Progesterone, prostaglandin F-2 alpha, PMSG and oestrone sulphate during early pregnancy in the mare.

The research focuses on examining the presence and variation of several hormones, including progesterone, prostaglandin F-2 alpha (PGFM), PMSG, and oestrone sulphate, in mares during different stages of their reproductive cycle — from the luteal phase to early pregnancy stages and up to about 150 days of gestation.

Hormonal Studies in Mares

• The study uses blood samples taken from 4 mares during various stages of the reproductive cycle: the late luteal phase, oestrus, early pregnancy stages, and up to about 150 days into gestation. These samples are analyzed for the presence and concentration of several hormones using radioimmunoassays. The hormones studied include progesterone, prostaglandin F-2 alpha (PGFM), PMSG, and oestrone sulphate.
• During the late luteal phase, the researchers observed peaks of PGFM, a type of prostaglandin that is primarily responsible for inducing labor. These peaks, interestingly, were recorded at the time of corpus luteum regression — which is when the corpus luteum (a hormone-secreting structure that develops in an ovary after an ovum has been discharged) begins to degenerate, leading to decreasing progesterone levels.

Changes in Hormone Levels During Early Pregnancy

• In contrast to the late luteal phase, these PGFM peaks were not detected during early pregnancy, which in this study means from mating and up to about Day 30. Instead, progesterone levels increased after mating and remained high throughout the observation period.
• Another hormone, PMSG, showed an increase around Day 40 of the early pregnancy phase, reaching maximum values between Days 50 and 80. The researchers point out that this is when the blood concentration was 1000 times higher than values during estrus — a time in the reproductive cycle when the female is sexually receptive or “in heat”. This suggests that PMSG levels might have a correlation with gonadotrophic activity because of their high cross-reaction with horse LH and horse FSH — two hormones necessary for the growth and development of follicles (immature eggs) in the ovaries.
• Oestrone sulphate, another hormone monitored in this study, showed about a 3-fold increase during estrus and a 10-fold increase at the peak of PMSG. Further into pregnancy (around Days 70-75), the levels of oestrone sulphate increased yet again, and high concentrations of more than 250 nmol/l were measured around Day 100 of pregnancy.