Stimulus-specific patterns of myosin light chain phosphorylation in smooth muscle of rabbit thoracic artery

When the rabbit thoracic artery was stimulated with submaximal concentrations of agonist [40 mM K+, 30 μM prostaglandin F2α (PGF2α) or 7 μM histamine], about 90% of a maximal contraction occurred. Each agonist induced a rapid development of contraction followed by a sustained response. The maximal rate of force generation stimulated with PGF2α was twice that seen with K+ or histamine. Stimulation with 40 mM K+ increased the extent of monophosphorylated 20 kDa myosin light chain (MLC-P) for up to 1 min to a maximal value of 38.8±1.0%, there was a subsequent rapid decrease and the MLC-P level remained just above the basal value for 40 min (6.8±3.0%). In the case of stimulation with 7 μM histamine, MLC-P level increased rapidly and was sustained for up to 40 min (28.0±4.9%). In contrast to the stimulation with K+ or histamine, PGF2α induced both mono- and diphosphorylated MLC20 (MLC-P and MLC-P 2 respectively) at a low concentration (3 μM). The monophosphorylation of MLC20 induced by 30 μM PGF2α reached the maximal value of 32.8±5.2%, and was sustained for up to 40 min (15.2±5.4%). The diphosphorylation of MLC20 increased rapidly (7.4±4.0% at 5 min), then decreased to the basal value within 40 min. These results suggest that different modes of stimulation of smooth muscle contraction produce different profiles of MLC20 phosphorylation. The implications of these observations are that the diphosphorylated form, specifically induced by certain agents, may modify the mode of contraction of the aortic artery.