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Onapristone (a progesterone antagonist) or ICI 182780 (an oestrogen antagonist) administered to guinea-pigs on days 11-14 of the cycle significantly reduced uterine PGF2 alpha output on day 15. Concentrations of progesterone in plasma of onapristone-treated and ICI 182780-treated guinea-pigs were still high on day 15 indicating that luteal regression had been prevented. These findings indicate that progesterone and oestradiol are necessary for increased PGF2 alpha production by the uterus towards the end of the cycle, and support the hypothesis that oestradiol acting on a progesterone-primed uterus is the physiological stimulus for increased uterine PGF2 alpha synthesis and release in guinea-pigs. The capacity of the endometrium to synthesize PGF2 alpha on day 15 was reduced by treatment with ICI 182780 and, unexpectedly, by treatment with onapristone, indicating that onapristone may also be antagonizing the release or action of oestradiol in some way. Tamoxifen was an agonist in guinea-pigs since it induced vaginal opening. It had no inhibitory effect on uterine PGF2 alpha output and did not delay luteal regression when administered between days 11 and 14 of the cycle. However, it redirected PG synthesis in homogenates of endometrium and myometrium from PGI2 (as indicated by 6-keto-PGF1 alpha) to PGF2 alpha. The output of 6-keto-PGF1 alpha from the uterus of day 15 guinea-pigs was reduced following tamoxifen treatment, but the high output of PGF2 alpha from the uterus was not affected.

Effects of onapristone, tamoxifen and ICI 182780 on uterine prostaglandin production and luteal function in nonpregnant guinea-pigs