Summary. A dose of 30 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 3β-HSD and 20α-HSD activities in Group 1, and implantation was significantly inhibited. The luteal 3β-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 3β-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 20α-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.
Keywords: RU 486; corpora lutea; luteolysis; 3β-HSD; 20α-HSD; rat