In the mechanism of action of a protein hormone ,one of the second messengers is

Options

A. cyclic AMP
B. Insulin
C. `T_(3)`
D. Gastrin

Answer

To solve the question regarding the second messenger in the mechanism of action of a protein hormone, follow these steps:

### Step 1: Understand the Role of Second Messengers
Second messengers are molecules that relay signals received at receptors on the cell surface to target molecules inside the cell. They play a crucial role in the signaling pathways initiated by hormones.

**Hint:** Look for molecules that act as intermediaries in hormone signaling.

### Step 2: Identify the Options Provided
The options given are:
1. Cyclic AMP (cAMP)
2. Insulin
3. T3 (Triiodothyronine)
4. Gastrin

**Hint:** Consider which of these options is known to function as a second messenger.

### Step 3: Analyze Each Option
- **Cyclic AMP (cAMP)**: This is a well-known second messenger that is produced from ATP and plays a significant role in the signaling pathways of many hormones, particularly protein hormones.
- **Insulin**: This is a protein hormone itself, not a second messenger.
- **T3 (Triiodothyronine)**: This is a thyroid hormone, not a second messenger.
- **Gastrin**: This is a hormone that stimulates gastric acid secretion, but it is not classified as a second messenger.

**Hint:** Recall the definition of second messengers and their examples in cellular signaling.

### Step 4: Conclude the Correct Answer
Based on the analysis, the only option that fits the definition of a second messenger in the mechanism of action of protein hormones is **Cyclic AMP (cAMP)**.

**Final Answer:** The correct option is **1. Cyclic AMP (cAMP)**.