Progesterone concentration in the peripheral plasma of the mare during the oestrous cycle and early pregnancy.

The research focused on studying the progesterone concentrations in a mare’s blood during its oestrous cycle and early pregnancy. By using a protein-binding test for daily peripheral plasma samples, the researchers found significant fluctuations in progesterone levels, particularly around ovulation and at different stages of early pregnancy.

Research Methodology

• The researchers monitored and collected peripheral plasma samples twice a day for four oestrous cycles from three mares.
• They also obtained sample once daily for the first seven weeks of pregnancy from four different mares.
• The progesterone concentrations were measured using a competitive protein-binding technique.

Results and Observations

• There were considerable variations in progesterone levels between morning and evening samples taken from the same mare on the same day.
• The progesterone concentration was the lowest around ovulation time.
• Within 24 hours of ovulation, there was a rise in progesterone levels, and two peaks were observed – one at five days and another at eight days post ovulation.
• Between 10 and 14 days after ovulation, a consistent decrease in the progesterone level was noticed, with a significant drop between days 14 and 16.
• An additional progesterone peak was found just before ovulation, which could likely be 17n-hydroxyprogesterone.

Progesterone Concentration during Early Pregnancy

• In the initial 10 days post ovulation, the progesterone concentration in pregnant and non-pregnant mares was more or less similar.
• Between days 10 and 14, a reduction in progesterone concentration was noted in pregnant mares too. However, there was an increase between day 17 and day 30.
• After an initial increase, there was a decrease in the progesterone concentration, first gradually and then rapidly, during the days leading up to ovulation, which typically occurred at around 42 days.

Cyclical Changes and Ovarian Activity

• The research found that the cyclical changes in progesterone concentration closely resembled the cyclical changes in the macro and microscopic morphology of the corpus luteum and the follicular activity of the ovaries.