1. Mechanism of Hormone action Lecture no: 2nd MBBS
Dr Muhammad Ramzan

2. Recommended books for Biochemistry/Endocrinology
Lippincott’s’’ Illustrated Reviews Biochemistry
Harper's Illustrated Biochemistry
Path physiology of Endocrinology system Colorado State University
Basic and Clinical Endocrinology – Francis Greenspan – David G.Gardner
Text book of Medical biochemistry – MN Chatterjea

3. Mechanism of action – the definition A result orientated process
It describes the effect produced by a molecule such as a hormone/drug through its action on :
A living organism or biochemical system
Like insulin reduces hyperglycemia (produces hypoglycemia)
www. Merrium Webster Dictionary.com

4. Mechanism of hormone action - the background
All hormones act through highly specific receptors in the hormone sensitive target organs by binding
This specificity allows the cells to respond to very low concentrations of a hormone
Drugs are formulated to have affinity and specificity for these receptors and regulate hormones

5. Types of Receptors - 3 There are mainly 03 types of receptors
Extra cellular/ membranous/ surface receptors
Intracellular/ Cytoplasmic / Cytosolic and
Nuclear receptors

6. Types of receptors – 3

7. Cytoplasmic receptors

8. Mechanism of Hormone action – 3 types after receptor binding
There are mainly 3 types of mechanism of action of a hormone after receptor binding:
Changes in permeability of the cell membrane 1
Activation of the Cytoplasmic enzymes
Activation of the genes(Receptors)

9. Changes in cell membrane permeability In general
Neurotransmitters like Acetylcholine, Catecholamine and Histamine act by this mechanism
Neurotransmitters are released by pre synoptic nerve endings ( Axons in neuromuscular junction) and
Act on the target cell membranes (Muscle cell) through surface receptors (GPCR) and:
Alter membranes permeability for muscle for contraction

10. Neuromuscular junction

11. Activates muscle contraction
Changes in membrane permeability – Neurotransmitters Acetyl Choline (Exocytosis)
Acetyl Choline is released from pre synaptic nerve fibers into the synaptic cleft on arrival of the impulse (1ST impulse)
Binds with the receptors at the muscle cell membrane to alter its permeability by depolarization (2nd impulse) and
Activates muscle contraction
It Is quickly degraded by the Cholinesterase – an enzyme located on the post synaptic membrane

12. Neuro muscular junction

13. Activation of Cytoplasmic enzymes 2nd messenger cAMP
All Protein hormones and Catecholamine act by binding with extra cellular receptor- GPCR
Activate G – Protein for α- GTP and Adenylate Cyclase, acts on ATP to produce cAMP - the 2nd messenger
Hormone being the 1st messenger
cAMP Phosphorylates a no of enzymes/proteins via Protein Kinases to carry out hormonal action

14. Protein hormones and Catecholamines act via GPCR

15. Activation of Nuclear genes in general
Thyroids and steroid hormones act by this method
Hormone binds with its receptor (Cytoplasmic/nuclear) to form Hormone Receptor Complex – HRC
HRC is translocated to the acceptor site at nuclear DNA → Hormone Response Element – HRE ( the gene)
HRE is activated and transcribes mRNA which translates proteins/enzymes to execute hormonal action

16. Steroid Hormone – Mechanism of action

17. Mechanism of action - Steroids
Steroids are available as free and bound form
Both enter the blood circulation after their release from their site of synthesis
Free steroids are biologically active and Lipophilic
Can cross cell membrane through diffusion and bind with the Cytoplasmic receptors to form:
Hormone receptor complex – HRC

18. Mechanism of steroids action cont.
HRC is then translocated to the acceptor site at the : Nucleus to form Steroid/Hormone Response element
HRE – the gene for the steroid at DNA strand
HRE transcribes mRNA which travels to Cytoplasm
mRNA translates proteins/ enzymes to:
Execute hormone action

19. Mechanism of action- steroid hormones

20. Thyroid hormones(TH)
TH are also available as free and bound forms and are
attached with Globulin and Albumen
Bound forms lose their attachment and cross the cell membrane through active transport:
Iodo- Thyronine transporters

21. Mechanism of Thyroids(TH)
TH, then enter the nucleus to bind with Retinoid X- receptors (RXR) to form HRC
HRC is then translocated to the acceptor site at DNA to form Hormone Response Element, HRE – Agene
HRE the transcribes mRNA that leaves for cytoplasm
Translation of mRNA synthesizes proteins/enzymes to carry out hormone actions

22. Thyroid hormones – Mechanism of action

23. Difference B/W Thyroids and Steroids
The mechanism of TH is similar to the mechanism of steroids : Activation of genes
TH differ from steroid hormones that :
TH cross the cell membrane by active transport T4 is deiodinated to T3 by Deiodenase of the target cell
Binds with Retinoid X Receptors instead of Cytoplasmic receptors. 3
HRC is then translocated to the - Hormone Response Element 4

25. Mechanism of action of TH – the difference