What is the action of a competitive antagonist at a receptor, and how can its effect be overcome? Example: flumazenil.

• A. It binds the receptor and competes with agonists; effects can be overcome by increasing agonist concentration.
• B. It irreversibly inactivates the receptor; no amount of agonist can overcome.
• C. It reduces receptor numbers via downregulation; requires receptor synthesis upregulation.
• D. It acts intracellularly to block second messengers; requires higher dose of antagonist to overcome.

Answer: (A)

A competitive antagonist binds reversibly to the same receptor site as the agonist, blocking receptor activation without permanently altering the receptor. Because the binding is competitive, increasing the concentration of the agonist raises the chance that the receptor is occupied by the agonist rather than the antagonist, allowing the response to be restored. Flumazenil exemplifies this at the benzodiazepine site on the GABA-A receptor: it competes with benzodiazepines for that site and can reverse their effects, illustrating how surmountable antagonism works when higher levels of agonist (or displacing the antagonist) can shift receptor occupancy back toward activation. This differs from irreversible antagonists, receptor downregulation, or intracellular blockade of signaling, whose effects aren’t overcome simply by increasing agonist concentration.