What is Pituitary Gland and its Secretions?
The human Pituitary gland is reddish–Grey, an oval structure about 10mm in diameter in the brain just behind the optic chiasma as an extension of the field the hypothalamus. The average weight of the gland in females is a 0.5–0.6gm and in males is 0.6 to 0.7 gm. The Pituitary gland location below the Hypothalamus of the brain.
The pituitary gland is made of 3 lobes namely,
- Anterior pituitary gland lobe (largest of all) [70%]
- Pituitary gland Middle lobe (5%)
- Posterior pituitary gland lobe (25%)
Anterior Pituitary Gland lobe
“Andreas Vesalius” described the pituitary body in 1543. The anterior pituitary hormones are tropic (trophikosG =nourishing) or tropins (troposG =turning) in nature, stimulating the secretion of all these hormones from target organs. Hormones secreted by anterior pituitary are,
- Growth Hormone (GH)
- Adrenocorticotrophic hormone (ACTH)
- Leutinizing Hormone (LH)
- Follicular Stimulating Hormone (FSH)
- Thyroid-Stimulating Hormone (TSH)
- Melanocyte-stimulating Hormone (MSH)
Posterior Pituitary Gland lobe
The Posterior pituitary gland contains two hormones.
- Oxytocin (or) Ocytocin
Anterior Pituitary gland lobe
Adrenocorticotrophic hormone (ACTH)
- It is a straight chain polypeptide with a molecular weight of about 4,500.
- It comprises 39 amino acids residues in mammals like a man, ox, sheep & Pig.
- The most potent segment of activity is from residue 15 to 18.
- ACTH increases the synthesis of corticosteroids by the adrenal cortex and also stimulates their release from the gland.
- ACTH is found to increase the transfer of cholesterol from plasma lipoproteins into the fasciculata cells.
- The ACTH induces a rise in cAMP, brings about phosphoryl and activation of “Cholesteryl esterase.”
- the enzyme action ultimately makes a large pool of free cholesterol.
- It activates the rate-limiting enzyme for conversion of cholesterol to pregnenolone (Cholesterol desmolase).
- It activates dehydrogenases of HMP to increase the concentration of NADPH required for hydroxylation.
- It activates “Hormone-sensitive lipase,” it involves which in lipolysis which increases the level of free fatty acids.
The circulating concentration of ACTH in normal is 0.1 to 2.0 m µ/dl. The pituitary gland stores 5 to 10mg.
Abnormality (or) Clinical importance:
- Oversecretion of ACTH leads to Cushing’s disease, because of tumor (or) hyperplasia of β-cells of the anterior pituitary gland lobe.
- It leads to hypersecretion of corticosteroids especially glucocorticoids.
- It produces symptoms like hyperglycemia, glycosuria, muscle wasting, atrophy of the skin, high urinary negative nitrogen balance.
- Abnormal retention of fats giving moon shape face appearance, retention of sodium ions in water and hypertension.
- LH is a glycoprotein in nature; it contains sialic acid, hexose, and hexosamine as the carbohydrate moiety (16%).
- The molecular weight of LH is 40,000
- LH is a dimer in nature; it consists of α-chain and β-chains linked non-covalently.
- The β-chain of human FSH and LH has 112 amino acid residues.
This hormone is also known as “Interstitial Cells Stimulating Hormone (ICSH).” In females,
- It causes the final maturation of graffian follicles and stimulates ovulation.
- Stimulates secretion of estrogen by the THECA and Granulosa cells.
- It helps in the formation and development of corpus lutem for leutinization of cells.
- In the ovary, it can stimulate the non-germinal elements, which contain the interstitial cells to produce the androgens, androstenedione, DHEA, and testosterone.
Follicle Stimulating Hormone (FSH)
- The molecular weight of FSH is 25,000.
- These are dimers of α-chain and β-chains linked non-covalently.
- α-Chain is identical to LH, TSH, but the β-chain of human FSH has 118 amino acid residues.
- Glycoprotein in nature, with sialic acid, hexose, and hexosamine as the carbohydrate moiety (16%)
It brings about its action by a specific receptor binding and cAMP.
- It promotes follicular growth
- It prepares the graffian follicle for the action of LH.
- It enhances the release of estrogen induces by LH.
- It stimulates seminal tubular and testicular growth.
- It plays an important role in the maturation of spermatozoa.
Thyroid-Stimulating Hormone (TSH)
- It is a glycoprotein with a molecular weight 30,000.
- Each molecule has 8 to 9 cysteine residues.
- It is produced by “Basophil cells” of anterior pituitary gland lobe.
- It consists of α-chain and β-chains linked non-covalently. • The α-chain subunit of TSH, LH, HCG & FSH is nearly identical.
- The α-subunit consists 92 amino acid & β-subunit consist 112 amino acids.
It stimulates the activity of thyroid gland and enhances the rate of specific reactions, such as
- Removal of iodine from blood by the thyroid.
- Conversion of iodide to thyroid hormones.
- The release of hormonal iodide from thyroid.
Melanocyte-stimulating Hormone (MSH)
The hormones secreted by intermediate lobe (or) middle lobe of the pituitary gland are called “MSH.”
- POMC (Proopiomelanocortin) is the precursor molecule, which is cleaved by “Proteases” to give ACTH, and β-MSH is familiar to both α-MSH and ACTH.
- The ACTH further cleaved to β-MSH, which has 13 amino acids.
- It also α-MSH which is present in more significant quantities.
- Amino acids 11 to 17 of β-MSH are common to both α-MSH and ACTH.
MSH darkens the skin and is involved in skin pigmentation by deposition of mammalian by melanocytes.
- MSH is in excess, which increases the synthesis of melanin resulting in brown pigmentation of the skin.
- Such a condition occurs in this disease.
Posterior Pituitary Gland Lobe
The Posterior pituitary gland lobe contains two hormones.
2) Oxytocin (or) Ocytocin
It is also called “Arginine Vasopressin”(AVP) • If “Arg” is replaced by “Lysine,” it is called “Lysine vasopressin” (LVP). • It is “Nonapeptide,” it consists of nine amino acids.
Mechanism of Action:
ADH binds to membrane receptor and activates adenyl cyclase. The cAMP thus produced will activate the protein kinase. In turn, it phosphorylates proteins of the microtubules and microfilaments. The net effect is the reabsorption of water.
Vasopressin causes “Antidiuresis.” It increases the permeability of the collecting ducts and tubules to water and allows most of the water to be reabsorbed as the tubular fluid passes through these ducts, thereby conserving water in the body and producing very concentrated urine.
b) Urea-retention effect:
Vasopressin increases the permeability of medullary collecting dust to urea. I lead to retention of urea and subsequently contributes to hypertonicity of the medullary interstitium.
c) Pressor effect: It stimulates the contraction of smooth muscles and thus causes vasoconstriction by increasing cytosolic Ca +2 contraction.
e) Glycogenolytic effect: By increasing intracellular Ca +2 concentration, it causes glycogenolysis.
Deficiency of ADH leads to “Diabetes insipidus,” it is characterized by excretion of large volumes of dilute urine. Excess secretion of ADH often results from the ectopic production of ADH by malignant tumors, referred to as the “Syndrome of Inappropriate secretion of ADH” (SIADH). There is the hypotonic expansion of extracellular volume, with hyponatremia.
Oxytocin (or) Ocytocin
The term means (Ocy G=quick; tokos G=birth) “to stimulate birth.” It is a polypeptide, each containing nine amino acids, so it calls Nona peptide.
Mechanism of action:
Oxytocin acts on an estrogen-primed uterus. When the uterus labor is near, the numbers of oxytocin receptors are increases to 100 to 200 times.
Oxytocin mainly causes smooth muscle contraction.
- Effect on the uterus and birth: Oxytocin causes the contraction of the pregnant uterus especially toward the end of gestation and is partially responsible for effecting delivery.
- Effect of oxytocin on milk ejection: Oxytocin affects the mammary gland. Suckling generates a neurogenic reflex, which stimulates the production of oxytocin of oxytocin. It causes contraction of the myoepithelial called expelling the milk into milk ducts from the acini.