The role of myosin light chain phosphorylation in regulation of the cross-bridge cycle

Ca ²⁺ binding to myofibrillar regulatory sites can produce conformational changes allowing cross-bridge attachment and cycling. Measurements of smooth muscle actomyosin ATPase activity suggested that Ca ²⁺ might act indirectly to mediate cross-bridge attachment by stimulating myosin light chain phosphorylation. However, the predicted obligatory relationship between developed force and myosin phosphorylation was not always observed in living smooth muscle. The observation that myosin phosphorylation was always tightly correlated with average cross-bridge cycling rates estimated from isotonic shortening velocities suggested that Ca ²⁺ has two regulatory roles. One action is exerted via a Ca ²⁺-binding protein whose identity is unknown in smooth muscle. This regulatory site acts like other Ca ²⁺-binding regulatory proteins in muscle to permit cross-bridge interaction and to determine active stress. The second regulatory role involves stimulation of myosin light chain kinase and light chain phosphorylation. Increasing the level of phosphorylated cross-bridges increases shortening velocities or rate of force development. We suggest that the dephosphorylated cross-bridges are noncycling or slowly cycling in activated smooth muscle. Smooth muscle may be a particularly favorable experimental preparation for demonstrating a general regulatory role of myosin phosphorylation in modulating the kinetics and energetics of muscle contraction.