Introduction: Parathyroid gland and hormone.
- Location
- Parathyroid hormone
- Functions of parathyroid hormone
- Mechanism of action
- Action of PTH (parathyroid hormone)
- Regulation of PTH secretion
The parathyroid gland is essential for life. It is a highly vascular and tiny gland measuring about 6 mm in length 3 mm in width and 2 mm in thickness its weight is only 120 mg it is present along with the thyroid gland.
Parathyroid glands are 4 in number. There are two on the left side and two on the right side. One parathyroid gland is present in the superior pole and one in the inferior pole of the thyroid gland on both sides.
Location
Location: In humans, there are 4 parathyroid glands. Two are present in the superior pole and two in the inferior pole of the thyroid gland. These glands are present on the posterior surface of the thyroid gland, covered by the internal and external layers of the thyroid capsule.
Two types of cells are present in the parathyroid glands
Oxyphil cells: Their functions are not well understood, but it is assumed that they support chief cells and may transform into chief cells.
Chief cells: secrete parathyroid hormone..
Parathyroid hormone
The biological half-life is less than 20 minutes; it is a polypeptide and is secreted in the blood vessels, like other hormones.
Functions of parathyroid hormone
The primary function is to regulate calcium concentration in both the extracellular and intracellular fluids. When the calcium level is low in the blood, it increases the calcium level in the blood. Its primary function is in bones, kidneys, and the gastrointestinal tract.
Mechanism of action
PTH receptors couple to the G protein, causing activation of adenylyl cyclase, which increases the formation of intracellular cAMP.
Action of PTH (parathyroid hormone)
The action of parathyroid hormone is described under the bones, Kidneys, gastrointestinal tract, and lactating mammary glands.
On bones
The action of PTH on bone is prolonged, slow, and less sensitive.
It causes bone reabsorption that promotes osteolytic action and increases plasma calcium and decreases plasma phosphate. This action is achieved by the following mechanism.
There are two types of cells in the bone responsible for the maintenance of bone density. They are osteoblasts, which increase calcium and phosphate in the bone and increase bone formation. It is responsible for providing power to the bone.
Osteoclasts are present in the bone, which is responsible for the reabsorption of the bone.
Parathyroid hormone goes to the osteoblast, where it gets attached to ‘parathyroid hormone receptors’ and stimulates the formation of Rank ligase osteoblasts, which trigger Rank receptors on the osteoclast.
This rank receptor stimulates the osteoclast to form various chemicals, including acid phosphatase and other proteolytic enzymes, facilitating the reabsorption of bone. Calcium phosphate and other chemicals are released into the circulation, thereby increasing calcium levels in circulation.
On kidneys
The action of PTH on the kidneys is quick and sensitive to minor changes in serum calcium levels. Increases calcium absorption in the distal convoluted tubule by an active process, so decreases calcium excretion in the urine.
Mechanism of action
The distal convoluted tubule is impermeable to Sodium, Potassium, calcium, and other minerals, so absorption of calcium is due to the presence of parathyroid hormone.
On the distal convoluted tubule, G receptors are present, which combine with the PTH and stimulate adenyl cyclase, which promotes the formation of cyclic AMP from ATP. Protein kinase is formed, which will cause transcription and translation that will create many calcium channels in the cell membrane of the distal convoluted tubule cells.
The calcium ions enter the distal convoluted tubule cells. The sodium-potassium channels in the distal convoluted tubule remove three sodium ions from the cells and two potassium ions into the cells.
From another channel, the sodium enters the cell in exchange for calcium, through the calcium sodium channel, which is a secondary active transport system. The primary active system is the sodium-potassium channel, and the energy produced by that is used in the calcium sodium channel. In this way, calcium is absorbed in the distal convoluted tubule.
Phosphate ions are secreted in the distal convoluted tubule. This is the Phosphaturic action of PTH.
On GIT
The phosphaturic action of PTH reduces the phosphate ion level in the blood, thereby increasing the production of 1,25-dihydroxycholecalciferol, the active form of Vitamin D.
Mechanism of action
7, dehydrocholesterol is supplied by diet milk, egg, etc. Ultraviolet rays of the sun convert it into cholecalciferol. Cholecalciferol enters the liver, where hydroxylation occurs by 25α-hydroxylase and forms 25-hydroxy cholecalciferol (25HCC) that will enter the kidneys.
In kidney’1,25α-hydroxylase, hydroxylates ‘25 hydroxy cholecalciferol into 1,25-dihydroxy cholecalciferol, the active form of D3, also known as Calcitriol.
The calcitriol enters the enterocytes –lining cells of the small intestine and binds with the cytoplasmic receptors. The receptor-calcitriol complex enters the nucleus, where it binds to the DNA gene, and a new mRNA is synthesized – transcription occurs. The mRNA initiates new protein synthesis by translation. The newly formed protein is expressed on the surface of the cell membrane. It opens several calcium channels, allowing more calcium ions to be absorbed in the enterocytes than in the blood.
On the lactating mammary gland
Parathyroid hormone reduces calcium secretion in milk.
Regulation of PTH secretion
A decrease in calcium ions in the blood stimulates PTH secretion, so that the level of calcium ions in the blood is maintained.
Increase serum phosphate level:
- Decreases serum calcium level, which will stimulate PTH secretion.
- Inhibits calcitriol formation, which will stimulate PTH secretion.
The product of serum calcium x serum phosphate in plasma remains constant.An increase in serum phosphate leads to a decrease in serum calcium and vice versa.
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