A full agonist is producing near-maximal contraction in an isolated smooth muscle preparation. When a partial agonist with affinity for the same receptor is added, the response falls below the level produced by the full agonist alone. This observation is best explained by:

• A The partial agonist has lower affinity for the receptor than the full agonist
• B The partial agonist competes for the receptor and produces a lower intrinsic activity, reducing the maximal response ✓
• C The partial agonist irreversibly blocks the receptor
• D The partial agonist depletes intracellular calcium stores

Explanation

A partial agonist has affinity for the receptor but intrinsic activity less than one. When it competes with a full agonist for the same receptor, it occupies receptors and drives them to an intermediate active state, so the net response falls. This is why partial agonists such as buprenorphine can precipitate withdrawal in opioid-dependent patients. Option A is wrong because the phenomenon depends on intrinsic activity, not affinity; a high-affinity partial agonist would displace the full agonist even more effectively.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.