Definition
- Second messengers are intracellular signal molecules generated after hormone-receptor (HR) complex formation, which relay and amplify the hormone's signal to change cell function.
- Major second messengers: cAMP, IP3, DAG, cGMP, intracellular Ca²⁺, phosphoproteins and newly transcribed mRNA.
Classification of Signal Transduction Pathways
Second messenger transduction systems
Mechanism
Ion channel gating
- Second messenger
- Membrane ion flux
- Example ligand
- Ach (nicotinic), NE (K+ channel)
Adenylyl cyclase (Gαs/Gαi)
- Second messenger
- cAMP
- Example ligand
- NE (β1, ↑), NE (α2, ↓), PTH, ADH (V2)
Phospholipase C (Gαq)
- Second messenger
- IP3, DAG
- Example ligand
- Angiotensin II, NE (α1), ADH (V1), TRH
Guanylyl cyclase
- Second messenger
- cGMP
- Example ligand
- ANP, nitric oxide
Tyrosine kinase / JAK-STAT
- Second messenger
- Phosphoproteins
- Example ligand
- Insulin, IGF, PDGF, EGF, M-CSF, GH
Serine or threonine kinase
- Second messenger
- Phosphoproteins
- Example ligand
- Inhibin, activin, TGF-β
Nuclear receptor
- Second messenger
- New mRNA, new protein
- Example ligand
- Steroid hormones, thyroid hormones