This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Increase in cytosolic calcium from intracellular storage, during smooth muscle contraction, is/are due to - 
Select and explain. 
A. cAMP, B. cGMP, C. cCMP, D. IP3-DAG
D: IP3-DAG
Answer: D: IP3-DAG 
Explanation: The release of Ca++ from intracellular source, that is sarcoplasmic reticulum, is by IP3-DAG second messenger system, whereas the pumping of Ca++ from extracellular space to cytosol is by Ca++ channels. 
IP3 when bound to its receptors on sarcoplasmic reticulum, causes the release of calcium into sarcoplasm.