Progesterone receptor is not required for progesterone action in the rat corpus luteum of pregnancy

In this study, we investigated whether progesterone exerts a local action r egulating the function of the corpus luteum of pregnancy in rats. The lutea l activities of the enzymes 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD ), involved in progesterone biosynthesis, and 20 alpha-hydroxysteroid dehyd rogenase (20 alpha-HSD), that catabolizes progesterone and reduces progeste rone secretion by the corpus luteum, were evaluated after intrabursal ovari an administration of progesterone in pregnant rats that had received a lute olytic dose of prostaglandin F-2 alpha (PGF(2 alpha)). Luteal 3 beta-HSD ac tivity decreased and 20 alpha-HSD activity increased after PGF(2 alpha) tre atment (100 mu g X 2 intraperitoneally on Day 19 of pregnancy at 12:00 p.m. and 4:00 p.m.) when compared with controls sacrificed at 8:00 p.m. on Day 20 of pregnancy. This effect of PGF(2 alpha) on the luteal 3 beta-HSD and 2 0 alpha-HSD activities was abolished in animals that also received an intra ovarian dose of progesterone (3 mu g/ovary on Day 19 of pregnancy at 8:00-9 :00 a.m.). In a second functional study, luteal cells obtained from 19-day pregnant rats responded to the synthetic progestin promegestone (R5020) in a dose-dependent manner, with an increase in the progesterone output. In ad dition, the glucocorticoid agent hydrocortisone did not affect progesterone accumulation in the same luteal cell culture. We also examined by immunocy tochemistry the expression of progesterone receptors (PR) in the corpora lu tes during pregnancy and demonstrated the absence of PR in this endocrine g land in all the days of pregnancy studied. In the same pregnant rats, posit ive staining for PR was observed in cells within the uteroplacental unit, s uch as cells of the decidua basalis and trophoblast giant cells of the junc tional zone. In addition, positive PR staining was observed in the ovarian granulosa and theca cells of growing follicles, but not in corpora lutes of ovaries obtained from cycling rats at proestrus. In summary, this report p rovides further evidence of a local action of progesterone regulating lutea l function in the rat despite the absence of a classic PR. (C) 1999 Elsevie r Science Inc. All rights reserved.