Regulation of glycerinated smooth muscle contraction and relaxation by myosin phosphorylation

Regulation of isometric force maintenance, isotonic shortening velocity, and muscle stiffness by myosin phosphorylation was examined during both contraction and relaxation of chemically permeabilized (glycerinated) rat uterine smooth muscle. Phosphorylation of the 20,000-Da light chain of myosin (LC20) was manipulated by varying the calcium activity of the bathing solution or by thiophosphorylation of LC20 in the presence of ATP gamma S. With saturating calcium and calmodulin, LC20 phosphorylation was 0.43 mol PO4/mol LC20. This increased to 0.92-0.96 mol PO4/mol LC20 on addition of ATP gamma S. Over the entire range of phosphorylation, there was a significant (P less than 0.001) linear correlation between force and phosphorylation. Stiffness increased monotonically with increasing force; however, the relationship was nonlinear, with stiffness increasing faster at lower levels of activation. Force, stiffness, shortening velocity, and LC20 phosphorylation were compared at identical calcium activities during steady-state conditions of partial contraction and partial relaxation. The ratio of the value of each parameter measured during relaxation to that measured during contraction was 1.11 for force, 1.09 for stiffness, 1.01 for shortening velocity, and 0.83 for LC20 phosphorylation. These results support the hypothesis that contraction and relaxation in glycerinated rat uterine muscle are regulated primarily by phosphorylation and dephosphorylation of LC20.