[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › Pharmacodynamics: receptors and signalling [Pharmacology](https://durgical.com/en/pharmacology/) · Topic # Pharmacodynamics: receptors and signalling What a drug does to the body: the molecular targets drugs bind, the signals they set off, and why responses fade with repeated use. Updated 2026-11-01 Pharmacodynamics studies how drugs produce their effects. Most drugs bind a protein target (a receptor, enzyme, ion channel or transporter) and change its activity. Receptors fall into a few families that differ in speed and in the signals they send, and the body adapts to repeated stimulation, which explains tolerance. ## Key points - Four receptor families: ligand-gated ion channels (milliseconds), G protein-coupled receptors (seconds), enzyme-linked receptors such as receptor tyrosine kinases (minutes), and intracellular receptors that change gene transcription (hours). - Drugs also act on enzymes (statins, aspirin), voltage-gated channels (lidocaine) and transporters (digoxin on the sodium pump). - Spare receptors: a full response may need only part of the receptors to be occupied, so potency can be high even with low occupancy. - Desensitisation is a fast loss of response (receptor phosphorylation, uncoupling); down-regulation is a slower loss of receptor number; tolerance is the clinical result. - Stopping a drug after up- or down-regulation can cause rebound, for example after sudden withdrawal of a beta blocker. ## Receptor families - Ligand-gated ion channels: nicotinic acetylcholine receptor, GABA-A receptor; binding opens a pore and ions flow - G protein-coupled receptors (seven transmembrane segments): adrenergic, muscarinic, opioid receptors; signal through Gs (cAMP up), Gi (cAMP down) or Gq (calcium up) - Enzyme-linked receptors: the insulin receptor is a tyrosine kinase - Intracellular receptors: steroid and thyroid hormones cross the membrane and act on DNA, so effects take hours to appear and last longer ## Second messengers and amplification One activated receptor can activate many G proteins, and each enzyme they turn on makes many second-messenger molecules (cAMP, IP3, calcium). This amplification is one reason a small amount of drug can cause a large response and why spare receptors exist. ## Tolerance, desensitisation and rebound With continued exposure, receptors are phosphorylated and uncoupled from their G proteins (minutes), then internalised and degraded (hours to days). Beta-2 agonists and opioids show this clearly. Long-term blockade has the opposite effect: receptors increase, so abrupt withdrawal of an antagonist such as propranolol can leave the tissue over-sensitive (rebound tachycardia, angina). ## Selectivity is relative A drug that is selective for one receptor subtype at usual concentrations can act on others at higher concentrations. Selectivity reduces, but does not remove, side effects mediated by other receptors. ## Memory aid Channel: instant. GPCR: seconds. Kinase: minutes. Nucleus: hours. ## Test yourself Think of the answer, then tap the question to see it. ## Why do steroid hormones take hours to produce their effects? They act on intracellular receptors that change gene transcription, so the response needs new proteins to be made. ## What is meant by spare receptors? A maximal response can occur when only part of the receptors are occupied, because the signal is amplified downstream. ## Why can stopping a beta blocker suddenly cause rebound tachycardia? Long-term blockade increases the number or sensitivity of beta receptors; when the blocker is removed, normal catecholamines act on more receptors. ## Which G protein raises cAMP and which lowers it? Gs stimulates adenylyl cyclase and raises cAMP; Gi inhibits it and lowers cAMP. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/pharmacology/topics/pharmacodynamics-receptors-and-signalling/) ## Related classes - [Beta blockers](https://durgical.com/en/pharmacology/classes/beta-blockers/) - [Opioids and antagonists](https://durgical.com/en/pharmacology/classes/opioids-and-antagonists/) - [Insulins](https://durgical.com/en/pharmacology/classes/insulins/) - [Beta-2 adrenergic agonists](https://durgical.com/en/pharmacology/classes/beta-2-adrenergic-agonists/) - [Neuromuscular blockers](https://durgical.com/en/pharmacology/classes/neuromuscular-blockers/) ## Example drugs - [Regular human insulin](https://durgical.com/en/pharmacology/regular-human-insulin/) - [Morphine](https://durgical.com/en/pharmacology/morphine/) - [Salbutamol](https://durgical.com/en/pharmacology/salbutamol/) - [Propranolol](https://durgical.com/en/pharmacology/propranolol/) - [Rocuronium](https://durgical.com/en/pharmacology/rocuronium/) - [Lidocaine](https://durgical.com/en/pharmacology/lidocaine/) [← All of pharmacology](https://durgical.com/en/pharmacology/) Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given. Sources: [9.1 Signaling Molecules and Cellular Receptors](https://openstax.org/books/biology-2e/pages/9-1-signaling-molecules-and-cellular-receptors) (OpenStax (Rice University), 2018); [15.1 Divisions of the Autonomic Nervous System](https://openstax.org/books/anatomy-and-physiology-2e/pages/15-1-divisions-of-the-autonomic-nervous-system) (OpenStax (Rice University), 2022); [15.4 Drugs that Affect the Autonomic System](https://openstax.org/books/anatomy-and-physiology-2e/pages/15-4-drugs-that-affect-the-autonomic-system) (OpenStax (Rice University), 2022). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/pharmacology/topics/pharmacodynamics-receptors-and-signalling/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/topics/pharmacodynamics-receptors-and-signalling/