Steroid synthesis by equine conceptuses between days 7 and 14 and endometrial steroid metabolism

The objective of this study was to determine if changes in steroid synthesis occurred in the horse blastocyst about the time of maternal recognition of pregnancy. Embryos collected between days 7.5 and 14.5 were incubated for 8 hr in vitro in HAM's F10 containing radiolabelled pregnenolone. The steroid metabolites in the incubation medium were separated by reverse phase HPLC and the major peaks expressed as a percentage of total metabolites. It was found that there were no major changes in the profile of metabolites throughout the period of study, although there was increased conversion as the conceptuses developed. It was found that the major metabolite produced was 17 alpha-hydroxyprogesterone and not estradiol as expected. A second experiment was conducted to determine if 17 alpha-hydroxyprogesterone was metabolized by endometrial tissue. Endometrial biopsies from anestrous mares and from pregnant and nonpregnant mares at day 11 were incubated with radiolabelled 17 alpha-hydroxyprogesterone, progesterone or pregnenolone. The 17 alpha-hydroxyprogesterone, but not progesterone nor pregnenolone, was converted to a more polar metabolite in all groups. Production of this metabolite was significant greater in the anestrous mares. This metabolite has not been unidentified conclusively. Thus, results of this study show that 17 alpha-hydroxyprogesterone is the major steroid synthesized by the equine blastocyst and that this steroid is further metabolized to an unidentified steroid by the endometrium. These steroids could play a role in conceptus development or maternal recognition of pregnancy.