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Cellular Mechanisms by Which Oxytocin Mediates Ovine Endometrial Prostaglandin F₂ Synthesis: Role of G_i Proteins and Mitogen-Activated Protein Kinases¹

^a Department of Animal Sciences ^b Animal Reproduction Laboratory, Department of Physiology, Colorado State University, Fort Collins, Colorado 80523 ^c Department of Animal Sciences, University of Kentucky, Lexington, Kentucky 40546

ABSTRACT

Oxytocin stimulates a rapid increase in ovine endometrial prostaglandin (PG) F₂ synthesis. The overall objective of these experiments was to investigate the cellular mechanisms by which oxytocin induces endometrial PGF₂ synthesis. The objective of experiment 1 was to determine whether G_i proteins mediate oxytocin-induced PGF₂ 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 G_i proteins, had no effect on the ability of oxytocin to induce PGF₂ 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 PGF₂ 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 PGF₂ 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 PGF₂ synthesis in a dose-dependent manner (P < 0.05). These results indicate that the ovine oxytocin receptor is not coupled to G_i proteins. These results indicate that oxytocin induces phosphorylation of ERK1/2 and that this MAPK appears to mediate oxytocin-induced PGF₂ synthesis in ovine endometrium.

FOOTNOTES

First decision: 18 September 2000.

¹ Supported in part by grants from the U.S. Department of Agriculture (96-35203-3495 to P.D.B.), Colorado Agriculture Experiment Station, and Kentucky Agriculture Experiment Station. Presented in part at the 1999 and 2000 American Society of Animal Science Western Section meetings.

² Correspondence. FAX 970 491 5326; pburns{at}ceres.agsci.colostate.edu

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