What is an example of a chemical antagonist?

• A. Protamine sulfate for heparin
• B. Flumazenil for benzodiazepines
• C. Naloxone for opioid receptor
• D. Atropine for muscarinic acetylcholine receptor

Answer: (A)

Chemical antagonism occurs when the antagonist neutralizes the agonist by direct chemical interaction rather than by blocking a receptor. Protamine sulfate fits this description because it binds to heparin in the blood, forming a stable complex that neutralizes heparin’s anticoagulant activity. This is different from receptor-based antagonism, where a compound binds to a receptor to prevent the agonist from eliciting a response without removing the agonist itself. For example, flumazenil acts at the benzodiazepine site on the GABA-A receptor, naloxone blocks opioid receptors, and atropine blocks muscarinic acetylcholine receptors. These are pharmacologic antagonists, not chemical antagonists.