Hormones are vital source for human physiological function and are associated with various mechanism and secondary messenger systems.

1. MECHANISM OF
HORMONES

2. Endocrine Glands
 Hormones are regulatory molecules secreted into the blood by
endocrine glands.
Chemical categories of
hormones
Steroids
Amines
Polypeptides
Glycoproteins

3. 1. Synergistic
2. Permissive
3. Antagonist
 Interactions between various hormones produce
effects that may be ;

4. Synergistic
When two or more hormones work
to produce a particular result, their
effect is noted as Synergistic.
Their effect may be additive or
complementary.
E.g, The action of epinephrine and
norepinephrine on the heart is a good
example of additive effect.
 Each of these hormones
separately produce an increase in
cardiac rate;

5. Other E.g, is Action of oestrogen,
cortisol, prolactin, oxytocin in the
secretion of milk by mammary glands.
 Their cooperative action is essential
at the time of lactation.
Permissive
 This effect is due to the action of a
second hormone. When it enhances
the responsiveness of a target organ
to the second hormone or when it
increases the action of second
hormone.

6.  Prior exposure of the
uterus to oestrogen,
 For E.g, induces the
formation of receptor
proteins for
progesterone, which
improves the response
of the uterus when it is
subsequently exposed
to progesterone.
 Thus oestrogen induces
a permissive effect on
the responsiveness of
the uterus to
progesterone.

7. Antagonistic
 In some situation, the action of one of the hormone antagonize the
effect of one another.
 E.g, Lactation during pregnancy is inhibited because of the high
concentration of oestrogen in the blood inhibits the secretion and
action of prolactin.
 Other E.g, Action of insulin and glucagon on adipose tissue; the
formation of fat is prompted by insulin, where as glucagon promotes
fat breakdown.

8. Effects of Hormone Concentrations on
Tissue Response
 Hormones do not generally accumulate in the blood because they are
rapidly removed by target organs and by the liver.
 The half-life of a hormone- the time required for the plasma
concentration of a given amount of the hormone to be reduce to half of
it’s physiological functions.
 This ranges from minutes to hours for most hormones; E.g, Thyroid
hormone has a half-life of several days.
 Hormones are removed from the blood by the liver are converted by
enzymatic reactions into less active products.
E.g, Steroids are converted into more water-soluble polar derivatives
that are released into the blood and excreted in the urine and bile.

9.  The effect of hormone is based on their concentration.
 When some hormones are taken abnormally high or pharmacological,
concentrations, their effects may be different from those produced
lower.
 Abnormally high concentration of a hormone may cause hormone to
bind to tissue receptor proteins of different but released hormones may
inhibit these different effect.
 Some steroid hormones can be converted by their target cells into
products that have different biological effects, the administration of
large quantities of one steroid can result in the production of a
significant quantity of other steroids with different effects.

10.  Pharmacological doses of hormones, particularly of steroids, can thus
have widespread and often damaging side effects.
 E.g; People with inflammatory diseases who are treated with high
doses of cortisone over long periods of time causes development of
Osteoporosis and characteristic changes in soft tissue structure.
Priming Effects
 Variations in hormone concentration within the normal, physiological
range can affect responsiveness of target cells.
 This is due in part to the effect of polypeptide and glycoprotein
hormones on the number of their receptor protein in target cells.
 More receptors may be formed in the target cells in response to
particular hormones.

11.  Small amounts of gonadotropin- releasing hormone (GnRH) secreated
by the hypothalamus.
 For example; increase the sensitivity of anterior pituitary cells to
further GnRH stimulation.
 This is a priming effect, sometimes also known as upregulation.
Desensitization and Downregulation
 Prolonged exposure to high concentrations of polypeptide hormones
has been found to desensitize the target cells.
 Subsequent exposure to the same concentration of the same hormone
thus produces less of a target tissue response.
 This desensitization may be partly due to the fact that high
concentrations of these hormones cause a decrease in the number of
receptor proteins in their target cells; this phenomenon is termed as
downregulation.

12.  E.g; In adipose cells exposed to high concentrations of insulin and in
testicular cells exposed to high concentrations of luteinizing
hormone(LH).
 In order to prevent desensitization from occurring under normal
conditions, may polypeptide and glycoprotein hormones are secreted
in spurts rather than continuously.
 E.g, In the hormonal control of the reproductive system. The pulsatile
secretion of GnRH and LH is needed to prevent desensitization.
 When these hormones are artificially presented in a continuous
fashion, they produce a decrease in gonadal function.

13. Mechanisms of Hormone Action
 Each hormone exerts a characteristic effects on target organs by acting
on the cells of these organs.
 Hormones of the same chemical class have similar mechanism of
action.
 Lipid-soluble proteins pass through the target cell’s plasma membrane,
bind to intracellular receptor proteins and act directly within the target
cell.
 Polar hormones do not enter the target cells, but instead bind to
receptor on the plasma membrane.
 This results in the activation of intracellular second- messenger
systems that mediate the actions of the hormone.

14.  Hormones under same chemical category have similar mechanism of
action.
 These similarities involve the location of cellular receptor proteins and
the events that occur in the target cells after the hormone has
combined with its receptor proteins.
 Hormones are secreted to blood by the glands but only target cells are
able to respond to these hormones.
 In order to respond to any given hormone, a target cell must have
specific receptor proteins for that hormones.
 Receptor protein-hormone interaction is highly specific.
 Specificity of hormone bind to receptor with high affinity and low
capacity.

15.  The location of a hormone’s receptor proteins in it’s target cells
depends on the chemical nature of the hormone.
 Since the lipophilic hormone( Steroids and Thyroxine) can pass
through the plasma membrane and enter their target cells, the receptor
proteins for lipophilic hormones are located within the cytoplasm and
nucleus.
 Water soluble hormones( catecholamines, polypeptides and
glycoproteins) cannot pass through the plasma membrane, their
receptors are located on the outer surface of the membrane.
 In these case the hormones require the activation of second messenger.

16. Hormones That Bind to Nuclear Receptor
Proteins
 Unlike water-soluble hormones, the
lipophilic steroid and thyroid
hormones do not travel dissolved in
the aqueous portion of the plasma.
 They are transported to their target
cells attached to plasma carrier
proteins.
 These hormones must then dissociate
from their carrier proteins in the
blood in order to pass through the
lipid component of the plasma
membrane and enter the target cell,
with which their receptor proteins are
located.

17.  The receptors for the lipophilic hormones are known as nuclear hormone
receptors because they function within the cell nucleus to activate genetic
transcription.
 The nuclear hormone receptors thus function as transcription factors that
first must be activated by binding to their hormone ligands.
 The newly formed mRNA produced by the activated genes directs the
synthesis of specific enzyme proteins that change the metabolism of the
target cell in ways that are characteristic of the effects of that hormone on
the target cell.
Nuclear Hormone Receptor
Ligand- Binding
Domain
DNA- Binding
Domain

18.  The receptor must be activated by binding to it’s hormone ligand
before it can bind to a specific region of the DNA, known as hormone-
response element.
 This is a short DNA span, composed of characteristic nucleotide bases,
located adjacent to the gene that will be transcribed when the nuclear
receptor binds to the hormone- response element.
Steroid
family Steroid
family

19.  In addition to the receptor for thyroid hormone, the latter family also
includes the receptors for the active form of Vitamin D and for
retinoic acid.
 Vitamin D and retinoic acid like the steroid and thyroid hormones, are
lipophilic molecules that play important roles in the regulation of cell
function and organ physiology.
 Scientists have currently identified the hormone ligand for only about
half of the approximately seventy different nuclear receptors that are
now known.
 The receptors for unknown hormone ligands are called orphan
receptors.
 For example, the receptor known as the retinoid X receptor was an
orphan until its ligand, 9-cis-retoinic acid.

20. Mechanism of Steroid Hormone Action
 Steroid hormones exert their effects by acting as ligands for nuclear
receptor proteins.
 Thus, they influence their target tissues by stimulating gene
expression.
 This is referred to as the genomic actions of steroids, and represents
their classical mechanism of action.
 Genomic mechanism is well understood by classical method.

21. In the classical,
 Genomic mechanism of steroid hormone action, the receptors for the
steroid hormones are located in the cytoplasm before the steroid
arrives.
 Depending on the steroid and the tissue, however, some unbound
steroid receptors may be located in the nucleus.
 When the cytoplasmic receptors bind to their specific steroid hormone
ligands, they translocate to nucleus.
 Once the steroid hormone- receptor protein complex is in the nucleus,
it’s DNA binding domain binds to the specific hormone- response
element of the DNA.

23.  The hormone- response element of DNA consists of two half-sites,
each six nucleotide bases long, separated by a three- nucleotide spacer
segment.
 One steroid receptor, bound to one molecule of the steroid hormone,
attaches as a single unit to one of the half-sites.
 Another steroid receptor, bound to one molecule of the steroid
hormone, attaches to other half-site of the hormone response element.
 The process of two receptor units coming together at the two half-
sites is called dimerization.
 Since both receptor units of the pair are the same, the steroid receptor
is said to form a homodimer.
 Once dimerization has occurred, the activated nuclear hormone
receptor stimulates transcription of particular genes, and thus
hormonal regulation of the target cell.

24.  Even this classical, genomic mechanism of steroid hormone action is
an oversimplification.
 For example; There are drugs such as tamoxifen, that act like estrogen
in one organ while antagonizing the action of estrogen in another
organ.
 Study of tamoxifen and other selective estrogen receptor modulators
(SERMs) has revealed that estrogen action requires more than twenty
different regulatory proteins called coactivators and corepressors.
 In addition to the estrogen receptor; these “third party” proteins have
stimulatory and inhibitory effects on the ability of estrogen to
stimulate genetic transcription.
 SERMs can have different effects in different organs because, even
though they bind to the estrogen receptor, they may enlist coactivator
proteins in one organ and corepressor proteins in another organ.

25. Mechanism of thyroid hormone
 Major hormone secreted by thyroid gland is thyroxine or
tetraiodothyronine (T₄).
 As of steroid hormones, thyroxine travels in the blood attached to
carrier proteins; primarily to thyroxine- binding globulin, or (TBG).
 The thyroid also secrete a small amount of triiodothyronine or (T₃).
 The carrier proteins have higher affinity for T₄ than for T₃.
 As a result the amount of unbound T₃ in the plasma is about 10 times
greater than the amount of free T₄.

26.  Approximately 99.96% of the thyroxine in the blood is attached to
carrier proteins in the plasma; the rest is free.
 Only the free thyroxine and T₃ can enter the target cell.
 The protein bound thyroxine serves as a reservoir of this hormone in
the blood.
 Once the free thyroxine passes into the target cell cytoplasm, it is
enzymatically converted into T₃.
 This T₃ is active than T₄ in target cells.
 The inactive receptor proteins for T₃ are located in the nucleus.
 Until they bind to T₃, the receptors are incapable of binding to DNA
and stimulating transcription.
 The T₃ may enter the cell from the plasma, or it may be produced in
the cell by conversion from T₄.

27.  In either case it uses some non-specific binding proteins as “stepping
stones” to enter the nucleus.
 It then binds to the ligand binding domain of the receptor.
 Once the receptor binds to T₃, its DNA binding domain can attach to
the half- site of the DNA hormone response element.
 The other half- site, however does not bind to another T₃ receptor
protein.
 Unlike steroid hormone receptors, the nuclear receptors in the non-
steroid family bind to DNA as heterodimers.
 Thyroid hormone receptor is one partner in the heterodimer; the other
partner is a receptor for the vitamin A derivative 9-cis-retinoic acid.
 Once bound to their different ligands, the two partners in the
heterodimer can bind to the DNA to activate the hormone response
element for thyroid hormone.
 In this way hormones stimulate transcription.

28. Hormones That Use Second Messengers
 Hormones that are catecholamines, polypeptides, and glycoproteins cannot
pass through the lipid barrier of the target cell’s plasma membrane.
 Although some of these hormones may enter the cell by pinocytosis, most
of their effects result from their binding to receptor proteins on the outer
surface of the target cell membrane.
 Since they exert their effects without entering the target cells, the action of
these hormones must be mediated by other molecules within the target cells.
 If hormones act as “messengers” from the endocrine glands, the
intracellular mediators of the hormone’s action can be called “Second
Messengers”.
 Second messengers are thus component of signal- transduction
mechanisms, since extracellular signals are transduced into intracellular
signals.

29.  When these hormones bind to membrane receptor proteins, there must
be active specific proteins in the plasma membrane in order to produce
the second messengers required to exert their effect.
 On the basis of the membrane enzyme activated, it is easy to
distinguish second- messenger systems that involve the activation of:
1. Adenylate cyclase
2. Phospholipase C
3. Tyrosine Kinase.

30. Adenylate Cyclase- Cyclic AMP
Second-Messenger System :-
 Cyclic adenosine monophosphate (cAMP) was the first “Second
messenger” to be discovered.
 When epinephrine and norepinephrine bind to their β- adrenergic
receptors, the effects of these hormones are due to cAMP production
within the target cells.
 It was later discovered that the effects of many polypeptide and
glycoprotein hormones are also mediated by cAMP.
 When one of these hormones binds to its receptor protein, it causes the
dissociation of a subunit from the complex of G- proteins.

31.  This G- protein subunit moves through the membrane until it reaches
the enzyme adenylate cyclase.
 The G- protein subunit then binds to and activates this enzyme, which
catalyzes the following reaction within the cytoplasm of the cell:
ATP cAMP + PPᵢ
 Adenosine triphosphate is thus converted into cyclic AMP(cAMP) and
two inorganic phosphates( pyrophosphate or PPᵢ).
 As a result of the interaction of the hormone with it’s receptor and the
activation of adenylate cyclase, hence the intracellular concentration of
cAMP is increased.
 Cyclic AMP activates a previously inactivate enzyme in the cytoplasm
called protein kinase.

32.  The inactive form of enzyme consists of two subunits:
Inactive
form of
enzyme
Catalytic
Inhibitory
Subunit

33. C
C

34.  Active protein kinase catalyzes the phosphorylation of different
proteins in the target cells.
 This causes some enzymes to become activated and others to become
inactivated.
 Cyclic AMP, acting through protein kinase, thus modulates the activity
of enzymes that are already present in the target cell.
 This alters the metabolism of the target tissue in a manner
characteristic of the actions of that specific hormones.
 Also cAMP must be rapidly inactivated for it to function effectively as
a second messenger in hormone action.
 This inactivation is accomplished by phosphodiesterase, an enzyme
within the target cells that hydrolyses cAMP into inactive fragments.

35.  The action of phosphodiesterase, the stimulatory effect of a hormone
that uses cAMP as a second messenger depends upon the continuous
generation of a new cAMP molecules.
 This also depends on the level of secretion of the hormone.
 In addition to cyclic AMP, cyclic guanosine monophosphate (cGMP)
functions as a second messenger in certain cases.
Example; The regulatory molecule nitric oxide exerts it’s effects on
smooth muscle by stimulating the production of cGMP in it’s target
cells.

36. Phospholipase C-Ca²⁺ Second Messenger
System
 The concentration of Ca²⁺ in the cytoplasm is kept very low by the
action of active transport; which carries calcium pumps in the plasma
membrane.
 Though the action of these pumps, the concentration of calcium is
about 10,000 times lower in the cytoplasm than in the extracellular
fluid.
 In addition, the endoplasmic reticulum of many cell contains calcium
pumps that actively transport Ca²⁺ from the cytoplasm into cisternae of
the endoplasmic reticulum.
 The steep concentration gradient for Ca²⁺ that results allows various
stimuli to bring a rapid effect.

37. For example;
1. The entry of Ca²⁺through voltage- regulated Ca²⁺ channels in the
plasma membrane serves as a signal for the release of
neurotransmitters.
2. When muscles are stimulated to contract, Ca²⁺ couples electrical
excitation of the muscle cell to the technical process of contraction.
 It is now known that Ca²⁺ serves as a part of a second-messenger
system in the action of a number of hormones.
 When epinephrine stimulates its target organs, it must first bind to
adrenergic receptor proteins in the plasma membrane of its target cells.

38. Adrenergic
receptors
α
β
 Stimulation of the beta- adrenergic receptors by epinephrine results in
activation of adenylate cyclase and the production of cAMP.
 Stimulation of alpha-adrenergic receptors by epinephrine results in
activates the target cell via the Ca²⁺ second- messenger system.

39.  The binding of epinephrine to it’s alpha – adrenergic receptor
activates, via a G- protein intermediate, an enzyme in the plasma
membrane known as phospholipase C.
 The substrate of this enzyme, a particular membrane phospholipids, is
split by the active enzyme into:
1. Inositol triphosphate(IP₃)
2. Diacylglycerol(DAG)
 Both derivates serve as second messengers.
 IP₃ leaves the Plasma Membrane and diffuses through the cytoplasm to
the endoplasmic reticulum.
 The membrane of endoplasmic reticulum contain receptor protein for
IP₃ which is a second messenger that carries hormones from the
plasma membrane to endoplasmic reticulum.

40.  Binding of IP₃ to it’s receptor causes specific Ca²⁺ channels to open, so
that Ca²⁺ diffuses out of the endoplasmic reticulum and into the
cytoplasm.
 As a result of these events, there is a rapid and transient rise in the
cytoplasmic Ca²⁺ concentration.
 This is incompletely understood, by the opening of Ca²⁺ channels in
the plasma membrane.
 This may be due to the action of different messenger sent from the
endoplasmic reticulum to the plasma membrane.
 The Ca²⁺ that enters the cytoplasm binds to protein called
Calmodulin.

41. Calmodulin
Ca²⁺
Activates
Calmodulin
Protein
Kinase
Modifies the action of
other enzymes
 Activation of specific calmodulin-
dependent enzymes is analogous to the
activation of enzymes by cAMP-
dependent protein kinase.

42.  Insulin promotes glucose and amino acid transport and stimulates
glycogen, fat, and protein synthesis in it’s target organs primarily the
liver, skeletal muscles, and adipose tissue.
 These effects are achieved by means of a mechanism of action that is
quite complex.
 It is known that insulin’s mechanism of action bears similarities to the
mechanism of action of other regulatory molecules known as growth
factors.
 Examples of this growth factors: Epidermal Growth Factor(EGF),
Platelet-Derived Growth Factor(PDGF) and Insulin-Like Growth
Factors(IGFs).
Tyrosine Kinase Second-Messenger System

43.  In the case of insulin and the growth factors, the receptor protein is
located in the plasma membrane and is itself a kind of enzyme known
as a tyrosine kinase.
 A kinase is an enzyme that adds phosphate groups to protein and a
tyrosine Kinase specifically adds these phosphate groups to amino
acid tyrosine within the proteins.
 The insulin receptor consists of two units that come together
(dimerize) when they bind with insulin to form an active tyrosine
kinase enzyme.
 Each unit of the receptor contain a site on the outside of the cell that
binds to insulin Ligand- Binding Site.
 A part that spans the plasma membrane, with an enzymatic site in the
cytoplasm.

44.  The enzymatic site is inactive until insulin binds to the ligand- binding
site and cause dimerization of the receptor.
 The activated tyrosine kinase receptor then phosphorylates other
proteins that serves as signalling molecules.
 Some of these signalling molecules are themselves kinase enzymes
that phosphorylate and activate other second- messenger systems.
 As a result of a complex series of activations, insulin and the different
growth factors regulate the metabolism of their target cells.
Example;
Insulin indirectly stimulates the insertion of GLUT-4 carrier proteins
into the plasma membrane of skeletal muscle, adipose and liver cells. In
this way, insulin stimulates the uptake of plasma glucose into the
organs.

45. Also, the binding of insulin to it’s receptor indirectly causes the
activation of glycogen synthetase, the enzyme in liver and skeletal
muscles that catalyzes the production of glycogen in these organs.
 The complexity of different second- messenger system is needed so
that different signaling molecules can have varying effects.
Example;
Insulin uses tyrosine kinase second messenger system to stimulate
glucose uptake into the liver and it synthesis into glycogen, whereas
glucagon promotes opposite effects ; the hydrolysis of hepatic glycogen
and subsequent secretion of glucose by activating a different second-
messenger system that involves the production of cAMP.

46. Reference
1. Human Physiology by STUART IRA FOX
2. https://my.clevelandclinic.org