3.3 Agonists & Antagonists
Drugs can activate (agonists) or block (antagonists) receptors. Understanding these classifications is essential for predicting drug effects and interactions.
Agonists
Full Agonist
Produces maximal response. Intrinsic activity = 1. Example: Morphine at ฮผ-opioid receptor.
Partial Agonist
Submaximal response even at full occupancy. IA < 1. Can antagonize full agonist. Example: Buprenorphine.
Inverse Agonist
Reduces constitutive receptor activity. IA < 0. Example: Some benzodiazepines at GABA-A.
Antagonists
Competitive Antagonist
Binds same site as agonist. Reversible. Shifts curve right, Emax unchanged. Surmountable.
Non-competitive Antagonist
Binds different site (allosteric). Reduces Emax. Not surmountable by increasing agonist.
Irreversible Antagonist
Covalent binding. Permanent inactivation. Recovery requires new receptor synthesis. Example: Phenoxybenzamine.
Clinical Examples
ฮฒ-Blockers
Competitive antagonists at ฮฒ-adrenergic receptors. Propranolol, metoprolol.
Naloxone
Competitive opioid antagonist. Reverses overdose. Short half-life.
Atropine
Competitive muscarinic antagonist. Mydriasis, dry mouth, tachycardia.
Aspirin
Irreversible COX inhibitor. Permanent platelet inactivation.