Smooth muscle contraction is primarily initiated via which complex?

Smooth muscle contraction is primarily initiated through the Ca2+-calmodulin complex. In smooth muscle cells, the contraction process begins with the influx of calcium ions (Ca2+) from the extracellular space and the release of Ca2+ from the sarcoplasmic reticulum. Once the calcium concentration increases, Ca2+ binds to calmodulin, which is a calcium-binding protein.

The binding of calcium to calmodulin forms the Ca2+-calmodulin complex, which then activates myosin light chain kinase (MLCK). This enzyme phosphorylates the myosin light chains, allowing myosin heads to interact with actin filaments, leading to contraction. This mechanism is key in smooth muscle physiology as it enables contraction in response to various stimuli such as hormonal signals, nerve stimulation, and stretch.

In contrast, other pathways such as those mediated by cAMP, IP3, or DAG play different roles in cellular signaling. For instance, cAMP primarily functions in mediating relaxation rather than contraction in smooth muscle. IP3 is involved in the release of Ca2+ from the sarcoplasmic reticulum but does not directly initiate the contraction process by itself, and DAG serves to activate protein kinase C, which has various roles in cellular