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

Carolina T C Okada,Martim Kaps,Camille Gautier,Christine Aurich,Javier Perez Quesada,Jörg Aurich

doi:10.3390/ani10122249

Carolina T C Okada, Martim Kaps + Show 4 more

Open Access

https://doi.org/10.3390/ani10122249

Copy DOI

Abstract

Simple SummaryDuring early pregnancy in mares, progestogen is synthesized by the primary corpus luteum, which is the only source of progestogen until endometrial cup and accessory corpus luteum formation, from day 36 of pregnancy onwards. In the present study, we investigated the hormonal profile (gonadotrophin and progestogen concentrations) of 11 mares after experimental reduction of primary corpus luteum function. Two pregnancies of each mare were assigned to the control and treatment groups, respectively, and were analyzed until day 34. Low plasma progestogen concentration caused by the treatment reduced the negative feedback on the hypothalamic-pituitary axis, stimulating gonadotropin release, and luteal tissue response. Progestogen concentration restoration soon after treatment suggests a rebound effect and the resurgence of luteal function. In addition, diestrous ovulation was observed between days 11 and 15 in five treatment pregnancies (5/11), but none of the controls (0/11). Although the total luteal area increased after diestrous ovulations, corpus luteum size was not correlated to progestogen secretion. Results suggest that diestrous ovulations during early pregnancy in mares may reflect low progestogen concentrations in the early postovulatory period. Spontaneous prolongation of the luteal phase has been described in horses, but the underlying causes are still unclear. The present study investigated details of gonadotrophin and progestogen secretion in pregnant mares (n = 11) with or without experimentally reduced early postovulatory luteal function. From days 0 to 3 after ovulation, they were treated with the prostaglandin F2α (PGF2α) analogue cloprostenol or left untreated. After conceptus collection on day 34, they were assigned to the opposite treatment. Mares were affiliated to the group primary corpus luteum (n = 6) or diestrous corpus luteum (n = 5) depending on diestrous corpus luteum (CL) detection in the PGF pregnancy. For statistical comparisons, a p-value < 0.05 was significant. There was an effect of treatment (p < 0.01), but not of group on progestogen concentration. The concentration of LH was higher in PGF-treated than in untreated pregnancies (p < 0.05), but did not differ between groups. The FSH concentration did not differ between groups nor treatments. The total luteal tissue area was greater in mares with a diestrous ovulation during the PGF treatment pregnancy. Low progestogen concentration in the early postovulatory phase diminish the negative feedback on the hypothalamic-pituitary axis in early pregnancy and, thus, stimulate a luteal tissue response. Detection of secondary CL at the time of pregnancy examination in mares may reflect that early post-ovulatory progestogen concentrations were low.

Highlights

In most animal species, prolongation of the luteal phase in the absence of pregnancy occurs only if experimentally induced or associated with uterine pathologies
There was an effect of treatment, but not of group on plasma progestogen concentration until day 34 of pregnancy (Figure 1a)
There was an interaction of treatment with day (p < 0.001) as well as group with day (p < 0.001). In both groups during treatment pregnancies, progestogen concentration rose steadily after day 3, but this increase was more pronounced in the group of mares with luteal phase ovulations

Summary

Introduction

Prolongation of the luteal phase in the absence of pregnancy occurs only if experimentally induced or associated with uterine pathologies (for review see [1]). Spontaneous prolongation of the luteal phase for periods of two to three months is not uncommon [1,2,3,4]. An association of this phenomenon with diestrous ovulations in at least part of the affected horses was suggested [1] and later on confirmed [4]. In approximately 10% of mares, luteal phase ovulations, i.e., ovulations occurring more than three days after a follicular phase ovulation, were detected in pregnant as well as non-pregnant mares [4]. If diestrous ovulations occur later than day 9, they will, result in prolonged luteal activity [2,3,4], because of non-responsiveness of the still immature luteal tissue at the time of endometrial prostaglandin release, the primary corpus luteum may undergo luteolysis [3,4]

Methods

Results

Discussion

Conclusion