To this end, 300 mg strips of endometrium were excised from cycling mares on various days postovulation and incubated (2 h at 37°C) with one of the following in vitro treatments: 1) no incubation (content); 2) no hormone (production); 3) 1.0 µg progesterone; and 4) 1.0 µg estradiol -17. Endometria were similarly collected from early pregnant mares and were analyzed for PGF content and production. To study steroid modulation of endometrial PGF production, 16 ovariectomized pony mares were assigned randomly to one of four exogenous hormone treatments for a 3 week period: 1) sesame oil; 2) estradiol valerate, 50 µg daily; 3) progesterone, 50 mg daily; 4) estradiol valerate for 7 days followed by progesterone for 14 days. On Day 21 of treatment, endometrial strips were incubated with one of the following in vitro treatments: 1) no incubation (PGF content); 2) no hormone (PGF production); 3) 1.0 µg progesterone; 4) 1000 ng estradiol-17; 5) 10.0 ng estradiol -17; 6) 1.0 ng estradiol -17. The concentration of PGF in homogenates was quantified with a validated radioimmunoassay.

In both the cycling and pregnant mare, the amount of PGF contained within the endometrium was lower but temporally mimicked production capacity trends. In the cycling mare, PGF production increased from Day 4 to Day 16 and then declined on Day 20; thus maximal PGF production corresponded to the expected time of luteolysis. Incubation of endometria from these mares with progesterone did not affect PGF production, whereas estrogen was stimulatory during late diestrus but had little or no effect during early diestrus. Estrogen appeared to be most stimulatory upon the progesterone-primed uterus. In the early pregnant mare, PGF production increased during the first 16 days of pregnancy in a pattern similar to that of the nonpregnant mare; however, concentrations of PGF continued to increase after Day 16. Prostaglandin F production was maximal, therefore, at a time of expected luteal maintenance. We suggest that the elevated levels of PGF in early pregnancy may be more directly involved with fetal-placental physiology than with luteal physiology.

In the ovariectomized mare, all in vivo hormone treatments stimulated PGF production above that of the sesame oil controls. Exposure of the tissue to estrogen in vitro resulted in a dose-dependent stimulation of PGF production, with the greatest response to estrogen in tissues from mares treated with progesterone in vivo. In contrast, exposure of the tissue to progesterone in vitro failed to alter PGF production. Thus maximal stimulation of endometrial PGF occurred after long-term (3 weeks) systemic progesterone exposure followed by short-term incubation (2 h) with estrogen, whereas systemic estrogen treatment followed by incubation with progesterone was without effect.