If both a partial agonist and a full agonist are present and the partial agonist concentration is high, what is the expected maximal response compared with full agonist alone?

If both a partial agonist and a full agonist are present and the partial agonist concentration is high, what is the expected maximal response compared with full agonist alone?

• A. The maximal response will exceed that of the full agonist alone.
• B. The maximal response remains unchanged.
• C. The maximal response will be reduced because partial agonist occupies receptors.
• D. The maximal response becomes unpredictable.

Answer: (C)

A partial agonist has lower efficacy than a full agonist, so each receptor it activates produces less signal. When both are present and the partial agonist is at a high concentration, it occupies many receptors, leaving fewer for the full agonist. Since those receptors activated by the partial agonist can’t deliver full activation, the total signal is reduced compared with what the full agonist alone could achieve. In other words, the partial agonist lowers the maximal response by occupying receptors without producing maximal activation, acting like a competitive antagonist in this setup. If there were receptor reserve, the full agonist might still reach near-max, but with high partial agonist occupancy the ceiling is still diminished.

A partial agonist has lower efficacy than a full agonist, so each receptor it activates produces less signal. When both are present and the partial agonist is at a high concentration, it occupies many receptors, leaving fewer for the full agonist. Since those receptors activated by the partial agonist can’t deliver full activation, the total signal is reduced compared with what the full agonist alone could achieve. In other words, the partial agonist lowers the maximal response by occupying receptors without producing maximal activation, acting like a competitive antagonist in this setup. If there were receptor reserve, the full agonist might still reach near-max, but with high partial agonist occupancy the ceiling is still diminished.