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Hormones: An overview
Paper: Zoology
Lesson: Hormones: An overview
Author: Dr. Smita Bhatia
College/Department: Ramjas College,
University of Delhi
Institute of Life Long Learning, University of Delhi
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Hormones: An overview
Table of Contents
 Introduction
 Type of hormones



Lipid soluble hormones

Steroid hormones

Thyroid hormones

Nitric oxide
Water Soluble hormones

Peptides and proteins

Biogenic amines

Eicosanoids
Mode of action of hormones

Action of Lipid-soluble hormones

Action of Water-soluble hormones
 Regulation of Hormone secretion
 Hormone interactions
 Summary
 Exercises
 Glossary
 References
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Hormones: An overview
Introduction
All functions of the body are very finely regulated to achieve a state of homeostasis. The
major regulatory systems of the body are the endocrine system and the nervous system
(and the interface between the two; the neuroendocrine mechanisms – where secretions of
the neurons act directly as hormones e.g. oxytocin and vasopressin or cause secretion of
another hormone e.g., stimulation of the sympathetic nervous system causes the release of
hormones from the adrenal medulla).
The endocrine system consists of glands that release their secretions (hormones) directly
into the blood stream as they do not have any ducts to convey their secretions to the target
organs (or cells). Endocrine glands are, therefore, called ‘Ductless Glands’ (Fig. 1). Also,
the target organ may be at a distant location or there may be multiple target organs for a
hormone where it may not be possible to reach through ducts.
Fig.1. Release and action of hormones
Value addition: Did you Know
Heading text: Types of secretion
Source: ILLL in house
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Hormones: An overview
While hormones (are released directly into the bloodstream (or in the interstitial fluid
from where they diffuse into the blood) and act on target cells located at a distance,
some secretions of the cells act on the target cells in the close vicinity. These
secretions are called Paracrine agents. Some cells produce secretions that regulate
the activities of the same cell. These secretions are known as Autocrine agents.
Paracrine and autocrine agents are local hormones.
Source: Author, ILLL in house
Table 1. Differences between the endocrine and nervous system
NERVOUS SYSTEM
ENDOCRINE SYSTEM
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Hormones: An overview
Neurotransmitters are released from
neurons
Hormones are released from
endocrine (ductless) glands
Neurotransmitters act locally
Neurotransmitters act on glands,
Hormones act on distant target
organs or cells
Hormones act on a variety of cells
muscle cells or other neurons
Action of neurotransmitters is very fast
Action of hormones may be fast or
slow
Effect of neurotransmitters lasts for a short
Effect of hormones may last a few
time
seconds, hours or several days
The various homeostatic functions of hormones include regulation of:

metabolism and energy balance

contractile activities of smooth and cardiac muscle

secretory activities of glands

growth and development

functioning of the reproductive system

establishment of circadian rhythms

chemical composition of various body fluids

some immune system activities
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Hormones: An overview
Fig.2. Location of endocrine glands in the body
Source: http://bio1152.nicerweb.com/Locked/media/ch45/45_06HumanEndocGlands.jpg
Free for educational purpose
Types of Hormones
Depending on their solubility, hormones can be categorized into two groups:
1. LIPID-SOLUBLE HORMONES

Steroid Hormones
These are derived from cholesterol e.g. male and female sex hormones and hormones of the
adrenal cortex.
a. Synthesis
They are synthesized from cholesterol (see Figure 3) which is usually picked up from the
plasma or may be synthesized de novo. Steroids are not stored in their ready form but are
derived from cholesterol esters stored in cytoplasmic vacuoles.
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Hormones: An overview
Fig.3. Synthesis of different steroids from cholesterol
Source:
http://en.wikipedia.org/wiki/Steroid_hormone#mediaviewer/File:Steroidogenesis.svg
Häggström M, Richfield D (2014). "Diagram of the pathways of human steroidogenesis".
Wikiversity Journal of Medicine 1 (1). DOI:10.15347/wjm/2014.005. ISSN 20018762.
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Hormones: An overview
b. Transport
Since steroid hormones are not water-soluble, they are not transported in dissolved form in
the plasma. Specific transport proteins in the plasma carry specific steroids e.g.
corticosteroid-binding-globulin (CBG, also known as transcortin) that transports 9095% of cortisol. The rest 5-10% of the hormone remains free in the plasma and is available
for binding to the receptors in the target cells. Equilibrium exists between the bound and the
free hormone.
As the free hormone is lost from circulation, more hormones are released
from the plasma proteins. Since lipid-soluble hormones are bound to plasma proteins they
are removed from the blood (by degradation in the liver and excretion by the kidneys) at a
lower rate and therefore, they have a longer half-life (half-life of a substance is the time
taken for the concentration of that substance to reduce to half of its original concentration).

Thyroid Hormones
a. Synthesis
Hormones
of
the
thyroid
gland,
Tri-iodothyronine
(T3)
and
thyroxine
(tetra-
iodothyronine; T4) are derived from the amino acid tyrosine by coupling of iodinated
tyrosine molecules. The phenolic rings of tyrosine and addition of iodine makes these
hormones lipid-soluble (Figure 4). These hormones are stored in the follicles of the thyroid
gland bound to the protein thyroglobulin. Upon stimulation, they are released by
proteolysis of thyroglobulin.
I
OH
I
CH
I
O
I
I
NH2
TYROSINE
HOOC
CH
I
I
NH2
HOOC
OH
I
I
CH
I
O
O
CH2
CH2
CH2
CH2
HOOC
OH
OH
NH2
HOOC
CH
NH2
TRIIDOTHYRONINE
(T3)
“REVERSE T3”
(INACTIVE)
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THYROXINE
(T4)
Hormones: An overview
Fig. 4. Structure of the thyroid hormones
Source: Author, ILLL in House
b. Transport
Since thyroid hormones are also water-insoluble (lipid-soluble), they are transported in the
plasma bound to the transport protein, thyroxine-binding globulin (TBG).
Value addition: Did you Know
Transport proteins for lipid-soluble hormones not only serve to carry these hormones
in the plasma but also:
• Provide a ready stock of hormones
• Increase the half-life of these hormones as they are not as readily filtered by the
kidneys
Source: Author
 Nitric Oxide (NO)
This is a gas, which acts as a hormone as well as a neurotransmitter.
a. Synthesis
It is not formed in advance but synthesized when needed. It is formed from the amino acid
arginine by the activity of enzyme Nitric Oxide Synthase (NOS).
b. Transport
Nitric oxide, known to act only locally till recently, has been found to be transported in blood
by hemoglobin and other proteins. Since it is lipid-soluble it easily diffuses into the target
cells. It lasts for about 6 seconds after which it reacts with water and oxygen to form inactive
nitrites and nitrates.
Value addition: Did you Know
NO was first discovered as a substance produced by endothelial cells of the blood
vessels where it acts as a vasodilator. It was then named as the Endothelium
Derived Relaxation Factor (EDRF).
Source: Author
2. WATER-SOLUBLE HORMONES
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Hormones: An overview

Peptides and proteins
These consist of amino acids joined together by peptide bonds. The number of amino acids
may vary from 3 to 200. Those containing less than 100 amino acids are called peptides e.g.,
oxytocin and vasopressin (each containing 9 amino acids) and those with more than 100
amino acids are called proteins e.g., growth hormone containing 191 amino acids.
a. Synthesis
They are synthesized by the rough endoplasmic reticulum as inactive preprohormones. They
undergo several modifications at different steps to finally give rise to the active hormone
(Figure 5).
Ca2+
Fig. 5. Synthesis and secretion of protein and peptide hormones
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Hormones: An overview
Source: Author, ILLL in house
The following steps show the formation and secretion of an active hormone:
Preprohormones (inactive)
synthesized by the rough
endoplasmic reticulum
A part of this molecule is cleaved
by the endoplasmic reticulum
Prohormones (inactive)
Enzymes in the Golgi apparatus cleave this
into a smaller, active hormone
Active Hormones
Packaged into secretory vesicles by the
Golgi apparatus
Secretory vesicles containing
active hormone stored in the
cytoplasm, some bound to the
cell membrane
Appropriate stimulus
The secretory vesicles release their
contents in the form of granules into
the interstitial fluid or the blood stream
by exocytosis.
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Hormones: An overview
Source: Author, ILLL in house

Biogenic Amines
a. Synthesis
These are the hormones derived from amino acids tyrosine and tryptophan. They retain an
amino group, hence called amines. Those derived from tyrosine are called catecholamines
(Figure 6) and include epinephrine, norepinephrine (both from adrenal medulla) and
dopamine (recall here that thyroid hormones are also derived from tyrosine but they are
not water-soluble because they do not retain the amino group but contain two phenolic rings
and iodine that makes them lipid-soluble- Figure 4). Serotonin and melatonin are derived
from the amino acid tryptophan. Biogenic amines also act as neurotransmitters.
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Hormones: An overview
O
OH
HO
TYROSINE
NH2
tyrosine-3-monoxygenase
Tetrahydrobiopterin
O
HO
L-DOPA
OH
NH2
HO
aromatic L-amino acid
decarboxylase
Pyridoxal phosphate
HO
HO
NH2
DOPAMINE
dopamine beta-hydroxylase
Ascorbate
OH
HO
HO
NH2
NOREPINEPHRINE
phenylethanolamine-N-methyltransferase
S-adenosylmethionine
OH
HO
NHCH3
EPINEPHRINE
HO
Fig. 6. Synthesis of Catecholamines from Tyrosine
Source: Author, ILLL in hous
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Hormones: An overview
 Eicosanoids
a. Synthesis
These hormones are derived from a 20-carbon fatty acid, arachidonic acid (‘Eicos’ meaning
twenty). These include prostaglandins, thrombokines and leukotrienes (Figure 7).
All water-soluble hormones are carried dissolved in plasma without any transport proteins.
LEUKOTRIENES
LINEAR PATHWAY
DIACYLGLYCEROL
Lipoxygenase
ARACHIDONATE
PGH2 Synthase
CYCLIC PATHWAY
Prostacyclin Synthase
PHOSPHOLIPIDS
PROSTAGLANDIN H2
Thromboxane Synthase
THROMBOXANES
PROSTACYCLINS
OTHER
PROSTAGLANDINS
Fig. 7 . Synthesis of Eicosanoids from Arachidonic acid
Source: Author, ILLL in house
b. Transport
All water-soluble hormones are carried dissolved in plasma without any transport proteins.
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Hormones: An overview
Value addition: Did you Know
Heading: How aspirin works
Though Aspirin (acetylsalicylic acid) has been used as an anti-inflammatory analgesic
for centuries, it was only in 1974 that John Vane discovered its mode of action.
Aspirin
works
by
inhibiting
the
enzyme
responsible
for
the
synthesis
of
prostaglandins that are involved in pain and inflammation.
Source: Author
Mode of Action of Hormones
Hormones manifest their effect by binding to specific receptors. These receptors are
intracellular for lipid-soluble hormones as these hormones can readily cross the lipid
plasma membrane. For water-soluble hormones, the receptors are present on the cell
surface since these hormones cannot cross the lipid plasma membrane of the cell.
As the hormone binds to its receptor, it activates the receptor to give rise to an activated
receptor-hormone complex. The sequence of events resulting in the ultimate response to the
hormone, starting with the formation of the activated hormone-receptor complex, is known
as the signal transduction pathway (where, hormone is the signal and the sequence of
events culminating in the response is the transduction).

Action of lipid-soluble hormones
Lipid-soluble hormones act directly by binding to intracellular receptors.
Steroid hormones have receptors in the cytoplasm while thyroid hormones have receptors
present inside the nucleus (Figure 8).
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Hormones: An overview
Fig. 8 . Action of steroids hormones
Source:
http://upload.wikimedia.org/wikipedia/commons/a/a9/1803_Binding_of_Lipid-
Soluble_Hormones.jpg
The action of these hormones can be shown by the following flow chart:
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Hormones: An overview
Value addition: Video
Heading: Mechanism of action of steroid hormone
Learn more about the mechanism of action of lipid soluble steroid hormones:
https://www.youtube.com/watch?v=Nm9u4lNCPyM
EMBEDTHE VIDEO
Source: YouTube

Action of water-soluble hormones
Water-soluble hormones bind to extracellular portions of transmembrane receptors
For water-soluble hormones, the receptors usually are transmembrane proteins with a part
of the receptor on the outer surface on the cell where the hormone binds and a part
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Hormones: An overview
protruding on the inner side of the cell membrane, which is responsible for bringing about
the effect of the hormone.
Fig. 10. Action of water soluble hormones
Source: http://cnx.org/content/m46667/latest/1804_Binding_of_WaterSoluble_Hormones.jpg
The inner portions of the membrane receptors for water-soluble hormones may act in
different ways to bring about the hormone-response:
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Hormones: An overview
1.
These receptors may directly function as enzymes that bring about a change in the cell
e.g.
Receptor tyrosine kinases that cause phosphorylation of its own tyrosine-
containing portions refer to Video 1).
Video 1. Action of tyrosine-kinase receptors. Signal molecule is the
water-soluble hormone. EMBEDTHE VIDEO
Source: ILLL in house
Action of receptor tyrosine-kinases can be summarized as follows:
Extracellular portion of the
receptor + hormone
Activated receptor-hormone
complex
Phosphorylation of the
tyrosine groups in the
intracellular portion of the
receptor protein
Binding of another
intracellular protein to the
phosphorylated receptor
protein
Phosphorylation of the
intracellular protein
Such
a
pathway
exists
for
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Stimulation
18of a cellular
activity
Growth
Hormones: An overview
Factors which stimulate cell proliferation and differentiation
2. These receptors may be associated with another group of cytoplasmic tyrosine kinases
called the Janus Kinases (JAKs). Different JAKs phosphorylate different proteins which act
as transcription factors stimulating synthesis of specific proteins. The signal transduction
pathway for these receptors is given below:
3. They
are
most
commonly
associated
with a hetero-trimeric protein, the G-protein (GTP-binding protein) and therefore, these
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Hormones: An overview
receptors are called G-protein-coupled receptors. The G-protein may cause opening of
certain ion channels e.g. Ca2+ ion channels or it may stimulate or inhibit an enzyme.
Activation of enzymes causes the formation of a second messenger usually cAMP (cyclic
Adenosine Mono Phosphate) which then brings about the hormone response (see Figure 9)
through a series of events shown below:
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Hormones: An overview
Extracellular
portion of the
receptor +
hormone
Activated receptorhormone complex
Intracellular (G-proteincoupled) part of the
receptor undergoes a
conformational change
G –protein
exchanges GDP
with GTP
G-protein gets associated with
another intracellular protein
e.g. enzyme Adenylyl (or
adenylate) cyclase and
activates it
Activated Adenylyl
Cyclase
Cyclic AMP (cAMP)
Cytoplasmic ATP
Degrades
Phosphodiesterase
Activates cAMPdependent Protein
Kinase
Activated Protein
Kinase
Phosphorylated
Enzyme 1
Enzyme 1
A cellular activity is
STIMULATED
(Stimulatory action
of the hormone)
Phosphorylated
Enzyme 2
A cellular activity is
INHIBITED
(Inhibitory action of
the hormone)
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Enzyme 2
Hormones: An overview
Those G-proteins that result in stimulation of a cellular activity are called stimulatory Gproteins and those causing inhibition of a cellular activity are called inhibitory G-proteins.
Fig.9: Action of G-protein-coupled receptors
Source:
http://www.rci.rutgers.edu/~uzwiak/AnatPhys/PPFall03Lect8_files/image002.jpg
permission
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written for
Hormones: An overview
Cyclic AMP (cAMP) is known as the second messenger as it brings about the response to
the hormone which is the first messenger. Cyclic AMP lasts for a limited period of time
after which it is destroyed by an intracellular enzyme, phosphodiesterase. For the action of
the hormone to continue, new molecules of the hormone need to bind to the receptors to
keep generating new cAMP. In addition to cAMP, other molecules like Ca2+ ions, inositol
triphosphate (IP3), diacylglycerol (DAG) and cyclic guanosine monophosphate (c GMP) are
also used as second messengers. The steps in the signal transduction pathway occur in
succession, each initiating or triggering the next step; one molecule activates another, which
in turn activates another forming a cascade of events. The reaction is amplified at each step
as each molecule activates hundreds of molecules with each activated molecule further
activating hundreds of another type of molecule.
Click the link and find out how the cAMP brings the response to hormone
http://www.youtube.com/watch?v=A3AUhMCE9n0
The responsiveness of the target cell to a particular hormone keeps changing from time to
time because the number of receptors for the hormone keeps changing. Generally, when the
concentration of the hormone increases resulting in an increased number of receptors
binding with the hormone, the cell responds by reducing the number of receptors. This is
achieved by internalization (where the receptor is taken inside the cell and is not available
for binding with the hormone), their destruction by lysosomes, and reduced production. This
process of reduction in the number of receptors following an increase in the hormone levels
is known as down regulation. Similarly, there may be an up regulation of the receptors
by causing an increase in the number of receptors or an increased responsiveness of the cell
to the hormone by increasing the synthesis of intracellular signaling molecules.
Regulation of Hormone Secretion
Hormones are secreted in short bursts and their concentration varies with the 24-hour cycle,
with age, season and state of sleep e.g., growth hormone levels rise during early stages of
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Hormones: An overview
sleep. The levels of hormones are regulated by negative and positive feedback mechanisms
with inputs from
1) Neurons
2) Chemical changes in the blood and
3) Other hormones.
When the concentration of a hormone increases beyond a specific level it inhibits its own
secretion by inhibiting the stimulus that caused its secretion. This is known as negative
feedback regulation (Fig. 10).
Stimulus
Endocrine Gland
Hormone Secretion
Increased
hormone levels
Negative Feedback (inhibition)
Fig.10. Negative feedback inhibition
Source: Author, ILLL in house
A positive feedback serves to increase the levels of a hormone e.g., during childbirth, the
levels of oxytocin keep increasing due to stretching of the cervix caused by uterine
contractions induced by oxytocin itself. The positive feedback cycle is broken by the event of
childbirth (Figure 11). Specific feedback mechanisms for regulating the secretion of different
hormones will be discussed in the subsequent chapters.
Click the link and find out about the positive and negative feedback regulation of
hormone
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Hormones: An overview
Fig.11. Positive feedback loop for oxytocin secretion during
childbirth
Source:
OpenStax
College.
Homeostasis.
OpenStax-CNX.
3
June
<http://cnx.org/content/m45989/1.3/>.
http://cnx.org/content/m45989/latest/106_Pregnancy-Positive_Feedback.jpg CC
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2013
Hormones: An overview
Hormone Interactions
Actions of a hormone on a target cell can be modified by another hormone. Depending on
the ways in which hormones interact with each other, two or more hormones together can
have a:

Permissive effect- where exposure to a specific hormone is necessary before another
hormone can act on the target cell. Here, the first hormone either causes the
development of receptors for the second hormone to act or it causes the synthesis of
some enzymes required for the action of the second hormone e.g. uterine cells have to
be exposed to estrogen before they can respond to progesterone because they develop
progesterone receptors only when exposed to estrogen.

Synergistic effect- when two hormones together can bring about a certain effect which
either one alone cannot e.g., development of ovarian follicles is brought about together
by FSH and estrogens and either of these alone cannot achieve that.

Antagonistic effect- where one hormone has an effect opposite to another hormone
e.g., calcitonin (hormone from the parafollicular cells of the thyroid gland) reduces blood
calcium levels while parathyroid hormone (hormone from the parathyroid glands)
increases them.
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Hormones: An overview
Summary
 Body functions are regulated by the endocrine system, the nervous system and the
interface between the two (neuroendocrine mechanisms)
 Endocrine glands are ductless glands that release their secretions (hormones) directly
into the blood stream from where they are carried to the various target organs.
 Hormones regulate various activities like metabolism and energy balance, contractile
activities of muscles, secretory activities of glands, immune system activities etc.
 Types of Hormones
Depending upon their solubility, hormones can be of two types--Lipid-soluble and watersoluble hormones.
1) Lipid-soluble hormones are not soluble in water. They include:
•
Steroid hormones- sex hormones and hormones of the adrenal cortex. They are
synthesized from cholesterol and transported in the blood bound to specific plasmaproteins.
•
Thyroid hormones- Tri-iodothyronine and thyroxine. These are iodinated molecules
derived from the amino acid tyrosine. They are transported in the blood bound to
thyroxine binding globulin.
•
Nitric Oxide (NO) - This is a gas which is synthesized from the amino acid arginine by
the activity of the enzyme Nitric Oxide Synthase (NOS). It is transported in the blood
bound to hemoglobin and other proteins.
2) Water-soluble hormones include:
•
Peptides and proteins- hormones containing less than 100 amino acids are called
peptides (e.g., oxytocin and vasopressin) and those with more than 100 amino acids are
called proteins (e.g., growth hormone).These hormones are synthesized as inactive
preprohormones by the rough endoplasmic reticulum and later modified into their active
forms.
•
Biogenic amines- they are derived from amino acids and retain an amino group.
These include catecholamines (epinephrine, nor-epinephrine and dopamine) derived from
the amino acid tyrosine. Serotonin and melatonin are derived from the amino acid
tryptophan.
•
Eicosanoids-hormones derived from a 20-carbon fatty acid, arachidonic acid. These
include prostaglandins, thromboxanes and leukotrienes.
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Hormones: An overview

All water-soluble hormones are transported by blood in their dissolved form (dissolved
in plasma).
 Mode of Action of hormones
Lipid-soluble hormones act on their target cells by binding to intracellular receptors as
they can cross the lipid membrane. Steroid hormones bind to receptors in the cytoplasm
while thyroid hormones bind to receptors present inside the nucleus. The hormonereceptor complex (activated) then binds to specific regulatory sequences of DNA
activating or inhibiting specific genes. This results in stimulation or inhibition of a cellular
activity through stimulation or inhibition of mRNA synthesis.
Water-soluble hormones act by binding to receptors on the surface of the target cells as
these hormones cannot cross the lipid membrane. The intracellular portions of the
receptors act in different ways to bring about the hormone response:
•
They may act as an enzyme (tyrosine kinase) to cause its own phoshorylation
resulting in stimulation (or inhibition) of a cellular activity e.g., receptors for growth
factors
•
They may activate cytoplasmic tyrosine kinases like Janus Kinases (JAK) that
phosphorylate other cytoplasmic proteins. These phosphorylated proteins act as
transcription factors stimulating protein synthesis that brings about a change in cellular
activities e.g., receptors for the hormone leptin from adipose tissue.
•
They are most commonly associated with the GTP-binding protein, the G-Protein,
hence these receptors are known as the G-Protein-coupled receptors. The G-Protein may
directly bring about a change by causing opening or closing of certain ion channels or it
may activate an enzyme (e.g., Adenylate cyclase) resulting in the formation of a second
messenger, cyclic-AMP which then brings about the response
The responsiveness of a target cell to a hormone keeps changing depending upon the
number of receptors that may increase with a reduction in the hormone concentration
(up-regulation) or decrease with an increase in the hormone concentration (downregulation).
 Regulation of hormone secretion
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Hormones: An overview
Hormone levels are regulated by negative feedback mechanisms with inputs from the
nervous system, other hormones and chemical changes in blood. When the concentration
of a hormone increases beyond a level it inhibits its own secretion by a negative
feedback. When its concentration falls below a certain level it stimulates its own secretion
by a positive feedback.
 Hormone Interactions
One or more hormones may interact with each other to produce an effect. These are:
•
Permissive effect- where action of one hormone is necessary before another hormone
can act on the target cells
•
Synergistic effect- where two or more hormones together can bring about a response
that each one of them alone cannot.
•
Antagonistic effect- when one hormone has an effect opposite to that of another
hormone
Glossary
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Hormones: An overview

Adrenocorticotropin
- (ACTH or corticotropin) anterior pituitary, peptide
hormone

Antidiuretic hormone - (ADH) hypothalamus, peptide hormone

Atrial natriuretic factor - (ANP) heart, , peptide hormone

Autocrine agents: Chemicals secreted by a cell or tissue that act on the same
cell or tissue that produces it.

Biogenic amines: Hormones derived from amino acids tyrosine and tryptophan
are called biogenic amines as they retain an amino group. These include
catecholamines (epinephrine, nor-epinephrine and dopamine that are derived from
amino acid tyrosine), serotonin and melatonin, both derived from amino acid
tryptophan.

Calcitonin - (CT) C cells of thyroid, peptide hormone

Corticosteroid binding globulin - (CBG) binds and transports glucocorticoids in
the plasma. Globin is synthesized in the liver.

Down-regulation: Reduction in the number of receptors for a hormone when the
hormone concentration is high.

Eicosanoids: These are the hormones derived from a 20-carbon fatty acid,
arachidonic acid. These include prostaglandins, thromboxanes and leukotrienes

Endocrine glands: Ductless glands that pour their secretions (hormones) into
the blood stream.

Follicle stimulating hormone - (FSH) pituitary, protein hormone

G-protein: A hetero-trimeric, GTP-binding protein associated with receptors for
water-soluble hormones that bring about the formation of a second messenger
like cyclic AMP.

G-protein-coupled receptors: Receptors for water-soluble hormones that are
associated with G-proteins.

Growth hormone - (GH) pituitary, peptide hormone

Hormones: Chemical substances secreted by ductless glands and released into
the blood stream.

Human chorionic gonadotropin - (hCG) pancreas glycoprotein hormone with 2
subunits (alpha and beta joined non covalently). Similar in structure to luteinizing
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Hormones: An overview
hormone (LH), hCG exists in multiple hormonal and non-endocrine agents (regular
hCG, hyperglycosylated hCG and the free beta-subunit of hyperglycosylated hCG).

Janus kinases: These are a type of cytoplasmic tyrosine kinases which are
activated by the receptors for the water-soluble hormones. They bring about
phosphorylation of specific cytoplasmic proteins affecting a cellular activity.

Lipid-soluble hormones: Hormones that are soluble in lipids and insoluble in
water
e.g.,
steroid
hormones.
They
need
specific plasma-proteins to be
transported in the blood as they are not soluble in blood plasma.

Luteinizing hormone - (LH) pituitary, protein hormone

Melanocyte stimulating hormone - (MSH) pituitary, peptide hormone

Negative feedback: When the concentration of a hormone increases beyond a
specific level it inhibits its own secretion by inhibiting the stimulus that caused its
secretion. This is known as negative feedback regulation.

Nitric Oxide: A gas which acts as a hormone and a neurotransmitter and is
synthesized when needed from the amino acid arginine by the activity of enzyme
nitric oxide synthase.

Paracrine agents: Chemicals secreted by a cell or tissue that act on the
neighboring cells to modify their activity.

Parathyroid hormone - (PTH) parathyroid, peptide hormone

Positive feedback: When the levels of a hormone are low, it enhances its own
secretion by stimulating the mechanisms causing its secretion. This is known as
positive feedback regulation.

Preprohormones: Protein and peptide hormones are synthesized by the rough
endoplasmic reticulum as inactive precursors called preprohormones that are
variously modified to form the active hormones.

Prohormones: During the synthesis of protein and peptide hormones, a part of
the preprohormone (the inactive precursor) is cleaved by the endoplasmic
reticulum to form a prohormone which is then converted into an active hormone.

Prolactin - (PRL) pituitary, peptide hormone

Protein and peptide hormones: Hormones consisting of 3 to 100 amino acids
are called peptide hormones while those containing more than 100 amino acids
are called protein hormones e.g., growth hormone.
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Hormones: An overview

Receptor tyrosine kinases: Receptors for water-soluble hormones that directly
function as enzymes causing phosphorylation of tyrosine-containing portions of
itself.

Second messengers: Molecules like cyclic AMP, diacylglycerol and inositol
triphosphate that are formed as a result of binding of a water-soluble hormone
(first messenger) to its receptor on the cell surface. Second messengers bring
about the response of the hormone.

Signal transduction pathway: The sequence of events resulting in the ultimate
response to a hormone in its target cell.

Steroid
hormones:
Lipid-soluble hormones derived from cholesterol. For
example, the sex hormones (estrogen, progesterone and testosterone) and the
hormones of the adrenal cortex (cortisol, aldosterone and androgens).

Thyroid hormone - (TH) thyroid, amino acid derivative.

Thyroid hormones: Hormones of the thyroid gland-- Tri-iodothyronine and tetraiodothyronine (thyroxine). These are formed by coupling of iodinated tyrosine
molecules.

Thyroid stimulating hormone - (TSH) pituitary, protein hormone

Up-regulation: Increase in the number of receptors for a hormone when the
hormone concentration is low.

Water-soluble
hormones: Hormones like oxytocin, growth hormone and
epinephrine that are soluble in water and can be transported in the blood without
any transport proteins.
Exercises
A. Multiple-choice questions
Q1 Hormones are secreted by
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Hormones: An overview
a) Endocrine glands
b) Exocrine glands
c) Extracellular tissue
d) Connective tissue
Q2 Which of the following contribute to homeostasis
a) Endocrine glands
b) Nervous system
c) Neuroendocrine mechanisms
d) All of these
Q3 Hormones are transported by
a) Blood
b) Lymph
c) Neurons
d) Cells
Q4 The advantage of release of hormones into the bloodstream and their transport by
blood is that they can
a) reach distant target organs/cells
b) act on multiple target organs/cells
c) bind to blood cells
d) both a and b
Q5 Paracrine agents act on
a) distant target organs/cells
b) neighboring organs/cells
c) same organ/cells
d) none these
Q6 Autocrine agents act on
a) neighboring organs/cells
b) same organ/cells
c) distant target organs/cells
d) none these
Q7 Steroid hormones are
a) lipid-soluble
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Hormones: An overview
b) derived from cholesterol
c) not stored in their ready form
d) all of these
Q8 Thyroxine is not
a) lipid-soluble
b) derived from tyrosine
c) stored in the thyroid follicle bound to thyroglobulin
d) transported dissolved in plasma
Q9 Plasma proteins that carry lipid-soluble hormones in the blood do not
a) increase their half-life
b) provide a ready stock of hormones
c) prevent them from being excreted by the kidney
d) decrease their efficacy
Q10 Peptide hormones are not
a) made of more than 100 amino acid residues
b) synthesized by the rough endoplasmic reticulum
c) synthesized as inactive preprohormones
d) all of these
Q11 Epinephrine is not a
a) biogenic amine
b) catecholamine
c) water-soluble hormone
d) tryptophan derivative
Q12 Nitric oxide is
a) synthesized from amino acid tyrosine
b) same as endothelium derived relaxation factor
c) water soluble
d) stored as a neurotransmitter
Q13 Eicosanoids are
a) derived from a 10-carbon fatty acid
b) lipid-soluble
c) prostaglandins, thromboxanes and leukotrienes
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Hormones: An overview
d) none of these
Q14 Receptors for water-soluble hormones are present
a) inside the cytoplasm
b) inside the nucleus
c) on the cell surface
d) on the nuclear membrane
Q15 Second messengers are required for the action of
a) lipid-soluble hormones
b) water-soluble hormones
c) steroid hormones
d) thyroid hormones
B. Short-answer questions
Q1
Differentiate between the role of nervous system and the endocrine system as
homeostatic systems.
Q2
How are lipid-soluble hormones transported in the blood?
Q3
How does aspirin work as a pain killer?
Q4
What are catecholamines?
Q5 Why are thyroid hormones lipid-soluble and catecholamines water-soluble when both
are derived from tyrosine?
Q6 What is meant by a ‘signal transduction pathway’?
Q7 What are second messengers?
Q8 Describe the negative and positive feedback regulation of hormones.
Q9 How is nitric oxide synthesized and transported?
Q10 Differentiate between up-regulation and down-regulation.
C. Long-answer questions
Q1 Describe the various hormones based on their solubility in blood plasma.
Q2 How do lipid-soluble hormones act on their target cells?
Q3 Discuss the mode of action of water-soluble hormones.
Q4 How are hormone levels regulated in the body?
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Hormones: An overview
Q5 How do hormones interact with each other to modify their actions?
References

Tortora, G.J. and Derrickson, B.H. (2009). Principles of Anatomy and Physiology,
XII Edition, John Wiley and Sons, Inc.

Widmaier, E.P., Raff, H. and Strang, K.T. (2008). Vander’s Human Physiology, XI
Edition, McGraw Hill

Guyton, A.C. and Hall, J.E. (2011). Textbook of Medical Physiology, XII Edition,
Harcourt Asia Pvt. Ltd/ W.B. Saunders Company
Weblinks
http://www.hartnell.edu/tutorials/biology/signaltransduction.html
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