Question

Name the hormones having cell surface receptors and the second messenger is calcium or phosphotidyl inositol.
(A). Thyrotrophic releasing Hormone (TRH)
(B). Gonadotropin Releasing Hormone (GRH)
(C). Calcitriol
(D). Calcitonin
Choose the correct answer from the options given below:

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

Hint:

Hypothalamic releasing hormones like TRH and GnRH classically utilize the $\text{G}_q$ pathway $\rightarrow$ PLC $\rightarrow$ $\text{IP}_3$/DAG $\rightarrow$ Calcium signaling cascade to trigger rapid exocytosis of pituitary hormones.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Hormones that cannot cross the hydrophobic plasma membrane bind to extracellular cell-surface receptors.
This binding activates intracellular G-proteins, which trigger enzymatic pathways to generate second messengers (such as cyclic AMP, calcium ions, or inositol trisphosphate) to propagate the signal inside the cell.

Step 2: Detailed Explanation:
Let us analyze each of the listed hormones:
- (A) Thyrotropin-Releasing Hormone (TRH): This peptide hormone binds to its cell-surface G-protein coupled receptor ($\text{G}_q$).
This activates phospholipase C (PLC), which cleaves phosphatidylinositol 4,5-bisphosphate ($\text{PIP}_2$) into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate ($\text{IP}_3$).
$\text{IP}_3$ triggers the release of calcium ions ($\text{Ca}^{2+}$) from the endoplasmic reticulum, which acts as the second messenger. (Correct)
- (B) Gonadotropin-Releasing Hormone (GnRH/GRH): GnRH similarly binds to a cell-surface $\text{G}_q$-coupled receptor and utilizes the phosphatidylinositol/calcium second messenger system to stimulate LH and FSH release from pituitary gonadotropes. (Correct)
- (C) Calcitriol (1,25-dihydroxyvitamin D3): This is a lipophilic steroid-like hormone. It diffuses into the cell and binds to intracellular (nuclear) receptors (Vitamin D Receptor, VDR) to directly regulate gene transcription, and does not use the calcium second messenger pathway. (Incorrect)
- (D) Calcitonin: While it binds to cell-surface GPCRs, its primary second messenger is cyclic AMP (cAMP) via the $\text{G}_s$ pathway, rather than primarily utilizing calcium/phosphatidylinositol.
Therefore, only TRH and GnRH fit all the criteria.

Step 3: Final Answer:
The correct combination is (A) and (B) only, corresponding to option (A).