Question

Scenario 2: You are informed that an endocrine organ in the body expresses four membrane receptors a) receptor-i which is Gs coupled b) receptor-ii which is Gi coupled c) receptor-iii which is Gq coupled d) receptor-iv is a receptor guanylyl cyclase You are further informed that an increase in Ca2+ concentration or an increase in the active PKA levels inside the organ both will result in release of hormone from this endocrine organ. On the other hand, PKG inhibits the release of hormone from this organ. What would be the expected outcome (increased blood concentration of hormone or decreased blood concentration of hormone) with the following drugs: Q4. Receptor 1 agonist? 1 point

Q5. Receptor 1 antagonist? 1 point

Q6. Receptor 2 agonist? 1 point

Q7. Receptor 2 antagonist? 1 point

Q8. Receptor 3 agonist? 1 point

Q9. Receptor 3 antagonist? 1 point

Q10. Receptor 4 agonist? 1 point

Q11. Receptor 4 antagonist? 1 point

Answer 1

We can write the data provided in points for easier interpretation:

1. The enzyme levels rise with rise in PKA and Ca ions.It decreases with rise in PKG.

2. Agonist produce action at the receptor and Antagonist inhibit it.

3. Function of receptors given and expected outcome:

i) Gi-inhibits cAMP formation from ATP and hence there is decrease in level of cAMP dependent PKA.

Agonist drug- Gi activated, PKA decreased , consequently enzyme level decrease in blood.

Antagonist drug- Gi inhibited, cAMP level rise=PKA level rise, hence enzyme level increase.

ii) Gq- activates phopholipase C which breaks PIP2 into IP3 and DAG. IP3 stimulates Ca ion release.

Agonist- increased IP3, increased Ca ions and hence increase in enzyme level

Antagonist - decreased IP3 , Decrease in Ca ions level and consequent decrease in enzyme level.

iii) Gs- increases adenyl cyclase activity which converts ATP into cAMP.

Agonist- increase in cAMP , imcrease in cAMP dependent PKA and hence increase in enzyme level.

Antagonist- decreased cAMP, decreased PKA and hence decreased enzyme levels.

iv) Receptor for guanyl cyclase- it convert GTP into cGMP. In turn cGMP activates PKG.

Agonist- increased cGMP, increased PKG and hence decreased enzyme levels.

Antagonist- decreased cGMP, decreased PKG and increase in enzyme levels.