Relationship between myosin phosphorylation and contractile capability of canine airway smooth muscle

To better understand excitation-contraction coupling in smooth muscle, myosin phosphorylation and force-velocity properties of canine tracheal muscle were compared during the rise and early plateau of force in electrically stimulated tetani. Velocity reached a peak of approximately 1.5 times plateau value when force had risen to approximately 45% of its maximum value and then declined progressively. Except early in the tetanus, when phosphorylation rose rapidly, maximum power and phosphorylation had nearly parallel time courses, reaching peaks of 1.2-1.3 times reference at 6-8 s before declining to the plateau level at approximately 12 s. Force, velocity, maximum power, and phosphorylation fell somewhat during the plateau, with the closest correlation between phosphorylation and power. These results suggest that 1) early velocity slowing is not associated with light chain dephosphorylation and 2) maximum power, which we use to signal changes in activation, is closely correlated with the degree of light chain phosphorylation, at least when phosphorylation level is not changing rapidly. Dissociation of these two properties would be expected early in the tetanus if phosphorylation precedes mechanical activity.