Endometrial oxytocin receptor and uterine prostaglandin secretion in mares during the oestrous cycle and early pregnancy

Circulating concentrations of 13,14-dihydro-15-ketoprostaglandin F2 alpha (PGFM) were measured before and after administration of oxytocin and after endometrial biopsy, with or without uterine flushing performed per vaginam, on days 10, 14 and 18 after ovulation in nine pregnant and nine cyclic mares. Concentrations of oxytocin receptor were measured in endometrial biopsy samples. Neither pregnancy status nor time after ovulation affected basal PGFM concentrations. PGFM concentrations were increased after oxytocin administration on each of the days studied in cyclic mares; on day 14 the mean response was 4.5 times higher than the mean response on days 10 and 18. In contrast, during pregnancy, responses to oxytocin administration occurred only on days 10 and 18. Marked increases in PGFM concentrations in response to endometrial biopsy occurred only on day 14 in cyclic mares and on day 18 in pregnant mares. Mean concentrations of oxytocin receptor were between 200 and 300 fmol mg-1 protein on day 10 in both pregnant and cyclic mares; in cyclic mares oxytocin receptor concentrations were increased approximately threefold on day 14 compared with days 10 and 18, but no such increase was evident during pregnancy. Total amounts of PGFM secreted after oxytocin treatment correlated with endometrial oxytocin receptor concentrations in cyclic (P < 0.001) but not in pregnant (P > 0.5) mares, and the same was true for PGFM release induced by endometrial biopsy (cyclic: P = 0.0025; pregnant: P > 0.5). The data support the hypothesis that endometrial concentrations of oxytocin receptor determine uterine prostaglandin F2 alpha secretion in cyclic mares and that endometrial oxytocin receptor concentrations are reduced in early pregnancy by a product of the conceptus. The increase in response of the pregnant uterus to oxytocin treatment or biopsy-flushing between days 14 and 18 was not due to an increase in the concentration of oxytocin receptors but presumably reflected increased receptor sensitivity.