Estrogen concentrations in semen of the stallion

Large amounts of estrogens are secreted by the testes of the mature stallion. In a recent study by Claus et al. [Claus, R., Dimmick, T., Gimenez, T., Hudson, L.W., 1992. Estrogens and prostaglandin F2α in the semen and blood plasma of stallions. Theriogenology 38, 687–693.], it was stated that high levels of estrogens were also present in semen. As a preliminary step to study possible implications for fertility in the stallion, we have measured estrone sulphate (E1S), the principal estrogen in blood, in both seminal plasma and spermatozoa. Semen was collected from four Standardbred stallions at each of two stud farms in Southern Ontario during the breeding season (March–May) in 1994 and 1995, respectively, and from five stallions at the second farm in 1996. Blood samples from the jugular vein were also taken at the time of semen collection in 1996. Gel-free semen samples (n = 98, 1994–1995, n = 12, 1996) and blood were stored at −20°C until analysis. Sperm was removed from thawed samples (1 ml) by centrifugation, washed 5 × with saline solution and extracted with 80% methanol. Seminal plasma (200 μl) was diluted with absolute methanol (800 μl), vortexed and centrifuged before aliquots were taken for radioimmunoassay (RIA). Blood plasma was diluted with RIA buffer (1:10), and all aliquots were assayed using an antiserum for direct measurement of E1S. Concentrations of E1S (ng ml−1) ranged from 0.73−8.15 (n = 110) in seminal plasma and from 64.7−153.5 (n = 12) in blood plasma. E1S concentration in the sperm pellet from 1 ml of semen had a mean value of 1.3 ng and a range of 0.54−2.48 ng (n = 53 from four animals). The amounts of E1S in total gel-free ejaculates (n = 22) from four stallions ranged from 26–121 ng. It was concluded that the high concentrations of E1S in peripheral blood of the stallion are reflected in lesser, but high levels of the steroid in the ejaculate. E1S concentrations were quite variable in seminal plasma among stallions but less so for collections from an individual animal. The presence of E1S in washed spermatozoa requires further study.