The secondary messenger which show antagonistic effect to c-AMP in heart muscle is

Options

A. `Ca^(2+)`
B. Inositol triphosphate `(IP_(3))`.
C. DAG
D. c-GMP

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the secondary messenger that has an antagonistic effect to cyclic AMP (cAMP) in heart muscle.

2. **Identifying cAMP**: cAMP stands for cyclic adenosine monophosphate. It is a secondary messenger that plays a crucial role in various biological processes, including the regulation of heart muscle contraction.

3. **Role of cAMP**: In the heart, cAMP is known to stimulate muscle contraction. It does this by activating protein kinase A (PKA), which then phosphorylates various proteins leading to increased calcium ion levels and enhanced contraction.

4. **Antagonistic Effect**: An antagonistic effect means that the secondary messenger works in opposition to cAMP. Therefore, we need to identify a secondary messenger that inhibits muscle contraction in the heart.

5. **Identifying Other Secondary Messengers**: The options provided are:
- Calcium ions
- Inositol triphosphate (IP3)
- Diacylglycerol (DAG)
- Cyclic guanosine monophosphate (cGMP)

6. **Analyzing cGMP**: cGMP, or cyclic guanosine monophosphate, is another secondary messenger. It is known to have a relaxing effect on smooth and cardiac muscle. Specifically, cGMP can decrease heart muscle contraction in response to certain signals, such as acetylcholine.

7. **Conclusion**: Since cGMP acts to decrease heart muscle contraction, it shows an antagonistic effect to cAMP, which promotes contraction. Therefore, the correct answer is cGMP.

**Final Answer**: The secondary messenger that shows an antagonistic effect to cAMP in heart muscle is **cGMP**.

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