EFFECT OF PROGESTERONE AND ESTRADIOL-17-BETA ON OXYTOCIN-INDUCED PGF(2-ALPHA) RELEASE AND ENDOMETRIAL OXYTOCIN RECEPTOR CONCENTRATIONS IN OVARIECTOMIZED GOATS

The effects of progesterone and estradiol-17 beta on oxytocin-induced prostaglandin (PG)F-2 alpha release and on endometrial oxytocin recept or concentrations were examined in ovariectomized goats. Each goat rec eived a progesterone + estradiol-17 beta pretreatment schedule for 6 d ays aimed at mimicking steroid hormone changes prior to estrus, and in ducing both a preovulatory-like LH surge and estrous behaviour. Goats were then assigned to treatment groups, i.e., 1) control, 2) progester one, 3) estradiol-17 beta, and 4) progesterone + estradiol-17 beta. In each group, (n = 4) on Days 3, 6, 9, 12, and 15 of the 15 day treatme nt period, each goat received 10 i.u. oxytocin i.v. Jugular lar blood samples were taken to monitor any changes in concentrations of 13, 14 dihydro-15-keto prostaglandin F-2 alpha (PGFM). No changes in PGFM wer e observed in any group in response to oxytocin on Days 3, 6, and 9. O n Day 12, however, there was a significant response in the progesteron e + estradiol-17 beta-treated goats (Group 4) and on Day 15, larger re sponses were observed in both Group 2 (progesterone-alone) and Group 4 . Both peak response (p < 0.05) and the area under the concentration v s. time curve (p < 0.001) for PGFM were greater in the latter group. I n a separate experiment, but using the same goats and steroid treatmen ts, endometrial oxytocin receptor concentrations were assessed on Day 15. Receptors were observed in all four groups. While progesterone (Gr oup 2) raised (p < 0.05) concentrations relative to those found in con trols and estradiol-treated goats, combined treatment with progesteron e and estradiol (Group 4) produced a further increase (p < 0.05). The results suggest that in the goat progesterone is capable of increasing oxytocin-induced PGF(2 alpha) release by inducing endometrial oxytoci n receptor formation. Estracliol-17 beta, while itself ineffective, ap pears to have a synergistic role in potentiating the effect of progest erone, producing an earlier and higher PGFM response to oxytocin.