Myosin phosphorylation, agonist concentration and contraction of tracheal smooth muscle.

Myosin phosphorylation plays an important part in excitation--contraction coupling in smooth muscle. Phosphorylation by a Ca2+, calmodulin-dependent kinase stimulates the actin-activated Mg2+-ATPase activity of smooth muscle myosin, suggesting that myosin phosphorylation regulates smooth muscle contraction. This hypothesis is supported by evidence that myosin is phosphorylated during contraction and dephosphorylated during relaxation of intact smooth muscles stimulated with a single agonist concentration. However, there is little information regarding the response to stimulation with various agonist concentrations. As the dose-response relationships for phosphorylation and tension should be similar if myosin phosphorylation does, in fact, regulate smooth muscle contraction, we studied myosin phosphorylation in tracheal smooth muscle stimulated with a broad range of concentrations of the cholinergic agonist, methacholine. The results of these experiments are consistent with the hypothesis that myosin phosphorylation regulates smooth muscle contraction but they indicate a relatively complex relationship between myosin phosphorylation and the generation of isometric tension.