# Hormones are informational molecules secreted by the endocrine cells in one part of the body and carried by blood to another part where they stimulate or inhibit specific physiological process. In other words the hormones are chemical messengers or informational molecules that regulate the biological processes and metabolism. Hormones organs called target organs. Targate cells have receptor proteins for specific hormone.
Discovery : First hormone discovered was secretin. It was discovered by two English physiologists : William M Bayliss and Ernest H. Starling in 1903.
Nomenclature : Term hormone was coined by starling (1905) from Greek work Hormaein means to excite. It is a mishomer because a number of hormones are known to have inhibitory effect (e.g. Somatostatin)
(i) Properties of hormones
(a) These are secreted by endocrine gland (biogenic in origin).
(b) Their secretions is released directly into blood (except local hormones e.g. gastrin).
(c) These are carried to distantly located specific organs, called target organ.
(d) These have specific physiological action (excitatory or inhibatory). These co-ordinate different physical, mental and metabolic activities and maintain homeostasis.
(e) The hormones have low molecular weight e.g. ADH has a molecular weight of 600–2000 daltons.
(f) These act in very low concentration e.g. around10–10 molar.
(g) Hormones are non antigenic.
(h) These are mostly short-lived. So have a no camulative effect.
(i) Some hormones are quick acting e.g. adrenalin, while some acting slowly e.g. ostrogen of ovary.
(j) Some hormones secreted in inactive form called Prohormone e.g. Pro-insulin.
(k) Hormones are specific. They are carriers of specific information to their specific target organ. Only those target cell respond to a particular hormone for which they have receptors.
# Classification of hormones
- (a) On the basis of chemical nature : On the basis of chemical composition hormones are classified into three categories.
(1) Amine hormones : These are derived form tyrosine amino acid and have amino group e.g. Thyroxine, Epinephrine, Nor-epinephrine.
(2) Steroids : These are fat soluble and have sterol group. These are derived from cholesterol e.g. hormones of adrenal cortex (cartisol, cartisone, carticasterone, aldasterone) testes (testosterone etc.) and ovaries (estrone, estradiol, progesterone etc.)
(3) Proteinous and peptide hormones : These are formed of 3 – 200 amino acids interlinked by peptide bonds and are water soluble e.g.
(i) Proteinous hormones like STH, TSH, FSH, LH etc. Out of these FSH and LH are glycoproteins.
(ii) Long peptide hormones like insulin and glucagon, ACTH, Paratharmone.
(iii) Short peptide hormones like oxytocin, ADH, MSH. These hormones formed of a few amino acids.
- (b) On the basis of mode of action
(1) Quick acting hormones : These hormones initiate immediate response from their target cells. There receptor is always located on the outer surface of plasma membrane of target cell because these are large sized. Hormone receptor complex activates a membrane enzyme adenyl cyclase which hydrolise ATP into cyclic AMP. Which acts as secondary messenger, c-AMP activates an inactive enzyme system by cassade effect. So their mode of action is called second messenger hypothesis. e.g. These includes proteinous, peptide and amine hormones.
(2) Short acting hormones : These hormones intiate response after some time. These are small sized so are diffusable through the plasma membrane of their target cell. These bind their proteinous receptor present in the cytosol. These always operate through de-novo synthesis of m-RNA by activation of certain genes. So their mechanism of action is called m-RNA hypothesis. e.g. These include steroid hormones of testes, ovary and adrenal cortex.
# Release of hormones : Hormones are released from endocrine glands by three types of stimuli.
(a) Specific metabolic : The presence of a specific metabolite in the blood elicits the hormone to deal with it. For instance excess of glucose in the blood causes the release of insulin from the pancreas.
(b) Other hormone : The presence of a specific hormone in the blood induces the release of another hormone. For example TSH stimulate thyroid gland to release thyroxine hormone.
(c) Neuronal impulse : Neurons of autonomic system stimulate hormone release from some glands. For example adrenaline and nor-adrenaline are released from adrenal medulla on the arrival of nerve impulses during anxiety, stress and danger.
# Hormones are informational molecules secreted by the endocrine cells in one part of the body and carried by blood to another part where they stimulate or inhibit specific physiological process. In other words the hormones are chemical messengers or informational molecules that regulate the biological processes and metabolism. Hormones organs called target organs. Targate cells have receptor proteins for specific hormone.
Discovery : First hormone discovered was secretin. It was discovered by two English physiologists : William M Bayliss and Ernest H. Starling in 1903.
Nomenclature : Term hormone was coined by starling (1905) from Greek work Hormaein means to excite. It is a mishomer because a number of hormones are known to have inhibitory effect (e.g. Somatostatin)
(i) Properties of hormones
(a) These are secreted by endocrine gland (biogenic in origin).
(b) Their secretions is released directly into blood (except local hormones e.g. gastrin).
(c) These are carried to distantly located specific organs, called target organ.
(d) These have specific physiological action (excitatory or inhibatory). These co-ordinate different physical, mental and metabolic activities and maintain homeostasis.
(e) The hormones have low molecular weight e.g. ADH has a molecular weight of 600–2000 daltons.
(f) These act in very low concentration e.g. around10–10 molar.
(g) Hormones are non antigenic.
(h) These are mostly short-lived. So have a no camulative effect.
(i) Some hormones are quick acting e.g. adrenalin, while some acting slowly e.g. ostrogen of ovary.
(j) Some hormones secreted in inactive form called Prohormone e.g. Pro-insulin.
(k) Hormones are specific. They are carriers of specific information to their specific target organ. Only those target cell respond to a particular hormone for which they have receptors.
# Classification of hormones
- (a) On the basis of chemical nature : On the basis of chemical composition hormones are classified into three categories.
(1) Amine hormones : These are derived form tyrosine amino acid and have amino group e.g. Thyroxine, Epinephrine, Nor-epinephrine.
(2) Steroids : These are fat soluble and have sterol group. These are derived from cholesterol e.g. hormones of adrenal cortex (cartisol, cartisone, carticasterone, aldasterone) testes (testosterone etc.) and ovaries (estrone, estradiol, progesterone etc.)
(3) Proteinous and peptide hormones : These are formed of 3 – 200 amino acids interlinked by peptide bonds and are water soluble e.g.
(i) Proteinous hormones like STH, TSH, FSH, LH etc. Out of these FSH and LH are glycoproteins.
(ii) Long peptide hormones like insulin and glucagon, ACTH, Paratharmone.
(iii) Short peptide hormones like oxytocin, ADH, MSH. These hormones formed of a few amino acids.
- (b) On the basis of mode of action
(1) Quick acting hormones : These hormones initiate immediate response from their target cells. There receptor is always located on the outer surface of plasma membrane of target cell because these are large sized. Hormone receptor complex activates a membrane enzyme adenyl cyclase which hydrolise ATP into cyclic AMP. Which acts as secondary messenger, c-AMP activates an inactive enzyme system by cassade effect. So their mode of action is called second messenger hypothesis. e.g. These includes proteinous, peptide and amine hormones.
(2) Short acting hormones : These hormones intiate response after some time. These are small sized so are diffusable through the plasma membrane of their target cell. These bind their proteinous receptor present in the cytosol. These always operate through de-novo synthesis of m-RNA by activation of certain genes. So their mechanism of action is called m-RNA hypothesis. e.g. These include steroid hormones of testes, ovary and adrenal cortex.
# Release of hormones : Hormones are released from endocrine glands by three types of stimuli.
(a) Specific metabolic : The presence of a specific metabolite in the blood elicits the hormone to deal with it. For instance excess of glucose in the blood causes the release of insulin from the pancreas.
(b) Other hormone : The presence of a specific hormone in the blood induces the release of another hormone. For example TSH stimulate thyroid gland to release thyroxine hormone.
(c) Neuronal impulse : Neurons of autonomic system stimulate hormone release from some glands. For example adrenaline and nor-adrenaline are released from adrenal medulla on the arrival of nerve impulses during anxiety, stress and danger.
