Calmodulin plays an important role in excitation contraction (EC) coupling and the initiation of the cross-bridge cycling in smooth muscle, ultimately causing smooth muscle contraction. In order to activate contraction of smooth muscle, the head of the myosin light chain must be phosphorylated. This phosphorylation is done by myosin light chain (MLC) kinase. This MLC kinase is activated by a calmodulin when it is bound by calcium, thus making smooth muscle contraction dependent on the presence of calcium, through the binding of calmodulin and activation of MLC kinase.
Another way that calmodulin affects muscle contraction is by controlling the movement of Ca across both the cell and sarcoplasmic reticulum membranes. The Ca channels, such as the ryanodine receptor of the sarcoplasmic reticulum, can be inhibited by calmodulin bound to calcium, thus affecting the overall levels of calcium in the cell. Calcium pumps take calcium out of the cytoplasm or store it in the endoplasmic reticulum and this control helps regulate many downstream processes.
This is a very important function of calmodulin because it indirectly plays a role in every physiological process that is affected by smooth muscle contraction such as digestion and contraction of arteries (which helps distribute blood and regulate blood pressure).