Effect of RU 486 on luteal function in the early pregnant rat

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Abstract
A dose of 300 mg RU 486/kg, an antiprogesterone, was administered to pregnant rats on Day 2 (Group 1) or Day 4 (Group 2) of pregnancy. RU 486 significantly changed serum progesterone and oestradiol concentrations and luteal 3.beta.-HSD and 20.alpha.-HSD activities in Group 1, and implantation was significantly inhibited. The luteal 3.beta.-HSD activity in Group 2 rats on Day 6 was significantly (P < 0.01) lower than the control value (7.5 .+-. 0.6 and 10.1 .+-. 0.6 mU/mg protein respectively). This decline in the 3.beta.-HSD activity was followed by a marked decrease in the serum progesterone concentration, resulting in a significant decrease of the progesterone/oestradiol ratio and implantation was completely inhibited. The 20.alpha.-HSD activity which could not be detected on Day 6 in the control rats, was twice as great in Group 2 than in Group 1 rats (17.5 .+-. 1.2 and 7.4 .+-. 3.1 mU/mg protein respectively). Ultrastructural examination of corpora lutea of Group 2 rats confirmed luteolysis. These results suggest that RU 486 has a luteolytic effect and its anti-implantation effect is concomitant with luteolysis of the corpora lutea of pregnancy.