61 Low plasma progestin concentration during the early luteal phase impairs equine conceptus development until placentation

Abstract

During the early luteal phase, low progesterone concentrations delay downregulation of endometrial progesterone receptors. This contributes to impaired histotroph production at Day 14 of pregnancy (Beyer et al. 2019 Theriogenology 125, 236-241). Until the beginning of placentation (i.e. Day 37 of pregnancy), nutritional supply of the equine conceptus depends on histotrophy alone. The aim of the present study was to analyse development of the equine conceptus under reduced plasma progesterone concentrations until shortly before placentation. Fertile Haflinger mares (n=11; 4-11 years old) were examined daily by transrectal ultrasonography, and when in oestrus, they were inseminated every 48h until spontaneous ovulation. Mares were randomly assigned to either the treatment group and received the prostaglandin F2α agonist cloprostenol (PGF2α; 125μg) once daily for 3 days after ovulation (Beyer et al. 2019) or the control group and left untreated. After conceptus collection on Day 34, mares were allowed one oestrous cycle for recovery and subsequently assigned to the opposite treatment, thus serving as their own controls. From Day 10 after ovulation, conceptus development including size, uterine fixation, and heartbeat detection was evaluated daily by transrectal ultrasonography. On Day 34, conceptus and fetal membranes were recovered transcervically. The recovered material was weighed, measured, and stored for further analysis. Conceptuses underwent microcomputed tomography that was evaluated by Amira (Thermo Fisher Scientific). Statistical comparison for differences between control and treatment pregnancies was performed by nonparametric Wilcoxon test or chi-square analysis. The day of first visualisation of the conceptus and of the embryo proper and heartbeat did not differ between treatments. Uterine fixation occurred on Day 19.3±0.5 in conceptuses from PGF2α-treated pregnancies but on Day 16.7±0.4 in controls (P<0.05). At 34 days of gestation, the conceptuses from PGF2α-treated mares were smaller (P<0.05) than control conceptuses when measured with ultrasound callipers in utero (maximal length: PGF2α 17.4±0.3mm, control 19.0±0.4mm). Conceptus weight determined after recovery was less in treated pregnancies (PGF2α 2.5±0.2 g; control 3.5±0.3 g; P<0.05). Microcomputed tomography analysis of selected inner organs showed some differences in development. Seven complete pairs of undestroyed conceptuses were available. Mean numbers of 33.4±3.2 and 18.8±7.5 bronchi were detected in control and PGF2α conceptuses, respectively (P<0.05). Heart volume did not differ, but communication between ventricles was detected in only 1/7 control but 5/7 PGF2α conceptuses (P<0.05). The footpad was present in 6/7 control and 0/7 PGF2α conceptuses (P<0.001). In conclusion, subphysiological progesterone concentration during the early luteal phase delays development of the equine conceptus before placentation. The condition may contribute to early conceptus loss in horses, which occurs in up to 20% of pregnancies.

Diestrous Ovulations in Pregnant Mares as a Response to Low Early Postovulatory Progestogen Concentration

“…Timely progesterone receptor downregulation is an important step in the preparation of the endometrium for early pregnancy and placentation [9,10]. This has also been demonstrated in horses [5,7,11].…”

“…When a dominant follicle >35 mm in diameter was detected along with uterine edema, mares were inseminated with raw semen (500 × 10 6 motile spermatozoa) from a fertile stallion at 48-h intervals until spontaneous ovulation (disappearance of the preovulatory follicle) occurred. On the ovulation day (day 0), the mares were randomly assigned to treatment (PGF) with the prostaglandin F2α (PGF 2α ) analogue cloprostenol (125 µg once daily; Estrumate, MSD, Vienna, Austria) from day 0 until day 3 or left untreated as control (CON), as previously described [5,7].…”

“…An association with low progesterone concentrations has been suggested [4]. There is, however, evidence that low postovulatory progesterone concentrations in mares may be also associated with increased pregnancy loss [5][6][7]. In several species, the concentration of plasma progesterone during the first days after ovulation substantially influences endometrial function with regard to conceptus nutrient supply [8] because it induces downregulation of endometrial progesterone receptors.…”

“…Treatment of mares with PGF 2α and its analogues during the early postovulatory phase does not induce complete luteolysis, but results in markedly reduced plasma progestogen concentrations, followed by a steady progestogen increase reflecting a certain resumption of luteal function [5,7,[12][13][14][15]. We could recently demonstrate that the experimentally reduced early postovulatory progestogen concentration in pregnant mares was not only associated with delayed conceptus development, but also with an increased incidence of luteal phase ovulations between days 11 and 15 after ovulation [7]. We therefore hypothesized that the reduced plasma progestogen concentrations due to a considerable reduction of the negative feedback on the pituitary [16,17] allow for increased gonadotrophin release and subsequent ovulation.…”

“…The detection of secondary ovulations at pregnancy examination may therefore suggest low progestogen concentrations during the early postovulatory phase, and thus be indicative of an increased risk for the pregnancy. In the present study, details of gonadotrophin and progestogen release together in pregnant mares with or without experimentally reduced early postovulatory luteal function [7] were investigated.…”