Increase in cytosolic calcium from intracellular storage, during smooth muscle contraction, is/are due to –
**Core Concept**
The contraction of smooth muscle is a complex process that involves the coordinated action of various ion channels, pumps, and receptors. One crucial step in this process is the increase in cytosolic calcium levels, which triggers the contraction of smooth muscle cells.
**Why the Correct Answer is Right**
The increase in cytosolic calcium from intracellular storage during smooth muscle contraction is primarily due to the release of calcium from the sarcoplasmic reticulum (SR), a type of smooth endoplasmic reticulum found in smooth muscle cells. This release is mediated by the ryanodine receptors (RyR), which are activated by the influx of calcium ions through voltage-gated calcium channels. The resulting increase in cytosolic calcium binds to calmodulin, leading to the activation of myosin light-chain kinase (MLCK) and the phosphorylation of myosin light chains, ultimately resulting in smooth muscle contraction.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the increase in cytosolic calcium during smooth muscle contraction.
* **Option B:** This option may be related to other cellular processes, but it is not directly involved in the increase in cytosolic calcium during smooth muscle contraction.
* **Option C:** This option is incorrect because the increase in cytosolic calcium is not due to the influx of calcium ions from the extracellular space.
**Clinical Pearl / High-Yield Fact**
The contraction of smooth muscle is a highly regulated process that involves the coordinated action of various ion channels, pumps, and receptors. Understanding the role of calcium in smooth muscle contraction is crucial for the management of various cardiovascular and gastrointestinal disorders.
**Correct Answer: C. Influx of calcium from extracellular space.