Mitogen-activated protein kinase cascade is involved in endothelin-1-induced rat puerperal uterine contraction

The regulation of mitogen-activated protein (MAP) kinase by endothelin-1 (E T-1) in cultured rat puerperal uterine myometrial cells was investigated. E T-1 caused the rapid stimulation of MAP kinase activity. ET-1-induced MAP k inase activation is neither extracellular Ca2+- nor intracellular Ca2+-depe ndent. ET-1 stimulation also led to an increase in phosphorylation of son-o f-sevenless (SOS), and transfection of dominant negative SOS attenuated the ET-1-induced MAP kinase activity. Phorbol-12-myristate 13-acetate (PMA) al so induced the MAP kinase activity, but pretreatment of the cultured cells with PMA, to down-regulate protein kinase C (PKC), did not abolish the acti vation of MAP kinase by ET-1. In addition, down-regulation of PKC had no ef fect on ET-1-induced SOS phosphorylation. Pertussis toxin, which inactivate s Gi/Go proteins, blocked the ET-1-induced MAP kinase activation but not th e PMA-induced MAP kinase activation. The results suggested that MAP kinase is acutely activated by ET-1 through a pertussis toxin-sensitive G protein and SOS, not through the PMA-sensitive PKC. In addition, although reverse-t ranscriptase PCR assays detected messenger RNA for both ET-1 receptor subty pes in cultured rat puerperal uterine myometrial cells, ET-1-induced MAP ki nase activity and uterine contraction were blocked by treatment with BQ485, an antagonist selective for an ET type A receptor (but not by BQ788, an ET type B receptor antagonist). Ritodrine, which is known to relax uterine mu scle contraction, attenuated ET-1-induced MAP kinase activity. We further e xamined the role of MAP kinase pathway in uterine contraction using an inhi bitor of MEK activity, PD098059. This inhibitor completely inhibited the ET -1-induced MAP kinase activation and partially, but significantly, inhibite d the ET-1-induced uterine contraction. These results indicate that ET-1-in duced MAP kinase signaling cascade may play an important role in the ET-1-i nduced uterine contraction.