Urinary and plasma estrogen conjugates, estradiol and estrone concentrations in nonpregnant and early pregnant mares

A direct radioimmunoassay for estrogen conjugates (EC) was applied to paired blood and urine samples collected from 20 mares and compared against estrone (E₁) and estradiol-17β (E₂) to monitor changes in estrogen production during ovulatory cycles and early pregnancy. Blood samples were taken daily from five mares through two consecutive ovulations and from six mares at 6-h intervals starting 48 hours prior to ovulation and continuing after ovulation had occurred. Blood samples were also collected daily or three times per week from conception until Day 60 of pregnancy in nine pregnant mares. The mean urinary EC, plasma EC and plasma E₂ dynamics were parallel in nonpregnant mares, with a 3-fold increase in mean urinary EC concentrations from baseline to the ovulatory peak, a 1.8-fold increase in mean plasma EC concentrations and a 1.4-fold increase in mean plasma E₂ concentrations. In early pregnancy, a two-fold increase in mean plasma E₁ and EC concentrations occurred in concert with a five-fold rise in mean urinary EC concentrations, whereas plasma E₂ did not change. Following hydrolysis and chromatographic separation, E₁ and E₂ were identified as the hydrolytic products in the urine of nonpregnant and pregnant mares; however, an unidentified estrogen was the major hydrolytic product in nonpregnant mares and pregnant mares prior to Day 38 of pregnancy. The increased resolution of the EC profiles compared with the profiles of other estrogen components indicates that the determination of EC in urine or plasma provides a useful alternative method for monitoring reproductive events in mares.