Ca2+ is a primary second messenger that binds to an intracellular receptor protein, calmodulin. Increases in cytosolic Ca2+ concentration mediated by activation of cell surface receptors result in the formation of a Ca2+ calmodulin complex that regulates many Ca2+-dependent cellular processes. In smooth muscle, Ca2+/calmodulin activates myosin light chain kinase, which phosphorylates the regulatory light chain of myosin. This phosphorylation reaction increases the actin-activated MgATPase activity of myosin and is associated with increases in contractile properties, including force, stiffness, and maximal shortening velocity. These biochemical and biomechanical responses occur rapidly (seconds) in response to physiological stimulation involving neurotransmitter activation of smooth muscle cells. Thus, the Ca2+-dependent phosphorylation of the myosin light chain is a primary event in activation of smooth muscle contraction.
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