ERK-mediated uterine artery contraction: Role of thick and thin filament regulatory pathways

We have demonstrated that extracellular signal-regulated kinase (ERK) plays an important role in the regulation of uterine artery contraction. The present study tested the hypothesis that ERK regulates thick and thin filament regulatory pathways in the uterine artery. Isometric tension, intracellular free Ca²⁺ concentration ([Ca²⁺]_i), and 20-kDa myosin light chain (LC₂₀) phosphorylation were measured simultaneously in uterine arteries isolated from near-term (140 days gestation) pregnant sheep. Phenylephrine produced time-dependent increases in [Ca ²⁺]_i and LC₂₀ phosphorylation that preceded the contraction, which were inhibited by the MEK (ERK) inhibitor PD-098059. In addition, PD-098059 decreased the intercept of the regression line of LC ₂₀ phosphorylation vs. [Ca²⁺]_i but increased the rate of tension development vs. LC₂₀ phosphorylation. In contrast to phenylephrine, phorbol 12,13-bibutyrate (PDBu) produced contractions without changing [Ca²⁺]_i or LC₂₀ phosphorylation. PD-098059 potentiated PDBu-induced contractions without affecting [Ca²⁺]_i and LC₂₀ phosphorylation. PDBu produced time-dependent increases in phosphorylation of p42 and p44 ERK and ERK-dependent phosphorylation of caldesmon at Ser⁷⁸⁹ in the uterine artery. PD-098059 blocked PDBu-mediated phosphorylation of p42 and p44 ERK and caldesmon. The results indicate that ERK may regulate force by a dual regulation of thick and thin filaments in uterine artery smooth muscle. ERK potentiates the thick filament regulatory pathway by enhancing LC₂₀ phosphorylation via increases in [Ca²⁺]_i and Ca ²⁺ sensitivity of LC₂₀ phosphorylation. In contrast, ERK attenuates the thin filament regulatory pathway and suppresses contractions independent of changes in LC₂₀ phosphorylation in the uterine artery.