Final answer:
In nonstimulated cells, calmodulin does not bind to calcium because the free concentration of Ca²+ is kept very low inside the cell through active transport mechanisms. It is only when signaling events trigger the influx of Ca²+ that calmodulin can bind calcium and initiate cellular responses such as muscle contraction.
Step-by-step explanation:
Why Calmodulin Does Not Bind to Calcium in Nonstimulated Cells
Calmodulin is a calcium-binding protein that plays a crucial role in various cellular processes, including the contraction of smooth muscle cells. In a nonstimulated cell, calmodulin does not bind to calcium because the free concentration of calcium ions (Ca²+) within the cell is maintained at a very low level. Calcium ions are continuously pumped out of the cell using ATP or are stored in cytoplasmic vesicles such as the endoplasmic reticulum. It is only when a signaling event occurs, such as the opening of ligand-gated calcium channels, that the Cafc influx raises the concentration of cytoplasmic Ca²+, enabling calmodulin to bind calcium and activate downstream signaling pathways, such as muscle contraction through the activation of myosin light chain kinase.
In smooth muscle cells, calmodulin, upon binding with Ca²+, forms a complex that activates myosin light chain kinase, resulting in the phosphorylation and activation of myosin heads that interact with actin to facilitate contraction. This process is distinct from skeletal muscle contraction, where the troponin-tropomyosin complex regulates cross-bridge formation.