Ovine Endometrium Synthesizes and Releases Platelet-Activating Factor, Which Can Cause the Release of Prostaglandin F2α by the Uterus in Situ1

Abstract

Explanted endometrial tissue from ovariectomized ewes that had received hormone replacement to mimic the luteal phase released platelet-activating factor (PAF) into medium in vitro. On Day 14, 310.1 (261-2185), and on Day 15, 424.4 (14.1-590.7) pg PAF (median 25th-75th percentile; p > 0.05) was released per 100 mg endometrial tissue after a 20-min incubation. A regulatory and final enzyme in the PAF biosynthetic pathway, lysoPAF:acetyltransferase, was also present with a specific activity of 0.533 +/- 0.124 pmol acetate/50 micrograms protein/30 min in Day 14 endometrial tissue and 0.810 +/- 0.468 in Day 15 tissue (mean +/- SEM; p > 0.05). PAF:acetylhydrolase, the metabolic enzyme regulating PAF's half-life, was present in uterine luminal washings, with a specific activity of 3.78 +/- 1.37 nmol acetate released/min/mg protein on Day 14 and 3.41 +/- 0.34 on Day 15 (mean +/- SEM; p > 0.05). Intrauterine infusion of 50-400 micrograms PAF caused a dose-dependent release of prostaglandin (PG) F2 alpha (measured as venous 14-dihydro-15-keto-prostaglandin F2 alpha, PGFM) within 10 min on Days 13-15. The enantiomeric form of PAF was significantly less effective in inducing a rise in venous PGFM. WEB 2086 is reported to be a competitive receptor antagonist for PAF, but a 10-fold excess of WEB 2086 failed to inhibit PAF-induced release of PGFM. It was observed that this dose of WEB alone induced PGFM release, suggesting that in this model this agent may not work as a true competitive antagonist. Repeat challenges at intervals of 0, 90, and 120 min with either 200 micrograms PAF or micrograms oxytocin resulted in a marked tachyphylaxis of response by the third challenge. This desensitization after repeated PAF challenge suggests a specificity of its actions. Infusion of 200 micrograms PAF on Day 14 or 15 at five 60-min intervals resulted in a tachyphylaxis such that by the fourth and fifth challenges, essentially no response was observed. The tachyphylaxis that was induced by four repeat challenges with PAF had no apparent effect on the capacity of the uterus to respond to a fifth challenge with oxytocin. Thus, PAF and oxytocin both caused homologous desensitization of their own capacity to mobilize uterine PGF2 alpha, but PAF did not cause heterologous desensitization of the response to oxytocin. This failure of heterologous desensitization suggests differences in the mechanism of action of the two ligands. Kinetic binding studies showed that PAF did not compete for the oxytocin receptor in vitro. This result demonstrated that the ovine endometrium produces PAF and responds to it by the release of PGF2 alpha in situ. The dynamics of the response elicited by PAF were similar to those of oxytocin, yet the two mediators apparently act separately. PAF may be a modulator of PGF2 alpha release by the ovine uterus during the luteal phase.

Actions and interactions of progesterone and estrogen on transcriptome profiles of the bovine endometrium

“…E 2 amplified the stimulatory effects of P 4 and oxytocin on endometrial PGF 2␣ synthesis, whereas E 2 alone had no effect on PGF 2␣ secretion by bovine endometrial tissue in short-term culture (46). E 2 stimulated both basal and oxytocin-regulated PGF 2␣ secretion in ovariectomized P 4 -primed cows (6). These data have demonstrated that E 2 did not stimulate PGF 2␣ release by bovine endometrium without P 4 pretreatment.…”

Components of a platelet-activating factor-signaling loop are assembled in the ovine endometrium late in the estrous cycle

“…Ewes were allowed to recover for Ն6 wk before treatment began. Hormone replacement of ovariectomized ewes with progesterone and estradiol-17␤ (Sigma Chemical, St. Louis, MO) was as previously described (2,4,11). This hormone regimen creates blood hormone levels similar to those achieved in the natural estrous cycle (11).…”

“…This 15-keto derivative is stable (26). Thus PGF2␣ release is conventionally assessed by measurement of PGFM in peripheral blood (2,4). The PGFM assay used was as previously described (4), and the assay sensitivity was 0.05 ng/ml.…”

Expression of platelet activating factors and its receptor in cat uterus during early pregnancy

“…However, it is also produced by a number of nonimmune cells such as endothelial (9), vascular, muscle (43), glomerular (13) and endometrial cells (4,5,7,45). PAF plays a role in many reproductive events (16), including ovulation (1), fertilization (27), implantation and early development of embryo (28).…”