Nonmuscle Myosin is Regulated During Smooth Muscle Contraction

The participation of nonmuscle myosin in force maintenance is controversial. To participate in force maintenance, nonmuscle myosin activity must be regulated in smooth muscle. To investigate this question, phosphorylation of the regulatory light chains of smooth muscle myosin (SMLC) and nonmuscle myosin (NMLC) were examined by two dimensional SDS‐PAGE. Intact mouse aortic smooth muscle strips were stimulated with either KCl depolarization or angiotensin II, and 4 distinct LC spots were resolved by two dimensional SDS‐PAGE. Using mass spectroscopy, these spots were identified as four distinct proteins; phosphorylated NMLC, non‐phosphorylated NMLC, phosphorylated SMLC and non‐phosphorylated SMLC. During KCl depolarization, inhibition of Rho kinase or PKC did not affect the increases in SMLC and NMLC phosphorylation. During angiotensin II stimulation, inhibition of PKC or Rho kinase reduced SMLC phosphorylation, while only inhibition of Rho kinase reduced NMLC phosphorylation. These data suggest that in smooth muscle, nonmuscle myosin is regulated by Ca 2+ ‐calmodulin activated MLCK during depolarization, and a Rho kinase mediated pathway during agonist stimulation.