1. ENDOCRINE SYSTEM
Ms. Dhruvi A. Prajapati
Assistant Professor
Dept. of Chemistry & QA
Babaria Institute of Pharmacy
BITS Edu campus - Vadodara
2. Content
Gtu Syllabus
Introduction to endocrine system
Classification of hormones
Mechanism of hormone action
References
Dhruvi A. Prajapati 2
4. INTRODUCTION
The endocrine system consists of glands secreting
hormones essential for maintenance of homeostasis
throughout the body.
Hormones are chemical messengers that act to control
and coordinate different functions of tissures and organs.
Various body activities like growth and development and
metabolism are also regulated by hormones.
Each hormone is secreted from a particular gland and is
distributed to the target tissues via blood.
Dhruvi A. Prajapati 4
5. Endocrine glands are ductless glands, thus release
their products directly into the blood stream & are
carried to their target cells.
On the other hand, exocrine glands secrete their
products (excluding hormones & other chemical
messengers) into the ducts, which are then
transported to the blood stream.
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7. These glands work in conjuction with the nervous
system, and therefore this complex of two systems is
referred to as the neuroendocrine system.
This system controls & coordinates various functions
of the body, maintaining homeostasis (constancy of
body fluids) within the body.
The term neuroendocrinology defines the study of
endocrine system in combination with the nervous
system.
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8. Classification of Hormones
On the basis of chemical composition, hormones are
categorized into lipid-soluble and water soluble
hormones.
This classification also describes the hormone
function since both the classes exert their effects
differently
1. Lipid-soluble Hormones
2. Water-soluble Hormones
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9. 1. Lipid-soluble Hormones
i. Steroid hormones:
Synthesized from cholesterol.
They are unique individually as different chemical groups are
attached at different sites on the four rings present in the core of
their structure.
Such minor differences results in varied functions.
e.g. Testosterone, Estrogen, Progesterone etc.
ii. Thyroid hormones:
The T3 and T4 are the two thyroid hormones with iodine linked to
tyrosine amino acid.
The benzene ring of tyrosine along with the iodine attached
increases the lipid solubility of T3 and T4 hormones.
iii. Nitric oxide:
Nitric oxide synthase enzyme aids in the synthesis of NO, This gas
behaves like a hormone as well as a neurotransmitter.Dhruvi A. Prajapati 9
10. 2. Water-soluble Hormones
i. Amine hormones:
Produced upon decarboxylation or modification of certain amino
acids.
The term amine is associated with these hormones because they
carry an amino group.
e.g. T3, T4, epinephrine, norepinephrine.
ii. Peptide hormones:
These hormones are amino acid polymers.
The smaller peptide hormones are made up of 3-49 amino acid
chains, whereas the larger ones contain 50-200 amino acid
chains.
e.g. Oxytocin and vasopressin both consist of only 9-amino acid
residues only.Dhruvi A. Prajapati 10
11. iii. Protein hormones:
These hormones are termed glycoprotein hormones since
thyroid stimulating hormone of this category contains
carbohydrate groups.
e.g. Insulin, glucagon, STH etc.
iv. Eicosanoid hormones:
Arachidonic acid acts as a precursor for these hormones.
They act as circulating and local hormoes.
e.g. Prostaglandins and leukotriens
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12. Local hormones
A number of body tissues not normally described as
endocrine glands secrete substances that act in tissues
nearby (locally).
Histamine
This is synthesised and stored by mast cells in the tissues
and basophils in blood.
It is released as part of the inflammatory response,
increasing capillary permeability and causing vasodilation.
It also causes contraction of smooth muscle of the bronchi
and alimentary tract and stimulates the secretion of
gastric juice.Dhruvi A. Prajapati 12
13. Serotonin (5-hydroxytryptamine, 5-HT)
This is present in platelets, in the brain & in the intestinal wall.
It causes intestinal secretion and contraction of smooth muscle.
Prostaglandins (PGs)
These are lipid substances found in most tissues. They act
nearby & have potent and wide-ranging physiological effects in:
the inflammatory response
potentiating pain
fever
regulating blood pressure
blood clotting
uterine contractions during labour.
Other chemically similar compounds include leukotrienes,
which are involved in inflammatory responses & thromboxanes,
e.g. thromboxane A2, which is a potent aggregator of platelets.
All of these active substances are found in only small amounts,
as they are rapidly degraded.
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14. Trophic hormone: A hormone that has its primary function the
regulation of hormone secretion by another endocrine gland.
Synergism: When different hormones work together and have
a greater effect than individual hormone action
Permissiveness: A small amount of one hormone allows a
second hormone to have its full effect on a target cell; i.e.
first hormone ‘permits’ the full action of the second
hormone.
Antagonism: One hormone produces the opposite effect of the
other
GENERAL PRINCIPLES OF HORMONE ACTION
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15. MECHANISM OF HORMONEACTION
The hormones fall into two general classes based on their
solubility in water.
1. Hydrophilic Hormone:
The water soluble hormone. They are transported simply
dissolved in blood.
Examples:The catecholamine's (epinephrine & nor
epinephrine) and peptide/protein hormones.
2. Lipophilic Hormone:
They are poorly soluble in water. So, they cannot be dissolved
in watery blood. They bind to plasma protein and present in
the blood in protein bound form. They are lipid soluble.
Examples:The lipid soluble hormones include thyroid hormone,
steroid hormones and Vitamin D3
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16. Broad Classification (Structure)
Hydrophilic
Proteins, peptide hormones &
catecholamine's
Primarily act through second
messenger system
Lipophilic
Steroid and thyroid hormones
Activate genes on binding with
receptors in the nucleus
Circulate mainly dissolved in the
plasma
Largely bound to plasma proteins
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17. Generally hormones work in two ways
Synthesis of new protein molecules
Changing cell permeability
Lipid-soluble hormones involves in ‘
whereas water-soluble hormones involves in ‘
MODE OF HORMONEACTION
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18. Action of Lipid soluble hormones
1. A free molecule of lipid soluble hormone resches the target cell by
diffusing in the interstitial fluid through the blood and then into the
cell through the lipid bilayer plasma membrane.
2. The hormone binds to the receptors present in the cytosol or
nucleus of the target cell. After binding, the receptors are
activated. This complex of activated receptor and hormone causes
alteration in gene expression by turning on or off a specific gene of
the nuclear DNA.
3. DNA transcription produces a new messenger RNA which enters the
cytosol from the nucleus. Here it regulates the synthesis of a new
protein or an enzyme on the ribosomes.
4. This new protein formed brings changes to the cell’s activity. It also
produces typical responses of that hormone.Dhruvi A. Prajapati 18
20. Action of water-soluble hormones
1. The first messenger diffuses in the intestinal fluid from the
blood, and then binds to the receptors present on the target cell
surface. The resultant hormone receptor complex activates the
G protein, which in turn activates adenylate cyclase enzyme.
2. Since the active site of this enzyme lies inner to the plasma
membrane, it transforms ATP into cAMP within the cytosol.
3. cAMP causes activation of one or more protein kinase enzyme,
which is either in free state in the cytosol or bound to the
plasma membrane.
4. This activates protein kinase causes phosphorylation of one or
more cellular proteins; whereby, activating some protein and
inactivating others. ATP donates the phosphate group and the
converts into ADP.
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21. 4. Phosphorylated proteins bring about reactions responsible
for physiological actions. Protein kinases differ in different
target cells and their different organelles. So, one protein
kinase may initiate glycogen synthesis, the other may
breakdown the triglyceride, a third may cause protein
synthesis and so on. Phosphorylation achieved by protein
kinase also inhibits some proteins.
For example, binding of epinephrine to liver cells inactivates
an enzyme required in glycolgen synthesis, thus unleashing
certain kinases.
5. After sometime phosphodiesterase enzyme inactivates
cAMP, halting the cell response till a new hormone molecule
binds to the receptors on plasma membrane.
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24. Reference
Anatomy and Physiology in Health and Illness,11th edition
by Ross and Wilson, Churchill Livingstone, New York, page
no. 207.
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