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Endocrine|Parathyroid gland and hormone
Keywords: Osteoblast|Osteoclast|
Parathyroid hormone| Oxyphil| Chief cell| Calcium sodium channel|sodium –potassium
channel| Parathyroid hormone|
Table of contents
|
1. |
Introduction |
|
2. |
Location |
|
3. |
Histology |
|
4. |
Functions Ø On bone Ø On Kidney Ø On GIT |
|
5. |
Phosphaturic action |
The parathyroid gland is essential for life. It is a highly vascular 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, 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:
In humans,
there are four parathyroid glands. Two are present in the superior and two in
the inferior poles of the posterior surface of the thyroid gland in between the
internal and external layers of the thyroid capsule.
Histology
Two
types of cells are present in parathyroid glands.
1. Oxyphil
cells’ functions are not precise, but it is assumed that they may transform into
chief cells.
2. Chief
cells secret parathyroid hormone.
Parathyroid
hormone is a polypeptide secreted directly in the blood vessels like other
hormones with a half-life of fewer than 20 minutes.
Functions of parathyroid hormone
The parathyroid
hormone primarily regulates calcium concentration in extracellular and
intracellular fluid. Its principal function is on bones, kidneys, and the gastrointestinal
tract.
It increases
the calcium level in the blood when calcium level falls in the blood.
Mechanism of action
PTH (parathyroid
hormone ) receptors coupled to G- protein causes activation of ‘adenylyl
cyclase,’ increasing the formation of intracellular cyclic AMP.
Actions of
parathyroid hormone are divided into three headings
1. Bones 2. Kidneys
and 3. Gastrointestinal tract.
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. The following mechanism achieves this action:
Two
types of cells in the bone maintain bone density. They are
1. Osteoblast
increases calcium phosphate in bone and initiates bone formation, providing
bone power.
2. Osteoclast
is responsible for bone reabsorption.
Parathyroid
hormone attaches to the ‘parathyroid hormone receptors’ of the osteoblast and
stimulates the formation of ‘rank ligase’ by the osteoblast. The rank ligase binds
to the ‘rank receptor on the osteoclast and stimulates the osteoclast to form
many types of proteolytic chemicals, including ‘acid phosphatase ‘and others,
so bone-reabsorption occurs. As a result, calcium phosphate and other chemicals
are released into the circulation, increasing blood calcium levels.
On kidneys
The action
of PTH on kidneys is quick and sensitive for minor changes of serum calcium
level. It increases calcium absorption in the distal convoluted tubule by an active
process, decreasing calcium excretion in the urine.
Mechanism of action
The distal
convoluted tubule of the nephron is impermeable to electrolytes, e.g., Sodium,
Potassium, Calcium, etc.
The distal convoluted tubule cells have ‘parathyroid
hormone G- receptors’ to which the parathyroid hormone binds and stimulates
‘adenyl cyclase.
The adenyl-cyclase
stimulates the formation of cyclic AMP from ATP and triggers sequences of
events leading to transcription and translation that will synthesize many
calcium channels in the cell membrane of the distal convoluted tubule cell.
Calcium
will enter through these calcium channels into the distal convoluted tubule
cells.
The distal
convoluted tubule cells have the sodium-potassium channel, removing three
sodium ions and adding two potassium ions to the cell.
The distal
convoluted tubule cells have another ‘Calcium sodium channel’ through which
sodium ions enter the cell in exchange for calcium. Calcium sodium channel’ is a secondary active transport system.
The Sodium Potassium channel is an active system, and
energy produced is used for the calcium sodium channel.
In this way,
calcium is absorbed in the distal convoluted tubule.
Phosphaturic
action of PTH
Phosphate
ions are secreted in the distal convoluted tubule.
On GIT
Phosphaturic
action of PTH causes the reduction of phosphate ion levels in the blood,
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’ hydroxylate ’25 hydroxy cholecalciferol’
into ‘1,25 dihydroxy cholecalciferol’, the active form of D3 known as ‘Calcitriol.’
The calcitriol’ enters the enterocytes –lining cell of the small
intestine and binds with the cytoplasm receptors. The receptor - calcitriol’
complex enters the nucleus and binds with the DNA gene to synthesize, a new
mRNA is –transcription occurs. The mRNA
initiates new protein synthesis by translation. The newly formed protein is
expressed on the surface cell membrane and opens several calcium channels so
that more and more calcium ions are absorbed in the enterocytes and then in the
blood.
On the
lactating mammary gland
Parathyroid hormone reduces calcium secretion in
milk.
Regulation
of PTH secretion
Decrease calcium ions in the blood stimulate PTH secretion so that the level
of calcium ions in the blood is maintained.
Increase serum phosphate level:
Ø decreases
serum calcium level that will stimulate PTH secretion.
Ø Inhibits
calcitriol formation that will stimulate PTH secretion.
Increased serum phosphate decreases the serum calcium and vice versa.
The product of serum calcium and serum phosphate in plasma remains
constant.
Hashtag: Osteoblast#Osteoclast#
Parathyroid hormone# Oxyphil# Chief cell# Calcium sodium channel# sodium –potassium
channel# Parathyroid hormone#
Internal link: https://blog.totalphysiology.com/2022/02/thyroid-hormonesendocrine-gland.html
blog.totalphysiology.com/2022/01/pituitary-glandsecretionhormones.html
https://blog.totalphysiology.com/2021/03/2021-google-ductlessglands-we-must-know.html
https://blog.totalphysiology.com/2021/03/gastrointestinal-hormones-you-must-know.html
External link: https://en.m.wikipedia.org>wiki
Very good article
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