Hormones act through specific receptors and pathways to regulate target tissues. Water-soluble hormones bind to cell membrane receptors and use second messengers like cAMP to trigger intracellular effects. Lipid-soluble hormones like steroids diffuse into cells, bind intracellular receptors, and form complexes that regulate gene expression. Hormone levels are maintained through feedback loops, with negative feedback inhibiting further hormone release and positive feedback amplifying it. Hormones are cleared by metabolic degradation, binding to tissues, and excretion by the liver and kidneys.

1. By
DR KHALED SALEH ALGARIRI
November 2015
HORMONE:
MECHANISM & ACTION

2. HORMONE
A hormone is chemical regulatory substance,
secreted by endocrine glands.
It passes through blood stream to reach the
tissues on which it acts. These tissues are called
“target tissues”.

3. TARGET TISSUE
Target tissue of a certain hormone is the
tissue, which contains the specific receptors
of that hormone

4. • 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
• The processes involve in both negative and positive feedback.
For example, if A>B>C>D, increase in D causes inhibition of A i.e.
negative feedback. If D decreases, A production is triggered, i.e.
positive feedback.
GENERAL PRINCIPLES OF HORMONE
ACTION

5. Pathways Insulin Glucagon Epi T3, T4 Cortisol GH
Glycolysis
Glycogenesis
Glycogenolysis
Gluconeogenesis
Lipogenesis
Lipolysis
Protein Synthesis
Pathology
(+)
(+)
(+)
(+)
(-)
(-)
(-)
(-)
(-)
(-)
(-)
(+)
(+)
(+)
(+)
(-)
(-)
(-)
(+)
(+)
(+)
(+)
(-)
(-)
(+)
(+)
(+)
(+)
(-)
(+)
(+)
(+)
(-)
(+)
(+)
(-)
(+)
(-)
(++)
(+)
(+)
(+)
“central”
“peripheral”
“Liver” “L & M”
DM,
Glucagonoma
Parkinson’s
Grave’s D.
Myxedema
Cretinism
Goiter
Cushing’s
Conn’s
Addison’s
Dwarfism
Gigantism
METABOLIC EFFECTS OF HORMONES
(+) Stimulated; (-) Inhibited

6. HORMONE RECEPTORS
• Definition:
Cell-associated recognition molecules which
are protein in nature.
• Functional sites:
Two functional sites:
 Recognition site: It binds the hormone
specifically.
 Signaling site: It couples hormone binding to
intracellular effect

7. HORMONE RECEPTORS
Location : Receptors may be:
 Intracellular receptors: (in the cytosol or
in the nucleus)
 Cell-membrane receptors: (in the plasma
membrane).

10. I- CLASSIFICATION ACCORDING
TO CHEMICAL NATURE
PROTEINS
AMINO ACIDS
DERIVED STEROIDS

11. I- CLASSIFICATION ACCORDING
TO CHEMICAL NATURE
A- PROTEIN HORMONES:
 Large polypeptides: e.g. insulin and
parathyroid hormone
 Small polypeptides: e.g. ADH (9a.a.),
oxytocin (9a.a.)
 Glycoprotein hormone: e.g. FSH, LH, TSH,
HCG

12. I- CLASSIFICATION ACCORDING TO
CHEMICAL NATURE
B- AMINO ACID DERIVED HORMONES:
Thyroid hormones and catecholamines are
derived from tyrosine.
Melatonin is derived from tryptophan

13. I- CLASSIFICATION ACCORDING TO
CHEMICAL NATURE
C- Steroid hormones:
• These hormones are derived from cholesterol. e.g.
 Glucocorticoids.
 Mineralocorticoids.
 Sex hormones.

14. II-CLASSIFICATION ACCORDING TO
MECHANISM OF ACTION
A- Hormones, which bind to intracellular
receptors.
B- Hormones, which bind to membrane receptors

15. Hormones bind to
intracellular RCs
Hormones bind to cell
membrane RCs
 LIPOPHYLIC (are poorly soluble
in water)
 HYDROPHILIC (The water
soluble hormone)
 NEED TRANSPORT PROTEINS
TO REACH TARGET TISSUES
 DO NOT NEED TRANSPORT
PROTEIN
 LONG PLASMA HALF-LIFE
(HOURS TO DAYS).
 SHORT PLASMA HALF-LIFE
 (MINUTES).
 ACTION IS MEDIATED BY
FORMING HORMONE-
RECEPTOR COMPLEX
 ACTION IS MEDIATED BY
 SECOND MESSENGER.
 INCLUDE: STEROID , THYROID ,
 CALCITRIOL and Vitamin D
INCLUDE: the catecholamines
(epinephrine and norepinephrine)
and peptide/protein hormones

16. THE SECOND MESSENGER
 Is the signal produced as a result of hormone binding to
its cell membrane receptor.
 It mediates the effects of the hormone.
 The second messenger may be:
Cyclic Adenosine Monophosphate (cAMP).
Cyclic Guanosine Monophosphate (cGMP).
Calcium or phosphatidyl inositol or both.
Protein kinase cascade.
• N.B. The hormone is considered to be the first messenger

17. Signal Amplification Via 2nd Messenger
Pathways
Initial signal is in the form of hormone
which acts as ligand whose concentration
is just one/per receptor. The hormonal
response has got multiple steps, and each
step multiplies the signal (cascading
effect) that finally leading to million fold
amplification, i.e. one hormone molecule
mediating its effect through million of
molecules. This process is known as
signal amplification.

19. RECEPTOR
INTERNAL
NUCLEAR
CYTOPLASMIC
EXTERNAL
CELL
MEMBRANE
TYPES OF RECEPTORS &
LOCATION

21. • Binding to specific cell receptor in the cell
membrane and form hormone-cell receptor
complex, which diffuses to nucleus
• The receptor is eventually released for re-use
• Steroid activates a specific gene to produce
mRNA
• mRNA pass out into the cytoplasm and initiates
protein [enzyme] synthesis
why do they penetrate the cell?
 the whole process is called mobile-receptor
hypothesis in which a steroid hormone is not
attached to the plasma membrane, but seem to move
freely in the nucleoplasm
HOW LIPID-SOLUBLE HORMONES WORK?

22. • Step1: Free lipophilic hormone (hormone not bound with its
plasma protein carrier) diffuses through the plasma
membrane of the target cell and binds with the receptor which
is intracellularly located inside the cytosol/or in the nucleus.
• Step2. Each receptor has specific binding region with
hormone and another region with binding with DNA. Receptor
alone cannot bind to DNA unless it binds to hormone. Once
the hormone is bound to receptor, the hormone receptor
complex binds to specific region of DNA known as Hormone
response element(HRE).
• Step3: Transcription of gene
• Step4: m RNA transported out of nucleus into the cytoplasm
• Step5: Translation at Ribosome
• Step6: Protein/enzyme released from ribosome
• Step7: protein/enzyme mediate ultimate response
HOW LIPID-SOLUBLE HORMONES
WORK?

23. STEROID HORMONES: MOLECULAR ACTION

24. Lipophilic hormone response mediated through Cytosolic
receptor/nuclear receptor

25. HOW LIPID-SOLUBLE HORMONES
WORK?

26. WATER-SOLUBLE HORMONES

27. HOW WATER-SOLUBLE HORMONES
WORK?

28. HOW WATER-SOLUBLE
HORMONES WORK?
• ●1 A water-soluble hormone (the first messenger) diffuses from
the blood through interstitial fluid and then binds to its receptor at the
exterior surface of a target cell’s plasma membrane. The hormone–
receptor complex activates a membrane protein called a G protein.
The activated G protein in turn activates adenylate cyclase.
• ●2 Adenylate cyclase converts ATP into cyclic AMP (cAMP).
Because the enzyme’s active site is on the inner surface of the
plasma membrane, this reaction occurs in the cytosol of the cell.
• ●3 Cyclic AMP (the second messenger) activates one or more
protein kinases, which may be free in the cytosol or bound to the
plasma membrane. A protein kinase is an enzyme that
phosphorylates (adds a phosphate group to) other cellular proteins
(such as enzymes). The donor of the phosphate group is ATP,
which is converted to ADP.

29. HOW WATER-SOLUBLE
HORMONES WORK?
• ●4 Activated protein kinases phosphorylate one or more cellular
proteins. Phosphorylation activates some of these proteins and
inactivates others, rather like turning a switch on or off.
• ●5 Phosphorylated proteins in turn cause reactions that produce
physiological responses. Different protein kinases exist within different
target cells and within different organelles of the same target cell. Thus,
one protein kinase might trigger glycogen synthesis, a second might
cause the breakdown of triglyceride, a third may promote protein
synthesis, and so forth. As noted in step ●4 phosphorylation by a
protein kinase can also inhibit certain proteins. For example, some of the
kinases unleashed when epinephrine binds to liver cells inactivate an
enzyme needed for glycogen synthesis
.
• ●6 After a brief period, an enzyme called phosphodiesterase inactivates
cAMP. Thus, the cell’s response is turned off unless new hormone
molecules continue to bind to their receptors in the plasma membrane

30. REGULATION OF HORMONE
SECRETION
FEEDBACK
CONTROL
involve the secretion of
catecholamines by
adrenal medulla.
NEGATIVE
FEEDBACK
POSITIVE
FEEDBACK

31. FEEDBACK CONTROL

32. REGULATION OF HORMONE SECRETION
Hormone Metabolism
Feedback Control
NEGATIVE FEEDBACK
HPT Axis
Hypothalamus
TRH
TSH
T3 , T4
+
-
+
+
-
Anterior
Pituitary
Thyroid
Increased
metabolism
 T3 and T4
inhibits secretion
of TRH and TSH
Long loop feedback involves
the hormone feedback all the
way back to the hypothalamic-
pituitary axis
Short loop feedback involves the
anterior pituitary hormone
feedback on the hypothalamus
to inhibit the hypothalamic
releasing hormones

33. Hormone Metabolism
Female HPG Axis
+
+
Hypothalamus
+
Anterior
Pituitary
Ovaries
Estrogen Progesterone
+
+
+
-
- FSHLH
GnRH
REGULATION OF HORMONE SECRETION
Feedback Control
involve the effect of estrogen
on secretion of FSH and LH by
anterior pituitary as well as the
effect of oxytocin.
Hypothalamic Pituitary Gonada Axis HPG Axis
POSITIVE FEEDBACK
The increase in contractions causes more
oxytocin to be released and the cycle goes
on until the baby is born. The birth ends
the release of oxytocin and ends the
positive feedback mechanism.

34. CLEARANCE OF HORMONES
Hormones are cleared by:
1.Metabolic destruction by tissues/ target cells.
2.Binding with the tissues
3.Excretion by liver into bile.
4.Excretion by kidneys into urine.
Water soluble hormones(peptides and
catecholamines) are degraded by enzymes in the
blood or tissues and are excreted by kidneys or liver.
They have a short half life.
Lipid soluble hormones(steroids) are protein
bound and are cleared slowly.