PARENT SESSION
FEMALE REPRODUCTIVE TRACT - B

Wednesday, August 4, 2004
10:30 AM–12:30 PM
Buchanan Courtyard

(809) RAPID PROGESTERONE ACTION INHIBITS OXYTOCIN-INDUCED SECRETION OF PROSTAGLANDIN F₂ BY OVINE ENDOMETRIAL EXPLANTS.

Bishop, Cecily¹, Stormshak, Fredrick¹, ¹ Departments of Biochemistry/Biophysics and Animal Sciences, Corvallis, OR

ABSTRACT- Progesterone (P4) regulates the expression of ovine uterine oxytocin receptor (OTR) mRNA via a genomic mechanism. However, recent research has demonstrated that P4 also acts nongenomically via a putative plasma membrane receptor to inhibit binding of oxytocin in the ovine endometrium. Oxytocin (OT) stimulates the release of prostaglandin F₂ (PGF₂) from ovine uterine endometrium during proestrus. Therefore, the objective of the present research was to determine whether a rapid action of P4 in vitro would suppress OT-induced secretion of PGF₂ in ovine endometrial explants. Five ewes were injected im with 250 g PGF₂ analogue (cloprostenol) on Day 14 to hasten onset of luteal regression and reduce systemic levels of P4. On Day 15 intercaruncular endometrial tissue was collected via midventral laparotomy. Explants (50-200 mg) were then cultured in Eagle's MEM in the absence or the presence of arachidonic acid [AA] (20 M) and cultured in 6-well plates for 2 h at 37°C in a humidified atmosphere (95% air, 5% CO₂). Explants were then subjected to a change of medium (3 ml) containing vehicle or 2.5 ng/ml P4, and cultured an additional hour at 37°C. The treatment regimen resulted in a 2×2 factorial arrangement of groups. All wells containing explants were then challenged with OT (5M) and cultured for an additional hour. A fifth group of explants was exposed only to medium for the entire experiment to serve as a negative control and establish basal PGF₂ release. After the final hour of culture, media were collected and analyzed for PGFM by RIA. Intra- and interassay coefficients of variation were 5.27 and 7.85 %, respectively. Prostaglandin F₂ secretion was expressed as pg PGFM/100l/mg tissue. Data on medium concentrations of PGFM were analyzed statistically by two-way ANOVA. Treatment of explants with AA increased PGF₂ compared to that of controls (P=0.06). Brief exposure to P4 significantly decreased OT-induced PGF₂ secretion (P<0.01) in explants previously exposed to medium or AA. Results of this experiment demonstrate a rapid, presumably nongenomic, effect of P4 in inhibiting OT-induced PGF₂ secretion by ovine endometrial explants.

KEY WORDS: endometrium, prostaglandin F₂, progesterone, oxytocin