Cellular Mechanisms by Which Oxytocin Mediates Ovine Endometrial Prostaglandin F2α Synthesis: Role of Gi Proteins and Mitogen-Activated Protein Kinases1

Oxytocin stimulates a rapid increase in ovine endometrial prostaglandin (PG) F2α synthesis. The overall objective of these experiments was to investigate the cellular mechanisms by which oxytocin induces endometrial PGF2α synthesis. The objective of experiment 1 was to determine whether Gi proteins mediate oxytocin-induced PGF2α synthesis. Uteri were collected from four ovary-intact ewes on Day 14 postestrus. Caruncular endometrial explants were dissected and subjected to in vitro incubation. Pertussis toxin, an inhibitor of Gi proteins, had no effect on the ability of oxytocin to induce PGF2α synthesis (P > 0.10). The objective of experiment 2 was to determine whether any of the three mitogen-activated protein kinases (MAPKs), extracellular signal regulated protein kinase (ERK1/2), c-Jun N-terminal/stress-activated protein kinase (JNK/SAPK), or p38 MAPK, mediate oxytocin-induced PGF2α synthesis. Eleven ovary-intact ewes were given an injection of oxytocin (10 IU; i.v.; n = 5) or physiological saline (i.v.; n = 6) on Day 15 postestrus. Uteri were collected 15 min after injection and caruncular endometrium was dissected. Endometrial homogenates were prepared and subjected to Western blotting. Membranes were probed for both total and phosphorylated forms of all three classes of MAPK. All classes of MAPK were detected in ovine endometrium, but oxytocin treatment had no effect on the expression of these proteins (P > 0.10). ERK1/2 was the only phosphorylated MAPK detected and its concentrations were higher in oxytocin-treated ewes (P < 0.01). The objective of experiment 3 was to further investigate the role of ERK1/2 during oxytocin-induced PGF2α synthesis. Uteri were collected from four ovary-intact ewes on Day 14 postestrus. Caruncular endometrial explants were dissected and subjected to in vitro incubation. PD98059, a specific inhibitor of ERK1/2 activity, blocked the ability of oxytocin to stimulate PGF2α synthesis in a dose-dependent manner (P < 0.05). These results indicate that the ovine oxytocin receptor is not coupled to Gi proteins. These results indicate that oxytocin induces phosphorylation of ERK1/2 and that this MAPK appears to mediate oxytocin-induced PGF2α synthesis in ovine endometrium.