How does the addition of a noncompetitive agonist influence the dose–response curve?

• A. Shifts the curve right and downward.
• B. Shifts the curve left and upwards, allowing maximal effect at lower concentrations.
• C. Shifts the curve to the right but leaves maximal effect unchanged.
• D. Has no effect on the curve.

Answer: (B)

A noncompetitive agonist can enhance receptor activation through an allosteric site, changing how the receptor responds to the primary agonist. This often makes the receptor more easily activated and can increase the maximum effect achievable. As a result, the dose–response curve shifts to the left (lower concentrations of the primary agonist are needed to produce the same effect, indicating increased potency) and upward (the maximal effect, Emax, can be higher because the receptor is more efficiently activated).

In contrast, a noncompetitive antagonist would lower the maximal effect (downward shift) and may shift the curve to the right, while a standard competitive antagonist shifts the curve right without changing the maximal effect. An agent with no effect would not alter the curve.