Question

Which of the following uses cAMP as a second messenger.
(A). Corticotropin
(B). Dopamine
(C). Histamine
(D). Progesterone
Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (A), (B) and (D) only.
• C. (A) and (D) only.
• D. (B), (C) and (D) only.

Hint:

Steroid hormones (such as progesterone, estrogen, and testosterone) are lipophilic and act directly on nuclear receptors to regulate gene expression, whereas peptide and amine hormones (such as ACTH and catecholamines) use GPCRs and secondary messengers like cAMP.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Hydrophilic hormones bind to cell-surface G-protein coupled receptors (GPCRs) and trigger secondary messenger cascades, such as cyclic AMP (cAMP), to relay signals inside the cell.

Step 2: Detailed Explanation:
- Corticotropin (ACTH) (A): Binds to GPCRs coupled to \(\text{G}_s\) proteins, which activates adenylyl cyclase to increase intracellular cAMP levels.
- Dopamine (B): Acts through GPCR sub-types (such as D_1 and D_5 receptors) that couple to \(\text{G}_s\) and stimulate cAMP production.
- Histamine (C): Acts through H_2 receptors, which are \(\text{G}_s\)-coupled and utilize cAMP as their primary secondary messenger.
- Progesterone (D): Is a hydrophobic steroid hormone that diffuses across the plasma membrane and binds to intracellular nuclear receptors to regulate gene transcription, rather than utilizing cell-surface receptors linked to cAMP.
- Thus, (A), (B), and (C) utilize the cAMP pathway.

Step 3: Final Answer:
The molecules that use cAMP as a second messenger are (A), (B) and (C) only.