Questions: Which best describes the role of calcium in smooth muscle? Promotes extensibility Triggers the release of ATP Interacts with contractile proteins Activates enzymes

The answer is the third one: Interacts with contractile proteins.

Explanation for each option:

1. Promotes extensibility: This is incorrect. Calcium does not primarily promote extensibility in smooth muscle. Extensibility refers to the ability of the muscle to be stretched, which is more related to the structural properties of the muscle and connective tissue rather than the role of calcium.

2. Triggers the release of ATP: This is incorrect. Calcium does not directly trigger the release of ATP. ATP release is more closely associated with cellular respiration and energy metabolism processes.

3. Interacts with contractile proteins: This is correct. In smooth muscle, calcium ions play a crucial role in the contraction process. When calcium levels increase in the cytoplasm, calcium binds to calmodulin, which then activates myosin light-chain kinase (MLCK). This enzyme phosphorylates the myosin light chains, allowing myosin to interact with actin filaments, leading to muscle contraction.

4. Activates enzymes: While this is partially correct, as calcium does activate certain enzymes (such as MLCK in smooth muscle), it is not the best description of its primary role. The interaction with contractile proteins is a more specific and accurate description of calcium's role in smooth muscle function.

Summary: Calcium in smooth muscle primarily interacts with contractile proteins to facilitate muscle contraction.