2. INTRODUCTION
Communication system to coordinate its biological
function in our body consists of :
– Nervous system: coordinates the body function through
the transmission of electrochemical impulses
– Endocrine system: acts through a wide range of chemical
messenger called hormones
3. HORMONES: DEFINITION
Organic substances
Produced in small amounts by specific tissues
(endocrine glands)
Secreted into the blood stream
Control the metabolic and biological activities in
the target cells
regarded as the chemical messengers involved in
the transmission of information from one tissue
to another and from cell to cell
4. Function of hormones:
HOMEOSTASIS / Maintenance of internal
environment (PTH, aldosterone, calcitriol,
calcitonin)
Reproduction
Growth and development
Production, utilization and storage of energy
(Insulin, glucagon, cortisol)
5. Transport of Hormones
After entering the blood hormones either remain in free from or
bind with a specific carrier protein.
A. Water Soluble Hormones
Peptide hormones & biogenic amines readily dissolve in the plasma.
So, they do not require special mechanism for their transport.
B. Transport of Steroid Hormones
Thyroxine, steroid hormones & vitamin D are relatively insoluble in
plasma & circulate in the bound form.
In blood, more than 90% of thyroid & steroid hormones are bound to
plasma proteins.
The transport proteins are specific for hormones.
However, few nonspecific proteins are also involved in the process of
hormone transport.
6. Classification of Hormones
A. Based on the chemical structure
B. Based on the mechanism of action
C. Based on the anatomical position
7. A) Classification based on the chemical nature
1) Protein or peptide hormones
• Hormones of the hypothalamus and pituitary,
Insulin, glucagon, the parathyroid hormone,
calcitonin,
2) Steroid hormones
• Adrenocorticosteroid hormones , sex hormones
3) Amino acid derivatives
• Epinephrine, norepinephrine, T3, T4
8. Group 1: Hormones that bind to intracellular receptors.
Group 2: Hormones that bind to cell surface receptors.
a) Second messenger is c AMP
eg. ACTH, ADH, PTH, TSH, glucagon
b) Second messenger is c GMP. eg. ANP, NO
c) Second messenger is calcium or phosphoinositol or both
eg. Ach, α Adrenergic, TRH
d) Second messenger is a kinase or phosphatase cascade.
eg. Growth factors , Insulin
Classification based on Mechanism of Action:
Two broad groups ( Group I & Group II) based on the location of
receptors to which they bind and the signals used to mediate
their action.
9. Hormone Receptors:
Hormone receptors are membrane proteins which are specific for
each hormone.
Their sensitivity or responsiveness is determined by:
Number of receptors; larger the number, greater is the
response.
Affinity of receptors; higher the affinity, greater is the response
The sensitivity of receptors can be changed by the change in
number of receptors, which can occur by:
Increase or decrease in synthesis of receptors
Increase or decrease in degradation of receptors.
10. In the surface of cell membrane, e.g. protein or
peptide hormones and catecholamines.
In the cytoplasm, e.g. steroid hormones.
In the cell nucleus, e.g. Thyroxine.
There are about 2000-100,000 receptors on each cell.
Site of Hormone Receptors:
11. Hormones that are not soluble in aqueous medium, but
soluble in lipid i.e lipophilic.
They can easily cross the cell membrane.
Thus, they can enter target cells and bind to intracellular
receptors to carryout their action.
• Examples: Thyroid hormones, Steroid hormones;
Group I /Lipophilic Hormones (Hydrophobic Hormones)
12. Hormones that are not soluble in aqueous medium, but
soluble in lipid i.e lipophilic.
They can easily cross the cell membrane.
Thus, they can enter target cells and bind to intracellular
receptors to carryout their action.
• Examples: Thyroid hormones, Steroid hormones;
Group I /Lipophilic Hormones (Hydrophobic Hormones)
13. – Pass through the cell membrane
– Binds to specific receptors.
– Then enters the nucleus to bind with the HRE of
cellular DNA which then activates certain genes
(Direct gene activation).
– mRNA is synthesized in the nucleus and enters
the cytoplasm and promotes protein synthesis
for:
• Enzymes as catalysts
• Tissue growth and repair
• Regulate enzyme function
Mechanism of action of steroid hormones
16. Hormones that are soluble in aqueous medium
They cannot cross the cell membrane,
Thus, they bind to receptor molecules on the
outer surface of target cells, initiating reactions
within the cell that ultimately modifies the
functions of the cells.
Examples: Insulin, Glucagon, Epinephrine.
Group II /Hydrophilic Hormones (Lipophobic Hormones)
17. The endocrine gland releases hormones that bind
with specific receptor on target cells and trigger
intracellular signals by:
Altering membrane permeability
Acting through 2nd messenger system
Activating specific genes to form new proteins
Group II hormones Contd:
18. The Receptor For Water Soluble Hormone-Cellular surface
receptors
Classified into three types:
1. Ion- channel linked receptors → Transmitter gated ion
channels Eg.Ach, Na+,Ca+
2. G- Protein linked receptors → The interaction between
the receptor and target protein is mediated by third
protein called G Protein
3. Enzyme linked receptors → When activated, either
function directly as enzyme or associated with enzymes.
19. The Receptor For Water Soluble Hormone-Cellular surface
receptors
Classified into three types:
1. Ion- channel linked receptors → Transmitter gated ion
channels Eg.Ach, Na+,Ca+
2. G- Protein linked receptors → The interaction between
the receptor and target protein is mediated by third
protein called G Protein
3. Enzyme linked receptors → When activated, either
function directly as enzyme or associated with enzymes.
20. React with specific receptors outside the cell.
This triggers an enzyme reaction with lead to the
formation of a second messenger (cAMP).
cAMP can produce specific intracellular functions:
Activates cell enzymes
Change in membrane permeability
Promote protein synthesis
Change in cell metabolism
Stimulation of cell secretions
Mechanism of action of Group II hormones via CAMP
