Expansion
Ion channel, G protein coupled, enzyme linked and intracellular receptors
Expansion
- Ligand-gated ion channels: milliseconds. Nicotinic acetylcholine, GABA-A, glutamate receptors
- G protein coupled receptors: seconds. The largest family and the target of about a
third of all drugs. Signal through second messengers
- Gs raises cyclic AMP (beta adrenoceptors)
- Gi lowers cyclic AMP (muscarinic M2, alpha-2)
- Gq raises inositol trisphosphate and calcium (alpha-1, muscarinic M1 and M3)
- Enzyme-linked receptors: minutes to hours. Tyrosine kinase receptors for insulin and growth factors
- Intracellular (nuclear) receptors: hours. Steroid and thyroid hormones, altering gene transcription
Second messengers amplify enormously: one receptor activates many G proteins, each producing many messenger molecules, so a few hormone molecules can produce a large cellular response.
Downregulation and desensitisation of receptors after prolonged stimulation explain tolerance to beta agonists and opioids, and the rebound that follows abrupt withdrawal of beta blockers.