<Table> <Tr> <Td> </Td> <Td> cAMP System </Td> <Td> Phosphoinositol system </Td> <Td> Arachidonic acid system </Td> <Td> cGMP System </Td> <Td> Tyrosine kinase system </Td> </Tr> <Tr> <Td> First Messenger: Neurotransmitters (Receptor) </Td> <Td> Epinephrine (α2, β1, β2) Acetylcholine (M2) </Td> <Td> Epinephrine (α1) Acetylcholine (M1, M3) </Td> <Td> Histamine (Histamine receptor) </Td> <Td> - </Td> <Td> - </Td> </Tr> <Tr> <Td> First Messenger: Hormones </Td> <Td> ACTH, ANP, CRH, CT, FSH, Glucagon, hCG, LH, MSH, PTH, TSH </Td> <Td> AGT, GnRH, GHRH, Oxytocin, TRH </Td> <Td> - </Td> <Td> ANP, Nitric oxide </Td> <Td> INS, IGF, PDGF </Td> </Tr> <Tr> <Td> Signal Transducer </Td> <Td> GPCR / G (β1, β2), G (α2, M2) </Td> <Td> GPCR / G </Td> <Td> Unknown G - protein </Td> <Td> - </Td> <Td> RTK </Td> </Tr> <Tr> <Td> Primary effector </Td> <Td> Adenylyl cyclase </Td> <Td> Phospholipase C </Td> <Td> Phospholipase A </Td> <Td> guanylate cyclase </Td> <Td> RasGEF (Grb2 - Sos) </Td> </Tr> <Tr> <Td> Second messenger </Td> <Td> cAMP (cyclic adenosine monophosphate) </Td> <Td> IP3; DAG; Ca2+ </Td> <Td> Arachidonic acid </Td> <Td> cGMP </Td> <Td> Ras. GTP (Small G Protein) </Td> </Tr> <Tr> <Td> Secondary effector </Td> <Td> protein kinase A </Td> <Td> PKC; CaM </Td> <Td> 5 - Lipoxygenase, 12 - Lipoxygenase, cycloxygenase </Td> <Td> protein kinase G </Td> <Td> MAP3K (c - Raf) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> cAMP System </Td> <Td> Phosphoinositol system </Td> <Td> Arachidonic acid system </Td> <Td> cGMP System </Td> <Td> Tyrosine kinase system </Td> </Tr> <Tr> <Td> First Messenger: Neurotransmitters (Receptor) </Td> <Td> Epinephrine (α2, β1, β2) Acetylcholine (M2) </Td> <Td> Epinephrine (α1) Acetylcholine (M1, M3) </Td> <Td> Histamine (Histamine receptor) </Td> <Td> - </Td> <Td> - </Td> </Tr> <Tr> <Td> First Messenger: Hormones </Td> <Td> ACTH, ANP, CRH, CT, FSH, Glucagon, hCG, LH, MSH, PTH, TSH </Td> <Td> AGT, GnRH, GHRH, Oxytocin, TRH </Td> <Td> - </Td> <Td> ANP, Nitric oxide </Td> <Td> INS, IGF, PDGF </Td> </Tr>

Why do some signals ( first messengers ) trigger second messengers to activate target cells