Effects of MEK inhibitors on ET-1-induced activation of ERK1/2 in HASMCs. Serum-starved cells were treated with U0126, PD98059 or SL327 for 30 min prior to addition of ET-1. A, bar graph shows inhibitory effects of MEK inhibitors on phosphorylated ERK1/2 induced by 10 nM of ET-1. Phosphorylated ERK1/2 was determined by immunofluorescence with an anti-phospho-ERK1/2 antibody. B, inhibitory effects of MEK inhibitors on phosphorylated ERK1/2 activity induced by 10 nM of ET-1. Phosphorylated ERK1/2 activity was determined by phosphoELISA assay as described in Methods. The upper panel of A indicates representative images of immunofluorescence showing the phosphorylated ERK1/2 from the samples treated with MEK inhibitors prior to addition of ET-1. The scale bar in each image represents 20 μm. Data represent mean ± S.E.M. ** p < 0.01, *** p < 0.001 compared with the ET-1-stimulated states after DMSO treatment. ### p < 0.001; p = phosphorylation, ns = non-significant.