Competitive antagonism can be distinguished from non-competitive (irreversible) antagonism on a log-dose-response curve by the fact that:

• A Competitive antagonism reduces both EC50 and Emax; irreversible antagonism only reduces EC50
• B Both types shift the curve to the right, but competitive antagonism is more potent
• C Competitive antagonism shifts the curve to the right without changing the maximum response (Emax); irreversible antagonism depresses Emax ✓
• D Irreversible antagonism shifts the curve to the right, while competitive antagonism reduces Emax only

Explanation

Competitive (surmountable) antagonists occupy the same receptor binding site as the agonist; increasing agonist concentration can overcome the blockade, shifting the dose-response curve parallel to the right (increased EC50) while the maximum response (Emax) remains unchanged. Non-competitive (insurmountable/irreversible) antagonists bind covalently or allosterically, permanently reducing the number of functional receptors; increasing agonist cannot restore Emax, so the maximum plateau is depressed even at saturating agonist concentrations.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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