Second Messengers
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Direct answer
A hormone that cannot cross the cell membrane has to shout through a second messenger. Gs-coupled receptors activate adenylyl cyclase, making cAMP and protein kinase A — the path of glucagon, β-adrenergic catecholamines, TSH, ACTH, LH, PTH, calcitonin and vasopressin V2. Gq-coupled receptors turn on phospholipase C, cleaving PIP2 into IP3 (releasing endoplasmic-reticulum calcium) and DAG (activating protein kinase C) — the path of α1 stimulation, vasopressin V1, GnRH, TRH, oxytocin and angiotensin II. cGMP carries atrial natriuretic peptide and nitric oxide through guanylyl cyclase; insulin and growth factors use receptor tyrosine kinases; growth hormone and cytokines use the JAK-STAT route. Cholera toxin locks Gsα on; pertussis toxin locks Gi off — both raise cAMP.
What you must remember
- Gs hormone list (cAMP): glucagon, adrenaline (β1, β2), TSH, ACTH, LH, FSH, hCG, PTH, calcitonin, ADH-V2, MSH — the single most examined mapping in endocrine pharmacology.
- Gq hormone list (IP3/DAG/calcium): adrenaline (α1), ADH-V1, angiotensin II, GnRH, TRH, oxytocin, histamine H1, muscarinic M1 and M3.
- cGMP systems: particulate guanylyl cyclase activated by ANP; soluble guanylyl cyclase activated by nitric oxide — the endothelial-relaxation cascade that sildenafil prolongs by inhibiting PDE5.
- Termination machinery: phosphodiesterases degrade cAMP and cGMP (blocked by caffeine and theophylline, which explains their pharmacology); GTPase activity of the α subunit switches G proteins off between signals.
- Toxin signatures: cholera toxin ADP-ribosylates Gsα (irreversibly active, sustained chloride secretion — rice-water stool); pertussis toxin ADP-ribosylates Giα (cannot inhibit adenylyl cyclase — the whoop comes from accumulated mucus and toxin action).
- Calcium as messenger: IP3-mobilised calcium binds calmodulin, activating kinases including myosin light-chain kinase — smooth-muscle contraction in one line.
- Amplification logic: one hormone-receptor complex activates many G proteins, each making many cAMP molecules, each kinase phosphorylating many targets — a cascade of three multiplications explains hormone sensitivity at picomolar concentrations.
A worked case: why oral rehydration works when cholera toxin does not
A patient with cholera loses a litre of rice-water stool hourly. The toxin's B subunit binds the ganglioside GM1 of the enterocyte, the A subunit enters and ADP-ribosylates Gsα; the permanently active Gs drives adenylyl cyclase, cAMP rises, protein kinase A phosphorylates the cystic fibrosis transmembrane conductance regulator, and chloride pours into the lumen with sodium and water following — a secretory, not invasive, diarrhoea. The therapeutic genius is that SGLT1, the sodium-glucose cotransporter at the brush border, is untouched by cAMP: glucose-coupled sodium absorption still works, which is exactly why WHO oral rehydration solution contains glucose and why it has saved more cholera patients than any antimicrobial. Antibiotics shorten the illness; the rice-based ORS deployed in Indian outbreak control exploits the same intact coupled transport through amino acid and glucose pathways.
Where students slip
Vasopressin is the double-agent: V2 receptors signal through cAMP (water reabsorption via aquaporin-2), while V1 receptors signal through IP3 (vasoconstriction) — stems exploit candidates who commit to a single pathway. Second, catecholamines are split by receptor, not hormone: β is cAMP, α1 is IP3, α2 is Gi (which lowers cAMP). Third, insulin is placed on cAMP; it signals through a receptor tyrosine kinase, and it actually lowers cAMP in adipose tissue — an examiner's favourite false pairing. Fourth, cholera and pertussis toxins are distinguished only by their G protein target, and both ultimately raise cAMP; attributing cholera to Gi causes the mark to fall. Fifth, sildenafil is called a "NO donor" — it inhibits the breakdown of cGMP (PDE5); the nitric oxide is the body's own, made from arginine by nitric oxide synthase.
Frequently asked questions
Which second messenger mediates β-adrenergic stimulation?
cAMP, generated by adenylyl cyclase through Gs and acting on protein kinase A.
How do the V1 and V2 vasopressin receptors differ in signalling?
V1 uses Gq with IP3 and calcium (vasoconstriction); V2 uses Gs with cAMP (aquaporin-2 insertion and water retention).
What does cholera toxin do at the molecular level?
It ADP-ribosylates the Gs α subunit, keeping adenylyl cyclase permanently active and driving cAMP-mediated chloride secretion.
How does nitric oxide relax vascular smooth muscle?
It activates soluble guanylyl cyclase, raising cGMP and dephosphorylating myosin light chain through protein kinase G.
Why is the cAMP system so sensitive to tiny hormone concentrations?
Cascade amplification — one receptor activates many G proteins, each making many cAMP molecules, each activating kinases that phosphorylate many targets.