Effects of Exogenous Insulin on Luteolysis and Reproductive Cyclicity in the Mare

The effects of obesity and insulin dysregulation on mare reproduction, pregnancy, and foal health: a review

Obesity is a growing welfare concern in modern equine populations and predisposes horses to disturbances in energy metabolism such as insulin dysregulation. However, equine metabolic syndrome has only been recognized in recent decades. Functioning energy metabolism is pivotal to normal body homeostasis and affects essentially all organ systems, including reproduction. Previous literature suggests that obesity has an effect not only on the reproductive processes in mares but also on offspring health, predisposing the offspring to later-onset orthopedic and metabolic problems. This review focuses on the effects of obesity, insulin dysregulation and hyperinsulinemia on the reproductive functions of mares and the implications on foal health before and after birth. The points of interest are the cyclicity and ovarian function, uterine environment, gestation, the postpartum period, and the newborn foal. The aim is to review the current state of knowledge, and identify outstanding questions that could stimulate future research. This topic is important not only from the equine industry and production perspective but is also relevant for the welfare of future populations and individuals.

Effects of Common Equine Endocrine Diseases on Reproduction

Endocrine diseases, such as equine metabolic syndrome and pituitary pars intermedia dysfunction, are common in domesticated horse populations, and the frequency with which these diseases are encountered and managed by equine veterinary practitioners is expected to increase as the population ages. As clinicians learn more about the effects of these diseases on equine reproductive physiology and efficiency (including effects on reproductive seasonality, ovulation efficiency, implantation, early pregnancy loss, duration of pregnancy, and lactation), strategies and guidelines for improving fertility in affected animals continue to evolve. It is hoped that further research will establish these recommendations more firmly.

Early pregnancy in the horse revisited - does exception prove the rule?

Early equine pregnancy shares many features with that of more intensively assessed domestic animals species, but there are also characteristic differences. Some of those are poorly understood. Descent of the equine conceptus into the uterine lumen occurs at day 5 to 6 after ovulation but is only possible when the embryo secretes prostaglandin E2. Although maintenance of equine pregnancy probably involves secretion of a conceptus derived anti-luteolytic factor, this agent has not been identified. Rapid growth, conceptus mobility and presence of an acellular capsule at the time of maternal recognition of pregnancy, i.e. between days 12 and 14, are prerequisites to avoid pregnancy loss. Progesterone together with 5α-pregnanes is secreted by the corpus luteum and induces the production of endometrial histotroph which is responsible for conceptus nutrition until placention. A stable contact between the outer trophoblast layer of the allantochorion and the luminal epithelium of the endometrium is not established before days 40 to 42 of pregnancy.

Progesterone and conceptus elongation in cattle: a direct effect on the embryo or an indirect effect via the endometrium?

The steroid hormone progesterone (P4) plays a key role in the reproductive events associated with pregnancy establishment and maintenance. High concentrations of circulating P4 in the immediate post-conception period have been associated with an advancement of conceptus elongation, an associated increase in interferon-τ production and higher pregnancy rates in cattle. Using in vitro and in vivo models and ∼8500 bovine oocytes across six experiments, the aim of this study was to establish the route through which P4 affects bovine embryo development in vitro and in vivo. mRNA for P4 receptors was present at all stages of embryo development raising the possibility of a direct effect of P4 on the embryo. Exposure to P4 in vitro in the absence or presence of oviduct epithelial cells did not affect the proportion of embryos developing to the blastocyst stage, blastocyst cell number or the relative abundance of selected transcripts in the blastocyst. Furthermore, exposure to P4 in vitro did not affect post-hatching elongation of the embryo following transfer to synchronized recipients and recovery on Day 14. By contrast, transfer of in vitro derived blastocysts to a uterine environment previously primed by elevated P4 resulted in a fourfold increase in conceptus length on Day 14. These data provide clear evidence to support the hypothesis that P4-induced changes in the uterine environment are responsible for the advancement in conceptus elongation reported previously in cattle and that, interestingly, the embryo does not need to be present during the period of high P4 in order to exhibit advanced elongation.

Prostaglandin H synthase Type 2 is differentially expressed in endometrium based on pregnancy status in pony mares and responds to oxytocin and conceptus secretions in explant culture

The equine embryo must signal its presence to the uterus for pregnancy to continue to term. Mobility of the conceptus throughout the uterus is crucial for its survival, and this action presumably permits the conceptus to transmit its antiluteolytic signal to the endometrium. Studies were completed to establish whether this unidentified antiluteolytic signal targets prostaglandin G/H synthase 2 (PGHS2), a rate limiting enzyme in converting arachidonic acid to prostaglandins (PGs). In the first study, quantitative RT-PCR was used to determine the relative abundance of PGHS2 mRNA in endometrium derived from estrous cyclic and pregnant mares on day 14 post-ovulation. PGHS2 mRNA abundance was substantially greater in endometrium from estrous cyclic mares. Additional studies were completed to better understand PGHS2 in equine endometrium. An estrogen and progesterone treatment regimen in ovariectomized mares was developed as a test model for detecting endometrial PGHS2 mRNA. Also, exposing endometrial explants to conceptus secretions (conditioned culture medium) decreased PGHS2 mRNA abundance whereas exposing explants to oxytocin increased PGHS2 mRNA abundance. Exposure to conceptus secretions also decreased PGF2alpha concentrations in explant-conditioned medium whereas oxytocin supplementation increased PGF2alpha concentrations in medium. These data support the hypothesis that PGHS2 is a target for the antiluteolytic signal produced by equine conceptuses during early pregnancy. Also, the endometrial explant culture system used for these studies can serve as a model for identifying and characterizing the maternal recognition of pregnancy factor in equids.