Homologous desensitization

• A. Adding an agonist for receptor A desensizes receptor B
• B. Desensitization occurs after receptor internalization
• C. Adding an agonist for receptor A desensizes receptor A
• D. Desensitization cannot occur in the absence of phosphorylation

Answer: (C)

Homologous desensitization is receptor-specific: when a receptor is activated by its own agonist, that same receptor becomes less responsive to further stimulation. This happens through mechanisms like phosphorylation by GPCR kinases followed by beta-arrestin binding, which uncouples the receptor from its G protein and often leads to internalization. Because the desensitization is specific to the activated receptor, receptor B is not affected.

The statement that fits this concept best is that adding an agonist for receptor A desensizes receptor A. It captures the idea that the same receptor being stimulated loses responsiveness.

Desensitization of a different receptor (receptor B) would be heterologous desensitization, not homologous. Desensitization can occur without internalization in some cases, so it’s not strictly required. And while phosphorylation is a common mechanism, desensitization is not universally impossible without it, since other uncoupling processes can occur in certain systems.