In homologous desensitization, what happens when an agonist activates receptor A?

• A. Only receptor A desensizes
• B. Receptor B desensizes
• C. All receptors desensitize
• D. No desensitization occurs

Answer: (A)

Homologous desensitization means the desensitization is specific to the receptor that was activated. When the agonist binds receptor A, receptor A is phosphorylated by GPCR kinases and binds arrestin, which uncouples it from the G protein and often leads to internalization. This reduces signaling from receptor A for subsequent stimulation, while other receptors (like receptor B) remain capable of signaling normally. So, only receptor A becomes desensitized. Desensitizing receptor B or all receptors would involve different, non-fixed cross-talk mechanisms (heterologous desensitization or global signaling effects), which is not the scenario here.