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M Shimada, N Kawano and T Terada

Steroid hormones, such as progesterone, oestrogen, androgen and meiosis activating sterols, are secreted from cumulus cells that are stimulated by gonadotrophins during maturation of oocytes in vitro. These steroid hormones may be absorbed by mineral oil or paraffin oil; however, in vitro maturation of pig oocytes is commonly performed using medium covered by oil. In this study, high concentrations of progesterone, oestradiol and testosterone were detected in the culture medium after pig cumulus-oocyte complexes (COCs) were cultured with FSH and LH for 44 h in medium without an oil overlay. However, high concentrations of these steroid hormones were not detected in medium when COCs were cultured with the mineral oil overlay. When high concentrations of these steroid hormones were secreted by COCs, germinal vesicle breakdown (GVBD) and the activation of p34(cdc2) kinase and mitogen-activated protein (MAP) kinase in oocytes occurred earlier in comparison with oocytes cultured in medium covered with mineral oil. Moreover, a decrease in p34(cdc2) kinase activity during meiotic progression beyond metaphase I was observed in oocytes cultured in conditions under which high concentrations of steroid hormones were secreted by COCs. In addition, the rate of development to the blastocyst stage after IVF was higher in oocytes matured in medium without an oil overlay. These adverse effects of oil may be explained by absorption by the oil of cumulus-secreted steroids or by the release of toxic compounds into the medium.

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T. Kawano, H. Okamura, C. Tajima, K. Fukuma and H. Katabuchi

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