A competitive antagonist shifts the agonist dose-response curve. Which statement CORRECTLY describes this shift?
- A The curve shifts to the right with a reduced maximum effect (increased EC50, decreased Emax)
- B The curve shifts upward with no change in EC50 (unchanged EC50, increased Emax)
- C The curve shifts to the left with unchanged Emax (decreased EC50, unchanged Emax)
- D The curve shifts to the right in parallel (increased EC50, unchanged Emax) ✓
Explanation
A competitive (reversible) antagonist competes with the agonist for the same receptor binding site. Increasing agonist concentration can overcome the competitive block—thus maximal efficacy (Emax) is preserved. The dose-response curve shifts to the right in a parallel manner: more agonist is needed to achieve 50% of the maximal effect, increasing the EC50 (or ED50). This rightward parallel shift with preserved Emax is the defining characteristic of competitive antagonism. Non-competitive (irreversible or allosteric) antagonism reduces Emax with variable EC50 changes because no amount of agonist can fully overcome blockade when receptors are permanently occupied.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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