Myosin light chain phosphorylation during phasic contractions of tracheal smooth muscle

Rapid, coordinated contractions of tracheal smooth muscle were elicited by either direct electrical depolarization of muscle cells or treatment with tetraethylammonium which produced spontaneous phasic contractile activity. Both types of contraction were blocked by the calcium channel antagonist verapamil, indicating that these contractions are supported primarily by calcium of extracellular origin. With direct electrical stimulation, force was biphasic and phosphate content of the phosphorylatable light chain (P-light chain) of myosin increased rapidly (∼2.5 s) from 0.1 to 0.4 mol phosphate/mol P-light chain, then decreased to levels above resting values. Phosphorylation increased more rapidly than force. Under conditions of spontaneous activity, phasic contractions occurred above a level of basal tone significantly greater than resting force, and minimum values of phosphorylation measured at the base of contraction were significantly greater than those observed in the resting muscle. Phosphorylation oscillated with force (from 0.2 to 0.4 mol phosphate/mol P-light chain) and peak values occurred during the rising phase of contraction. Time courses of phosphorylation and force showed evidence of a prolonged state of activation of myosin following dephosphorylation. These results suggest that phosphorylation and dephosphorylation of myosin P-light chain are sufficiently rapid to participate in regulation of contractility during phasic mechanical activity.