Hormone Action and Second Messengers
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Direct answer
Cholera toxin keeps the stimulatory G protein permanently switched on by ADP-ribosylating its alpha subunit, and the resulting unrestrained cAMP production in the gut pours chloride and water into the lumen — the clearest demonstration that hormone action is signal transduction. Water-soluble hormones (peptides, catecholamines) bind membrane receptors whose intracellular arms generate second messengers such as cAMP, cyclic GMP, IP3, diacylglycerol and calcium; lipophilic hormones (steroids, thyroid hormone) diffuse in and bind nuclear receptors that switch genes on directly. Matching a hormone to its pathway is the question the exam will ask.
What you must remember
- cAMP system (Sutherland's Nobel discovery): seven-transmembrane G protein-coupled receptor, Gs or Gi, adenylyl cyclase, protein kinase A; phosphodiesterase terminates the signal (inhibited by caffeine and theophylline).
- Hormones working through cAMP: ACTH, TSH, LH, FSH, hCG, glucagon, PTH, calcitonin, ADH (V2 receptor) and beta-adrenergic agonists.
- IP3–DAG–calcium system (Gq-coupled): alpha-1 adrenergic, vasopressin V1, oxytocin, angiotensin II, GnRH, TRH; IP3 releases calcium from endoplasmic reticulum, DAG activates protein kinase C, calcium-calmodulin activates kinases including myosin light-chain kinase.
- cGMP: atrial natriuretic peptide at the membrane receptor and nitric oxide at soluble guanylyl cyclase (vasodilatation; sildenafil inhibits PDE5).
- Receptor tyrosine kinases: insulin and IGF-1 (IRS-PI3K-Akt arm), EGF, PDGF; JAK-STAT pathways serve growth hormone, prolactin, leptin, erythropoietin and cytokines (JAK2 V617F drives polycythaemia vera).
- Steroid hormones and thyroid hormone act on intracellular receptors binding hormone response elements in DNA; effects take hours and require protein synthesis, though thyroid hormone is an amino-acid derivative that nonetheless behaves like a steroid.
- Toxin pharmacology: cholera toxin locks Gs on; pertussis toxin locks Gi off (whooping cough); both ADP-ribosylate GTP-binding proteins.
- Receptor dynamics: down-regulation with sustained exposure explains tachyphylaxis; beta-arrestin-mediated desensitisation follows beta-agonist overexposure.
- Amplification: one hormone molecule activating one receptor can generate thousands of cAMP molecules — the cascade principle hormones exploit for sensitivity.
Mapping three patients onto three pathways
A patient in septic shock gets noradrenaline: at alpha-1 receptors, Gq activates phospholipase C, IP3 releases calcium from the sarcoplasmic reticulum, and calcium-calmodulin activates myosin light-chain kinase — vasoconstriction through the very same messenger that powers smooth-muscle contraction. A patient with symptomatic bradycardia gets glucagon (bypasses the blocked beta receptor by acting at its own Gs-coupled receptor to raise cAMP in myocardium), while an asthmatic gets salbutamol for the same cAMP target in bronchial muscle — one pathway, opposite clinical goals. A woman with a growth-hormone-secreting adenoma illustrates the third design: GH binds its receptor, associated JAK2 phosphorylates STAT proteins, which dimerise, enter the nucleus and transcribe IGF-1; the measured IGF-1 reflects pathway output. Finally, a patient on prednisolone demonstrates the slow arm: the drug diffuses into cells, binds the glucocorticoid receptor, and the complex binds glucocorticoid response elements to induce anti-inflammatory gene transcription — hours, not seconds, which is why status asthmaticus needs hours for steroids to work.
Where students slip
Students list PTH under calcium-messenger hormones because PTH raises calcium; its receptor, however, is Gs-coupled and works through cAMP in bone and kidney — the messenger name and the ion moved are different things. Second, they misplace insulin: it uses a receptor tyrosine kinase and never generates cAMP, which is why its actions are not blocked by cholera or pertussis toxins. Third, thyroid hormone's classification: an amino-acid-derived hormone acting at a nuclear receptor like a steroid — a deliberate exam exception. And when asked why steroid hormones need carrier proteins, the answer is their hydrophobicity, not instability: free hormone is the active fraction, bound hormone is the reservoir.
Frequently asked questions
What is the second messenger for glucagon and beta-adrenergic stimulation?
cAMP. Receptor-Gs activation of adenylyl cyclase raises cAMP, activating protein kinase A, which phosphorylates target enzymes such as phosphorylase kinase in liver and lipase in adipose tissue.
How do cholera and pertussis toxins affect G proteins?
Cholera toxin ADP-ribosylates the Gs alpha subsite preventing its GTPase activity, so adenylyl cyclase runs unchecked (secretory diarrhoea); pertussis toxin ADP-ribosylates Gi and prevents it from inhibiting adenylyl cyclase.
Which hormones signal through the IP3–DAG pathway?
Alpha-1 adrenergic agonists, vasopressin (V1), oxytocin, angiotensin II, GnRH and TRH — Gq-coupled receptors activating phospholipase C, with IP3 releasing endoplasmic calcium and DAG activating protein kinase C.
Why does hormone-receptor binding produce such a large intracellular response?
Cascade amplification: one ligand-receptor complex activates many G proteins, each adenylyl cyclase makes many cAMP molecules, and each kinase phosphorylates many substrates — sensitivity scaling of a million-fold is achievable.
How do steroid hormones differ in mechanism from peptide hormones?
They cross the membrane, bind cytosolic or nuclear receptors, and the complex acts as a transcription factor at hormone response elements; responses therefore begin in 30 minutes to hours and depend on new protein synthesis, unlike the seconds-long second-messenger effects of peptide hormones.