ENDOTHELIN-1 AND ENDOTHELIN RECEPTORS ARE PRESENT IN THE SHEEP UTERUSAND CONCEPTUS AT IMPLANTATION

Previous studies have demonstrated that endothelin is present in the o vine endometrium and increases at around the expected time of implanta tion. To characterize further uterine endothelin at the time of establ ishment of pregnancy in sheep, endothelin was measured by radioimmunoa ssay in uterine flushings obtained during the oestrous cycle and in pr egnant ewes up to the time of implantation (day 16). During the oestro us cycle, the highest amounts of endothelin were present in uterine fl ushings on day 14 (1.1 +/- 0.2 ng endothelin/uterus). During early pre gnancy, basal levels of endothelin (0.5-0.6 ng endothelin/uterus) were present in uterine flushings for the first 10 days and then increased on day 14 to levels similar to those found at the equivalent stage of the oestrous cycle. On days 15 and 16 of pregnancy, endothelin conten t in the uterine lumen increased to significantly (P < 0.05) higher co ncentrations (2.9 +/- 0.4 ng endothelin/uterus) when compared with the non-fertile cycle. The principal isoform present in flushings at the time of implantation was endothelin-1, as determined by reverse-phase HPLC. Endothelin was released principally by purified endometrial epit helial cells in culture, with barely detectable amounts released by en dometrial stromal cells or conceptus tissue, which is consistent with the epithelium being the principal source of endothelin in the uterine lumen. Endothelin binding sites were present in endometrium and myome trium, as demonstrated by specific binding of I-125-labelled endotheli n-1, which was saturable and displaced by endothelin-1. Both endotheli n(A) and (B) sub-types of receptors were present as demonstrated by th e biphasic displacement of I-125-labelled endothelin-1 binding by the specific endothelin(B) agonist BQ3020. These were localised principall y on luminal and glandular epithelium and in the vasculature of the en dometrium and myometrium as shown by autoradiography. Endothelin recep tors were also present on the conceptus obtained at the time of implan tation. In the day 20 conceptus, endothelin immunostaining was localis ed principally in the heart, in trophoblast in uninucleate but not in binucleate cells, and in fetal membranes. This immunostaining of the c onceptus may represent binding to receptor sites. It is concluded that endothelin-1 is present in the uterine lumen and may play an importan t role in the paracrine regulation of the conceptus and endometrium at the time of rapid embryo development, implantation and early placenta tion.