How do receptor reserves influence the interpretation of EC50 vs EC100 in pharmacology?

• A. EC50 can be lower than the concentration needed for full effect due to spare receptors.
• B. EC50 must always equal EC100.
• C. EC50 is irrelevant when receptor reserves exist.
• D. Receptor reserves eliminate the need to consider EC100.

Answer: (D)

Receptor reserves mean you don’t need to occupy all the receptors to achieve the full cellular response because downstream signaling amplifies the signal. Because of this amplification, a smaller fraction of receptors being activated can produce a large effect, so the concentration that produces 50% of the maximal response is reached with far fewer receptors engaged than the concentration that produces the full, 100% response. In other words, EC50 is lower than EC100 when spare receptors are present.

This doesn't mean EC100 becomes irrelevant. EC100 still defines the concentration needed to reach the maximal effect, which is important for understanding the drug’s ceiling of response and its therapeutic potential. Receptor reserves change the linkage between receptor occupancy and observed effect, but they do not make EC100 disappear from consideration.