Smooth muscle: regulation by calcium and phosphorylation.

Skinned smooth muscle cell bundles were used to study the mechanisms by which Ca2+ and protein phosphorylation regulate smooth muscle contraction. The sarcolemma of skinned cells is rendered permeable to ions and proteins by chemical treatment, allowing the investigator to control the ionic and protein environment within the cells and to determine the effect of this environment on the activation of contraction and protein phosphorylation. Results from these studies show that there is a strong correlation between myosin light chain phosphorylation and force development in skinned smooth muscle cells. Both protein phosphorylation and the activation of tension are affected in the predicted manner by agents known to affect myosin light chain kinase activity. Phosphorylation or thiophosphorylation of myosin light chains in the absence of Ca2+ is sufficient to fully activate contraction in skinned smooth muscle cells. Subsequent dephosphorylation of the myosin light chains by phosphatase results in muscle relaxation. Activation of contraction in skinned smooth muscle cells requires phosphorylation of myosin light chains by the Ca2+- and calmodulin-dependent myosin light chain kinase.