Actin cytoskeleton reorganization is regulated by various actin-binding proteins. Cofilin is the principal filament-depolymerizing protein, whose activity is reduced upon phosphorylation by LIMK. Thus, LIMK and cofilin comprise a signal transduction module regulating actin turnover and myogenic tone in healthy vasculature. Novel functions of smooth muscle cells (SMCs) in the hypertensive pulmonary artery, such as increased motility and proliferation, are supported by the actin cytoskeleton. We therefore hypothesized that bioactive peptides that affect these SMC functions may also result in an upregulation of LIMK and cofilin expression. Semiquantitative RT-PCR and immunoblotting indicated that LIMK2 and cofilin mRNA and protein expression is upregulated in canine pulmonary artery SMCs (PASMCs) exposed to PDGF or IL-1β (10 ng/ml). Inhibition of ERK MAPKs (U-0126, 10 µ M) or p38 MAPK (PD-169316, 10 µ M), but not PI3Ks (LY-294002, 50 µ M), reduced LIMK2 and cofilin gene expression stimulated by PDGF or IL-1β. Inhibition of ROCK (Y-27632, 10 µ M) reduced only the IL-1β-stimulated LIMK2 and cofilin expression. These novel observations in PASMCs indicate that LIMK2 and cofilin expression can be induced by PDGF or IL-1β. This parallel upregulation of LIMK2 and cofilin may have potentially broad functional significance for the progress of pulmonary artery disease.