Short-term mode of secretion of equine chorionic gonadotrop in and the effect of GNRH

Five mature Quarterhorse mares were bled every 30 min for 25 h on day 50 of pregnancy to determine the short-term mode of secretion of equine chorionic gonadotropin (eCG). Three other mares with persistent endometrial cups after abortion were administered gonadotropin releasing hormone (GnRH; 1.0 mug/kg of body weight) and were bled immediately prior to and at 15, 30, 45, 60, 90, 120, 180 and 240 min after GnRH. Concentrations of eCG in plasma of pregnant mares were constant over the 24-h period; the variation of each mare's individual values was no greater (P>.05) than the predicted random variation of the radioimmunoassay. Administration of GnRH had no significant effect on eCG concentrations of mares with persistent endometrial cups over a 4-h period. These data are consistent with a model of eCG secretion in the pregnant mare in which there is little short-term regulation of secretion other than the factors which affect the number of healthy endometrial cup cells within the uterus.