Myosin phosphorylation, force, and maximal shortening velocity in neurally stimulated tracheal smooth muscle

Field stimulation of intrinsic nerves in bovine tracheal smooth muscle strips elicited atropine-sensitive contractions that were more rapid than those obtained by addition of carbamylcholine to the bathing medium. These stimulus conditions were used to improve estimates of maximal rates of activation as indicated by myosin light chain phosphorylation, maximal shortening velocity (Vo), and isometric force. Maximal values of Vo [0.25 X optimal muscle length X s-1] and light chain phosphorylation (0.65 mol phosphate/mol light chain) were attained after 5 s of stimulation and preceded maximal force (60 s). Force gradually fell to 0.85 times maximal values during 30 min of stimulation, while both light chain phosphorylation and Vo declined to 0.3 times the maximal value. The temporal correlation between light chain phosphorylation and Vo supports the hypothesis that myosin phosphorylation in smooth muscle functions in regulating cross-bridge cycling rates. Myosin was dephosphorylated during relaxation with a half time of 2.7 s. Calculated maximal cellular rates of light chain phosphorylation were similar to measured values, indicating that most of the kinase was activated on stimulation.