The up-regulation of oxytocin (OT) receptors in rabbit amnion at the end of gestation is associated with a large increase in the ability of OT to stimulate PGE2 synthesis. The purpose of these investigations was to determine what other factors contribute to this increase. OT enhanced PGE2 synthesis at several levels. The concentrations of cytosolic phospholipase A2, which generates arachidonic acid for PGE2 synthesis, and PGH endoperoxide synthases (types 1 and 2), which catalyze the conversion of arachidonic acid to prostanoids, rose substantially in rabbit amnion at term. OT stimulated translocation of cytosolic phospholipase A2 to the cell particulate fraction, presumably by a Ca2+-mediated process, and phosphorylation of cytosolic phospholipase A2 via the extracellular regulated protein kinase 2/1-mediated pathway. OT-stimulated increases in intracellular Ca2+ concentrations and extracellular regulated protein kinase 2/1 phosphorylation were both mediated by G(q/11) activation. OT also increased the expression of PGH endoperoxide synthase-2 after treatment of amnion cells in culture for 2 h; however, PGE2 release in response to OT was virtually immediate. These findings show that the rapid stimulation of PGE2 synthesis by OT occurs through cytosolic phospholipase A2 activation and PGH endoperoxide synthase-1 activity, both of which, along with OT receptor concentrations, are considerably up-regulated in the amnion at the end of gestation.
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