Identify the chemical which is antagonistic to adrenaline in relation to heart beat

Options

A. Serotonin
B. Melatonin
C. Inhibin
D. Acetylcholine

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a chemical that has an antagonistic effect to adrenaline specifically concerning heart rate. Adrenaline (also known as epinephrine) typically increases heart rate and blood pressure.

2. **Analyzing the Options**: We need to evaluate each option given in the question to determine which one decreases heart rate, thus acting as an antagonist to adrenaline.

3. **Evaluating Option 1 - Serotonin**: Serotonin is known to increase heart rate. Therefore, it does not have an antagonistic effect to adrenaline. This option is incorrect.

4. **Evaluating Option 2 - Melatonin**: Melatonin is involved in regulating sleep cycles and can also increase blood pressure and heart rate. Thus, it does not oppose the action of adrenaline. This option is also incorrect.

5. **Evaluating Option 3 - Inhibin**: Inhibin is primarily involved in the regulation of reproductive hormones and does not have a direct role in heart rate regulation. Therefore, it is not relevant to the question.

6. **Evaluating Option 4 - Acetylcholine**: Acetylcholine is a neurotransmitter of the parasympathetic nervous system. It is known to decrease heart rate by promoting relaxation of the heart muscles and dilating blood vessels. Thus, it acts as an antagonist to the effects of adrenaline.

7. **Conclusion**: Based on the evaluations, the correct answer is **Acetylcholine**, as it counteracts the effects of adrenaline on heart rate.

**Final Answer**: Acetylcholine

---