The indirect (second-messenger) mechanism
Extracellular fluid
Cytoplasm
Cell membrane
Receptor
G protein
Adenylate cyclase
ATP
Protein kinases
1
Hormone binds a surface receptor
First messenger
2
G protein activated
3
Adenylate cyclase switched on
4
ATP → cAMP (second messenger)
1 hormone → many cAMP (amplification)
cAMP = second messenger
5
Protein kinases → cell responses
Enzyme activated
Filament contracts
Channel opens
Secretion
6
cAMP broken down → effect stops
More hormone needed to continue
Enzyme breaks down cAMP
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