The vascular smooth muscle cell is a highly specialized cell whose primary function is contraction and relaxation. It expresses a variety of contractile proteins, ion channels, and signalling molecules that regulate contraction. Upon contraction, vascular smooth muscle cells shorten, thereby decreasing the diameter of a blood vessel to regulate the blood flow and Pressure. Contractile activity in vascular smooth muscle cells is Initiated by a Ca2+-calmodulin interaction to stimulate phosphorylation of the light chain of myosin. Ca2+-sensitization of the contractile proteins is signaled by the RhoA/Rho-kinase pathway to inhibit the dephosphorylation of the light chain by myosin phosphatase, thereby maintaining force. Removal of Ca2+ from the cytosol and stimulation of myoson phosphatase Initiate the relaxation of vascular smooth muscle.