Mechanism of Action of Hormones Binding to Cell Membrane Receptors
(mainly for peptide/protein hormones and some amines like epinephrine)
Why they need membrane receptors
These hormones are water-soluble, so they cannot pass through the lipid cell membrane. Instead, they bind to specific receptors located on the external surface of the target cell.
What happens after binding
- The receptor changes its shape (activation)
- This triggers intracellular signaling cascades
- The response inside the cell is carried out by second messengers
The hormone itself never enters the cell.
It just acts as a first messenger.
Hormone-Receptor Interactions
The interaction is very specific.
A receptor fits its hormone like a lock fits a key.
Affinity and sensitivity matter
- Small amounts of hormone can produce large effects
- Receptors can up-regulate (increase) or down-regulate (decrease) based on body needs
Example: In insulin resistance, receptors are down-regulated.
This receptor-hormone binding is the first step that triggers everything else.
The Role of Second Messenger cAMP
cAMP is one of the main internal chemical messengers used by cells to carry out the hormonal signal.
How cAMP pathway works
- Hormone binds to G-protein–coupled receptor (GPCR) on the cell membrane.
- This activates the G-protein inside the membrane.
- Activated G-protein turns on the enzyme Adenylate Cyclase.
- Adenylate Cyclase converts ATP → cAMP.
- cAMP activates Protein Kinase A (PKA).
- PKA triggers phosphorylation of many proteins inside the cell.
- Cellular responses occur:
- Enzymes get activated or inhibited
- Ion channels open
- Metabolic pathways switch on or off
So cAMP is like a loudspeaker inside the cell, amplifying the hormonal signal.
Example Hormones using cAMP system
- Glucagon (stimulates liver to release glucose)
- Epinephrine (β-receptors) (activates heart, bronchi, glycogen breakdown)
- TSH, ACTH, LH, FSH from the pituitary
These all work very fast because they do not require gene expression.