Hypotensive Agents

Smooth muscle contraction differs somewhat from skeletal muscle. Ca is involved in the initiation of contraction as it is in skeletal muscle. However, the myosin in smooth muscle must be phosphorylated for activation of the myosin ATPase. Phosphorylation and dephosphorylation of myosin also occurs in skeletal muscle, but phosphorylation is not necessary for activation of the ATPase. In smooth muscle, Ca binds to calmodulin, and the resulting complex activates myosin light chain kinase, the enzyme that catalyses the phosphorylation of myosin. Actin then slides on myosin, producing contraction. This is in contrast to skeletal and cardiac muscle, where contraction is triggered by the binding of Ca to troponin C.