RU486 antagonizes the inhibitory action of progesterone on prostacyclin and thromboxane A₂ synthesis in cultured rat myometrial explants

A myometrial explant culture system was developed to investigate the effect of progesterone and the antiprogestagen RU486 on prostacyclin (PGI₂) and thromboxane A₂ (TXA₂) synthesis by the rat myometrium. After culture, eicosanoid synthesis was stimulated with arachidonic acid (AA) and the calcium ionophore A₂3187 (A₂3187). Spontaneous release of eicosanoids was also studied. Progesterone inhibited the spontaneous release of PGI₂ and TXA₂ release by myometrial explants in a concentration- and time-dependent manner. Adequate inhibition of myometrial eicosanoid synthesis by physiological concentrations was achieved at 18 h of culture: All subsequent studies were carried out after an 18-h culture of explants. A23187- and AA-stimulated PGI₂ and TXA₂ synthesis were inhibited equipotendy by progesterone. 17β-Estradiol alone was without effect on spontaneous AA- or A23187-stimulated PGI₂ or TXA₂ synthesis and was without effect on progesteroneelicited inhibition of eicosanoid synthesis in the myometrial explants. The protein synthesis inhibitors, actinomycin D and cycloheximide, did not block the inhibitory action of progesterone on A23187- or AA-stimulated eicosanoid synthesis by the myometrial explants and alone mimicked the inhibitory action of progesterone. The inhibitory action of progesterone on AA- and A23187-stimulated PGI₂ and TXA₂ synthesis was antagonized in a concentration-dependent manner by RU486. These data indicate that within this ex vivo system, progesterone probably inhibits myometrial cycloxygenase, that progesterone may exert this action through inhibition of a modulating or permissive protein, and that the antiprogestagen RU486 is an effective in vitro antagonist or progesterone.