Hormone
Substances released by an endocrine gland and
transported through the bloodstream to another tissue
where it acts to regulate functions in the target
tissue.
• Paracrine: Hormones that act locally on cells
that did not produce them.
• Autocrine: Hormones that act on cells that
produced them.

Endocrine Blood vessel
Distant target cells
Hormone secretion
into blood by
endocrine gland
Paracrine
Secretory cell
Autocrine
Target sites on same cell
Adjacent target cell
Receptor
Hormone or other extra
cellular signal

General characteristics of hormones
 Hormones are molecules synthesized by specific tissue.
Classically these tissues were called glands.
 Hormones are secreted directly into the blood which
carries them to their sites of action.
 Hormones are present at very low levels in the
circulatory system.
 Hormones specifically affect or alter the
activities of the responsive tissue (target
tissue).
 Hormones act specifically via receptors located on or in,
target tissue.

Mechanisms of Hormone Action
1. Fixed Membrane Receptor Mechanism
• This type of mechanism is shown by the water-soluble hormones
that are amines or proteins in composition such as the growth
hormone, oxytocin, ADH, etc.
• These hormones can’t pass through the lipid membrane. They
have their target receptor on the cell membrane to which the
hormone binds.
• When the hormone binds to the specific target receptor, the
enzyme adenyl cyclase in the cell membrane is activated. This
helps in the production of cyclic AMP (cAMP).
• cAMP acts as the secondary messenger. It diffuses through the cell
membrane and activates several enzymatic reactions to cause
biochemical changes.
• The target cell responds to these changes and cAMP is deactivated
by the enzyme phosphodiesterase.

2. Mobile Receptor Mechanism
• This type of mechanism is shown by lipid-soluble hormones
such as steroids that can easily pass through the plasma
membrane.
• They possess intracellular receptors. The hormones bind to the
target receptor that activates the enzymatic activity of the cell to
bring about biochemical changes.
• Transcription of DNA is initiated by the hormone-receptor
complex.
• The mRNA is translated into protein. This protein causes
biochemical changes inside the cell.

Receptors
A molecule inside or on the surface of a cell that
binds to a specific substance and causes a specific
effect in the cell.
Types of receptors
1. Ion channel receptors
2. G-protein coupled receptors
3. Enzyme-linked receptors
4. Nuclear receptors/ intracellular receptors

1. Ion channel receptors
 Ion channel receptors are usually proteins located in the
plasma membrane.
 Each of these proteins arranges itself so that it forms a
passageway or pore extending from one side of the
membrane to the other.
 These passageways, or ion channels, have the ability to
open and close in response to a chemical or mechanical
signals.
 When an ion channel is open, ions move into or out of the
cell. Individual ion channels are specific to particular
ions.

2. G-protein coupled receptors
 Present in the membrane. Some part is exposed to the
membrane and other is exposed to the cell.
 When a drug bind to the outside, the inner surface
become open and G-protein is attached to that surface.
Fragmentation of G-protein occurs.
 Fragmentation leads to the activation of
 Adenylyl cyclase enzyme
 Phospholipase
 cAMP and IP3 are secondary messengers which cause
a biological effect.

3. Kinase linked receptors
 Membrane-bound receptor.
 Inner surface contains a binding site for the
enzyme at which the enzyme gets attached and
activated.
 Examples include insulin receptors, oxytocin
receptors, etc.

4. Intracellular receptors
 Present within the cell such as in the cytoplasm or
in the nucleus of the cell.
 Drug molecule which acts on the intracellular
receptors should be lipophilic so it can cross the
cell membrane.
 Steroidal hormones such as estrogen hormone act
via binding to intracellular receptors.

Hormones
Reproduction Growth & Development
Maintenance of
internal environment
Energy production,
utilization & storage
The four primary arenas
of hormone action

Classification of hormones

Polypeptides
Steroids
Amino acid derivatives
Insulin
glucagon
somatotropin
FSH
LH
Oxytocin
Estrogen
testosterone
cortisol
Aldosterone
Progesterone
Epinephrine
norepinephrine
Thyroxine, T3 and T4
Melatonin
Serotonin

1. Polypeptide hormones
1. Insulin
 Insulin is a peptide hormone produced by beta cells of
the pancreatic islets encoded in humans by the INS gene.
 Insulin acts on all cells of the body, binding to receptor
proteins on the surface of cells and enabling the uptake
of glucose.
 Peptide hormones have played an important role in
modern medicine including the use of insulin to treat
diabetes and the therapeutic application of ACTH and
growth hormone.

2. Glucagon
 Glucagon is a peptide hormone, produced by alpha cells
of the pancreas.
 It raises the concentration of glucose and fatty acids in
the bloodstream and is considered to be the main
catabolic hormone of the body.
3. Somatotropin
 Somatotropin, also known as growth hormone is a
peptide hormone that stimulates growth, cell
reproduction, and cell regeneration in humans and other
animals. It is thus important in human development.

5. FSH and LH
 Luteinizing hormone is a hormone produced by
gonadotropic cells in the anterior pituitary gland. The
production of LH is regulated by a gonadotropin-
releasing hormone from the hypothalamus.
 In females, an acute rise of LH known as an LH surge
triggers ovulation and development of the corpus luteum.
 Follicle-stimulating hormone is a gonadotropin, a
glycoprotein polypeptide hormone.
 FSH is synthesized and secreted by the gonadotropic
cells of the anterior pituitary gland and regulates the
development, growth, pubertal maturation, and
reproductive processes of the body.

6. Oxytocin
 Oxytocin is a peptide hormone and neuropeptide
normally produced in the hypothalamus and released by
the posterior pituitary.
 It plays a role in social bonding, reproduction and
childbirth

2. Amino acid derivatives
1. Melatonin
 The pineal gland in the brain produces melatonin, which
is instrumental in your sleep/wake cycles and your
internal body clock.
 As the light of day becomes the dark of night, your brain
amps up your melatonin levels to prepare you for sleep.
Interruptions to natural darkness impair melatonin levels
and sleep quality.
2. Serotonin (Happy hormone)
 Serotonin carries messages between nerve cells in the
brain and throughout your body. Serotonin plays a key
role in such body functions as mood, sleep, digestion,
blood clotting, and sexual desire.

3. Epinephrine
 Epinephrine is released by your adrenal glands in
response to stress. This reaction causes a number of
changes in your body and is known as the fight-or-flight
response.
 It is released in high-stress conditions or in excitement or
fear.
4. Thyroxine
 Produced by the thyroid gland.
 controls the rate of metabolic processes (how energy is
used) in the body and influences physical development
 People may not produce enough of this hormone and get
a condition known as hypothyroidism. They can take
thyroxine to treat this condition.

3. Steroidal hormones
1. Aldosterone
 Aldosterone (ALD) is a hormone that helps to regulate
blood pressure by managing the levels of sodium (salt)
and potassium in the blood and impacting blood volume.
 Produced by adrenal glands in kidney.

• Where it comes from:
ovary (where an egg
was released)
• Where it acts: uterus
• What it does: controls
menstruation in women
and plays a role in
pregnancy.
• One of the components
of birth control pills

• Where it comes from: testicles
• Where it acts: body-hair cells,
muscle, reproductive structures
• What it does: stimulates
development of male sexual
characteristics
• Testosterone is a steroid and has
been administered to athletes in
order to improve performance. This
is considered to be a form of doping
in most sports and is a very
dangerous practice.
• Females also produce small
amounts of testosterone in their
ovaries that affect muscle
development and other body
functions.

• Where it comes from: ovary
• Where it acts: breast tissue,
reproductive structures in
female
• What it does: stimulates
development of female
sexual characteristics
• Estrogen levels may be
related somehow to migraine
headaches in women.

• Where it comes from:
outer part of adrenal
gland
• Where it acts: multiple
tissues
• What it does: mental
stimulation, breaks down
fat and protein to glucose,
anti-inflammation
• It is usually referred to as
the "stress hormone" as it
is involved in response to
stress and anxiety.