Question

The function of \(\mathrm{G}\) protein-coupled receptors, such as the opioid and cannabinoid receptors, can be regulated by the process of heterologous desensitization. Which of the following statements is true about this regulatory mechanism? a. This process occurs when the activation of one receptor leads to an increased expression of a second receptor. b. This process only occurs between receptors expressed on the surfaces of adjacent cells. c. This process cannot occur between receptors expressed on the same cell. d. This is a process that appears to involve transphophorylation of \(\mathrm{G}\) protein-coupled receptors. e. All of the above

Short answer

Answer: Heterologous desensitization of G protein-coupled receptors is a process that appears to involve transphosphorylation of the receptors.

Step-by-step solution

a. Activation of one receptor increases expression of a second receptor.

Heterologous desensitization is a process in which the phosphorylation of one GPCR results in the reduced responsiveness of other GPCRs that can recognize the same G-protein. This statement implies an increase in the expression of a second receptor upon activation of the first receptor, which is not true for heterologous desensitization. The correct answer does not describe the process accurately. #Step 2: Evaluate statement b.#

b. Heterologous desensitization only occurs between receptors on adjacent cells' surfaces.

This statement is also incorrect, as heterologous desensitization can occur within the same cell. It doesn't necessarily require the involvement of adjacent cells' surfaces, as it is a regulatory mechanism that occurs at the level of a single cell. #Step 3: Evaluate statement c.#

c. The process cannot occur between receptors on the same cell.

Contrary to this statement, heterologous desensitization is a process that occurs within the same cell. As mentioned before, it is a regulatory mechanism in which the phosphorylation of one GPCR reduces the responsiveness of other related GPCRs within that cell. Thus, this statement is incorrect. #Step 4: Evaluate statement d.#

d. Transphosphorylation is involved in GPCR heterologous desensitization.

This statement is true. Heterologous desensitization is a process that involves the transphosphorylation of G protein-coupled receptors by kinases, particularly GPCR kinases (GRKs) and second messenger-dependent kinases. These kinases phosphorylate specific serine or threonine residues in the receptor, which leads to the receptor's reduced responsiveness to its agonist. Therefore, option (d) is the correct answer. #Step 5: Evaluate statement e.#

e. All of the above

Since statements a, b, and c are incorrect, the correct answer cannot be "All of the above." We already identified the correct statement as option d. #Conclusion# The correct answer is option (d): Heterologous desensitization of G protein-coupled receptors is a process that appears to involve transphosphorylation of the receptors.