss.
- Despite the unclear role, the body does demonstrate physiological responses to calcitonin, which implies it has effects that are not yet fully understood or are more complex than currently known.
5. ðĶī Calcitonin's Mechanism in Bones and Kidneys
- Calcitonin lowers blood calcium levels by targeting bones and kidneys.
- In bones, calcitonin binds to receptors on osteoclasts, the cells responsible for bone resorption.
- Osteoclasts typically form a ruffled border to secrete acid and break down bone, releasing calcium and phosphate into the blood.
- When calcitonin binds to its receptor on osteoclasts, it triggers a G protein that activates adenylate cyclase, converting ATP to cyclic AMP (cAMP).
- Increased cAMP levels cause osteoclasts to reduce arm formation, preventing the maintenance of the ruffled border.
- This leads to decreased bone resorption and subsequently lowers blood calcium levels.