Effects of exogenous insulin on luteolysis and reproductive cyclicity in the mare.

This paper investigates the role of insulin in horse reproduction, particularly if it signals receipt of a conceptus (embryonic structure). The findings indicate that injecting insulin does not notably impact the reproductive cycle and it is unlikely to function as a signal for maternal recognition of pregnancy.

Objective and Research Methodology

• The research aimed to explore if insulin, a hormone traditionally known to regulate carbohydrate and fat metabolism, also plays a role in equine reproduction, specifically in maternal recognition of pregnancy (MRP).
• A previous study had indicated insulin production in horse conceptuses between the tenth and eighteenth day, which motivated this investigation.
• The research involved six Warmblood mares undergoing two successive oestrous cycles. In one cycle they were given insulin, and in the second, they were provided a control saline solution.
• Ovulation time and corpus luteum (an essential part of reproduction and pregnancy) surface area were monitored through serial transrectal ultrasonographic examination. Simultaneously, daily blood samples were tested for progesterone and luteinizing hormone (LH) concentrations.
• We also measured systemic 15-ketodihydroprostaglandin F(2 alpha) (PG-metabolite) release in response to oxytocin stimulation. Oxytocin influences the reproductive cycle and, in this case, was used to examine the luteolytic drive (regression of the corpus luteum).
• Furthermore, insulin concentrations were analyzed within the yolk sac fluid recovered from 32 day 10 to 14 equine conceptuses.

Findings and Conclusion

• The study found that the administration of insulin had no significant effect on luteal size, duration of dioestrus (the stage in the sexual cycle following ovulation), the interval between ovulations, or circulating LH concentrations.
• Insulin injection did not suppress the release of PGF(2 alpha) when stimulated by oxytocin and seemed to decrease systemic progesterone concentrations rather than increase them. This result contradicts the role of progesterone in maintaining pregnancy.
• Importantly, insulin was not detected in the fluid of the yolk sac of day 10 to 14 equine conceptuses, contrary to the preliminary study that suggested its presence.
• Basing on these results, the study concluded that insulin- when administrated daily during specific days of dioestrus- does not have a significant effect on the reproductive cycle of mares. Furthermore, insulin is unlikely to be the MRP signal, contradicting the assumption made at the onset of the study.