EFFECT OF ESTRADIOL ON OXYTOCIN-STIMULATED PROSTAGLANDIN-F2-ALPHA RELEASE IN MARES

The objective of this study was to clarify the role of oestradiol in l uteal function by examining its effect on the oxytocin stimulation of 15-keto-13,14-dihydro-prostaglandin F2alpha (PGFM) concentrations in c yclic mares. In the first experiment, three groups of mares (4 per gro up) were given a bolus injection of 17alpha-oestradiol (1 mg), oestrad iol (1 mg) or vehicle on days 7, 9, 11, 13 and 15 of the cycle. Six ho urs later the mares were challenged with 10 iu oxytocin intravenously and frequent blood samples were taken from 15 min before to 15 min aft er for measurement of PGFM. Results showed a significant stimulatory e ffect of oestradiol (five times greater than controls at day 11; P < 0 .05), but not of 17alpha-oestradiol, on the oxytocin stimulation of PG FM. As a relatively large dose was given systemically in this experime nt, a second experiment was performed to introduce a dose that was mor e physiological into the uterus. Small Silastic spheres (1 cm diameter ) were impregnated with or without oestradiol at a concentration that gave a release rate similar to that of embryos at day 12 (10 ng h-1). These were inserted (one per mare) into the uterus of two groups of ma res (five per group) on day 7. The mares were challenged with oxytocin on days 9, 11, 13 and 15 of the cycle and blood samples were taken as before for determination of PGFM. The results showed that oestradiol enhanced (four times greater than controls at day 13; P < 0.05) the ox ytocin stimulation of PGFM concentrations. These data show that oestra diol enhances the oxytocin stimulation of prostaglandin release at the time of luteolysis. Thus, oestradiol produced by the embryo does not appear to prevent luteolysis or to be the signal involved in maternal recognition of pregnancy.