Increase in cytosolic calcium from intracellular storage, during smooth muscle contraction is/are due to: (PGI Dec 2008)
**Core Concept**
Smooth muscle contraction involves a complex interplay of signaling pathways and calcium regulation. The correct answer highlights the role of intracellular stores in increasing cytosolic calcium levels, a crucial step in the contraction mechanism.
**Why the Correct Answer is Right**
During smooth muscle contraction, calcium ions (Ca2+) play a central role. The correct answer, **1,4,5-IP3 (DAG)**, refers to the inositol trisphosphate (IP3) and diacylglycerol (DAG) pathway. When a smooth muscle cell is stimulated, phospholipase C (PLC) is activated, breaking down phosphatidylinositol 4,5-bisphosphate (PIP2) into IP3 and DAG. IP3 binds to IP3 receptors on the endoplasmic reticulum, causing a release of Ca2+ into the cytosol. This increase in cytosolic calcium triggers the contraction of smooth muscle cells. The DAG molecule, on the other hand, activates protein kinase C (PKC), which contributes to the contraction mechanism.
**Why Each Wrong Option is Incorrect**
**Option A:** Cyclic adenosine monophosphate (cAMP) is a key second messenger involved in smooth muscle relaxation, not contraction. It activates protein kinase A (PKA), which phosphorylates and relaxes smooth muscle.
**Option B:** Cyclic guanosine monophosphate (cGMP) is also involved in smooth muscle relaxation, particularly in the context of nitric oxide signaling. It activates protein kinase G (PKG), leading to smooth muscle relaxation.
**Option C:** There is no such molecule as CCMP (cyclic cytidine monophosphate). This option is likely a distractor with no basis in physiology.
**Option D (correct answer not repeated):**
**Clinical Pearl / High-Yield Fact**
In smooth muscle physiology, the IP3/DAG pathway is a critical regulator of calcium release from intracellular stores. This pathway is also involved in other cellular processes, including cell growth and differentiation. Understanding the role of IP3 and DAG is essential for grasping the complex mechanisms underlying smooth muscle contraction and relaxation.
**✓ Correct Answer: D. 1,4,5-IP3 (DAG)**