CONCEPTUS INTERFERON IN UTERINE FLUSH, ENDOMETRIAL CONCENTRATIONS OF OXYTOCIN RECEPTORS AND PROSTAGLANDIN-F2-ALPHA RELEASE IN-VITRO AFTER TRANSFER OF CONCEPTUSES TO EWES INDUCED TO OVULATE AT 28 DAYS POSTPARTUM

We examined the key events underlying maternal recognition of pregnanc y and the prevention of luteolysis in early postpartum ewes by synchro nously transferring single expanded blastocysts recovered from control ewes on day 11 of pregnancy into the uterus of either postpartum reci pients that had been induced to ovulate 28 days after lambing (n = 12) or control recipients (n = 11). Conceptus development, uterine flush interferon (oTP-1) concentrations, endometrial oxytocin receptor conce ntrations and endometrial prostaglandin F-2 alpha (PGF(2 alpha)) relea se in vitro were determined 5 days later (corresponding to day 16 of t he ovarian cycle). By this stage, both conceptus mass and oTP-1 conten t of total uterine flush in the eight postpartum recipients that remai ned pregnant were significantly lower (P < 0.01) than in the eight pre gnant control ewes (524 +/- 116.6 versus 959 +/- 80.6 mg and 968 +/- 1 6.9 versus 1512 +/- 106.2 ng oTP-1 for postpartum and control recipien ts, respectively). These effects were independent of ovulation rate an d daily peripheral progesterone concentrations after blastocyst transf er, which were similar between groups. Endometrial oxytocin receptor d ensity was variable in both groups when they were killed, and was gene rally higher in pregnant postpartum than in control recipients, and wa s significantly different (P < 0.05) when the values for the transfer but not the contralateral uterine hems were compared. Similarly, basal and oxytocin-stimulated endometrial PGF(2 alpha) release during a 4 h incubation were higher (P < 0.01) in pregnant postpartum versus contr ol recipients. Irrespective of treatment group and when expressed per uterine hem, conceptus mass was highly negatively correlated with numb er of oxytocin receptors and PGF(2 alpha) release in vitro. The result s of this study imply that suboptimal conceptus growth rates and secre tion of oTP-1, resulting in an inability to regulate endometrial oxyto cin receptor-mediated PGF(2 alpha) secretion, may be central to pregna ncy failure in the early postpartum ewe.