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The following points highlight the nine main hormones released by anterior pituitary gland. The hormones are: 1. Tropic Hormone 2. Adrenocorticotropic Hormone 3. Lutenizing Hormone (LH) 4. Follicle Stimulating Hormone (FSH) 5. Growth Hormone (GH, Somatotroph in) 6. Prolactin (PL) 7. Neurohypophysis (The Posterior Lobe of the Pituitary) Oxytocin 8. Vasopressin 9. Middle Lobe of the Pituitary.
Anterior Pituitary Gland: Hormone # 1. Tropic Hormone:
Thyrotropic Hormone; Thyroid Stimulating Hormone (TSH):
Chemistry:
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a. It is a glycoprotein of molecular weight about 30,000.
b. It consists of two polypeptide chains termed TSH-cc and TSH-0.
c. It is very rich in sulphur containing amino acids consisting of 11 disulphide residues and contains fucose, mannose, galactose, glucosamine and galactosamine.
Functions:
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a. It binds to specific membrane receptors and activates thyroidal adenylate cyclase causing an increased cellular cAMP.
b. It increases the rates of the removal of inorganic iodide from blood by thyroid and incorporation of iodide in the thyroid hormone.
c. It increases the release of thyroxine from the thyroid gland to circulation.
d. It is used clinically to differentiate primary hypothyroidism (myxedema) from secondary hypothyroidism (pituitary insufficiency).
Control of Release:
The hypothalamus secretes thyrotrophic releasing hormone (TRH), a regulatory factor. It is a tripeptide containing pyro-glutamic acid, histidine, and prolinamide. This factor is abundant in the pineal gland and frog skin.
Inhibitors of protein synthesis have no effect on the synthesis of this factor from the hypothalamus which indicates that the process is non-ribosomal. Its action is calcium dependent. This factor specifically acts on the thyrotropin secreting cells. TRH also stimulates prolactin secretion.
Anterior Pituitary Gland: Hormone # 2. Adrenocorticotropic Hormone:
Chemistry:
a. It is a straight chain polypeptide of molecular weight 4,500 containing 39 amino acids.
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b. Only the first 23 amino acids are required for activity. These 23 amino acids in the peptide chain is the same in all species; whereas the remaining biologically inactive 16 amino acid chain varies according to the animal source.
Functions:
a. It stimulates the synthesis of corticosteroids by the adrenal gland and also enhances their release from that gland.
b. It increases the total protein synthesis.
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c. It causes the synthesis of steroid hormone from cholesterol.
d. It has a mild stimulating effect in dispersing melanin granules on the skin.
e. The administration of ACTH causes:
(i) The increased excretion of nitrogen, potassium and phosphorus,
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(ii) Retention of sodium, chloride and secondary retention of water,
(iii) Elevation of fasting blood sugar,
(iv) Increase in circulating free fatty acids,
(v) Increased excretion of uric acid,
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(vi) Decline in circulating eosinophil’s and lymphocytes.
f. Since it activates adenylate cyclase and increases intracellular levels of cAMP, it can increase lipolysis in adipose tissue and stimulate insulin secretion from the pancreas.
g. Puromycin, the blocking agent of protein synthesis, inhibits the effect of ACTH on the adrenal gland.
Control of ACTH Secretion:
a. It is controlled by corticotropin-releasing hormone (CRH) of the hypothalamus.
b. Activation of the hypothalamic centres takes place via neurotransmitters in the central nervous system (e.g., acetylcholine and serotonin). Therefore, stresses such as cold, insulin hypoglycemia, epinephrine, estrogens, psychic stimuli cause an increased production of ACTH leading to increased adrenal cortical activity. High levels of ACTH inhibits the further synthesis of ACTH.
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The normal level of ACTH in serum is 15-17 pg/ml.
Anterior Pituitary Gland: Hormone # 3. Lutenizing Hormone (LH):
Functions:
a. In the female, it stimulates final maturation of the graafian follicle, ovulation, and .the development of the corpora lutea. The secretion of estrogen and progesterone is also stimulated.
b. In the male, it stimulates testosterone production by the testis which maintains spermatogenesis and causes the development of accessory sex organs such as the vas deferens, prostate, and seminal vesicles.
c. It increases cAMP. Both cAMP and LH are blocked by puromycin.
In serum the normal level of LH in males 1-15 mlU/ml & in females <1-30 mlU/ml.
Anterior Pituitary Gland: Hormone # 4. Follicle Stimulating Hormone (FSH):
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Functions:
a. In the female, it stimulates the growth and maturation of graafian follicles and prepares them for ovulation and for the action of LH and enhances the release of estrogens. This hormone is very active during menstrual cycle.
b. In the male, it stimulates seminal tubule and testicular growth and also the early stages of spermatogenesis.
c. The secretion of this hormone is inhibited by the administration of testosterone, progesterone and high concentration of FSH.
Anterior Pituitary Gland: Hormone # 5. Growth Hormone (GH, Somatotroph in):
Chemistry:
a. It is a single polypeptide with a molecular weight of about 21,500.
b. The human growth hormone consists of 191 amino acids.
c. The activity of hormone resides only in a portion of the molecule.
d. Partial hydrolysis of the hormone does not stop its activity.
e. The growth hormone preparations from monkey and human are active in both humans and rats.
Functions:
a. It increases total growth and causes gigantism in children. Its deficiency in children causes dwarfism.
b. It stimulates production of Somatomedins (Sulfation factors) from liver and kidney, somatomedins is similar to serum insulin-like activity.
c. It stimulates protein synthesis causing an increase in nitrogen and phosphorus retention. Blood amino acids and urea are decreased. Growth hormone increases synthesis of DNA and RNA in all tissues. It stimulates erythropoiesis.
d. It accelerates the mobilization of fat from the adipose tissues and the free fatty acids in the blood. It enhances oxidation of fats in liver and muscle. Increased ketogenesis occurs in the deficiency of insulin.
e. In muscle, it functions against the action of insulin causing the decrease in the utilization of glucose and stimulates the secretion of glucagon resulting in the increase in the blood glucose level. In liver, it inhibits hexokinase reaction and thus opposes the action of insulin.
f. It increases intestinal absorption of calcium as well as its excretion. As it stimulates the growth of the long bones at the epiphyses as well as the growth of soft tissues, retention of other minerals such as potassium, phosphorus, sodium, magnesium and chloride occurs. It causes the formation of sulphate esters for incorporation into cartilage by increasing somatomedin from the liver.
Control of secretion of growth hormone:
a. The control of growth hormone is exerted by a specific growth hormone releasing factor (GHRF) also termed growth hormone releasing hormone (GRH) which is extracted from the hypothalamus.
b. Growth hormone release-inhibiting hormones (GH-RIH, GIH) or Somatostatin release-inhibiting hormone (SRIH) is negative modulator of growth hormone.
c. Secretion of growth hormone is influenced by exercise, hypoglycemia, starvation, excitement and exposure to cold.
Anterior Pituitary Gland: Hormone # 6. Prolactin (PL):
Chemistry:
a. It is a protein with a molecular weight of about 23,000.
b. It is produced by the pituitary acidophil cells.
Functions:
a. It activates the corpus luteum and stimulates progesterone production by the developed corpus luteum.
b. It also stimulates enlargement of crop gland and formation of “crop milk” in pigeons.
c. It increases during pregnancy and stimulates mammary development and growth hormone like metabolic changes.
d. It is inhibited by a hypothalamic factor, prolactin inhibiting factor.
Anterior Pituitary Gland: Hormone # 7. Neurohypophysis (The Posterior Lobe of the Pituitary) Oxytocin:
Chemistry:
a. It is a cyclic polypeptide containing 8 amino acids.
b. Its molecular weight is about 1000.
c. Its structure is quite similar to that of vasopressin. The differences are isoleucine of oxytocin which is replaced by phenylalanine of vasopressin and leucine of oxytocin is replaced by lysine of vasopressin.
Functions:
a. It causes contraction of the smooth muscles in the mammary gland resulting in milk excretion. Its level is increased by suckling.
b. It is increased during labour. It causes uterine contraction and is used in obstetrics when induction of uterine contraction is required.
c. It stimulates the contraction of gall bladder, intestines and urinary bladder.
Anterior Pituitary Gland: Hormone # 8. Vasopressin:
Chemistry:
a. It is a cyclic polypeptide containing 8 amino acids.
b. Its structure is quite similar to that of oxytocin except isoleucine which is replaced by phenylalanine and leucine which is replaced by lysine.
Functions:
a. It has marked effect on the kidney tubules accelerating the rate of water reabsorption from the distal tubules and thus produces a marked antidiuretic effect. Hence, it is termed as antidiuretic hormone (ADH).
b. It is an effective inhibitor of the gonadotrophins, particularly LH.
c. ADH formation is prevented by the tumor in the hypothalamus or injury to hypothalamus. Diabetes insipidus occurs in the absence of ADH which is characterized by the large volume of excretion of urine—up to 30 L of urine per day. Alcohol also inhibits ADH secretion.
d. ADH secretion is increased by emotional and physical stress, electrical stimulation, acetylcholine, nicotine and morphine. These stimulations are associated with an increase in RNA synthesis in the neuron indicating an increased protein synthetic activity.
Anterior Pituitary Gland: Hormone # 9. Middle Lobe of the Pituitary:
Intermedin or melanocyte-stimulating hormone (MSH)
Chemistry:
a. There are two peptides (α-MSH and β- MSH). β-MSH is 50 times more than α- MSH.
b. α-MSH is smaller containing only 13 amino acids. Both have structural similarity with that of ACTH.
c. α-MSH is identical to the first 13 amino- acids of ACTH.
d. Amino acids 11 -17 of β-MSH are common to both α-MSH and ACTH.
e. α-MSH has some corticotropic activity but not of β-MSH.
Functions:
a. This hormone increases the deposition of melanin by the melanocytes of the human skin.
b. In Addison’s disease when the production of the corticosteroids is inadequate, melanocyte stimulating hormone (MSH) is secreted causing the increased synthesis of melanin accompanied by brown pigmentation.
c. Both cortisone and hydrocortisone, epinephrine and norepinephrine inhibit the action of MSH.
Abnormalities of Pituitary Functions:
A. Hyperpituitarism:
Gigantism:
a. It occurs from the hyperactivity of the gland during childhood or adolescence.
b. The long bones increase in length so that the individual attains an unusual height.
c. The growth of the legs and hands are relatively greater than the trunk.
d. The individuals are mentally subnormal.
Acromegaly:
a. This condition occurs in adults after the epiphyses have closed and growth has ceased.
b. The individual exhibits enlarged nose, growth and enlargement of the hands, feet and thickening of the skin.
c. Sexual function is increased. Raised BMR, hyperglycemia and glycosuria result. Excess production of ACTH produces Cushing’s disease.
B. Hypopituitarism:
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Dwarfism:
a. It occurs as a result of hypo-activity of the gland in childhood.
b. Cessation of growth and sexual retardation.
Frohlich’s syndrome:
a. This occurs in pituitary destruction in childhood.
b. The children become stunted and quite stupid.
c. There are deposits of fats all over the body.
d. Obesity and diabetes insipidus occur.
e. Disturbances in sleep mechanism and body temperature occur. These children spend most of their time asleep.
f. The obesity is due to increased appetite and reduced energy output.
Simmond’s Disease (Pan-hypopituitarism):
a. It occurs due to the deficiency of the function of the hypophysis in the adult.
b. BMR is reduced, subnormal body temperature and the heart rate is low.
c. Carbohydrate metabolism is entirely disturbed and the individual develops severe and sometimes fatal hypoglycemic coma. Myxedema is due to lack of thyrotropin.
