# Second Messengers

> Second messengers for FMGE Biochemistry: cAMP, IP3-DAG and cGMP pathways, hormone mapping and cholera versus pertussis toxin.

- Canonical URL: https://prepelephant.com/topics/fmge/biochemistry/second-messengers-fmge
- Exam / course: FMGE · Subject: Biochemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Second Messengers", PrepElephant, https://prepelephant.com/topics/fmge/biochemistry/second-messengers-fmge

## 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.