LOCALIZATION OF 15-HYDROXY PROSTAGLANDIN DEHYDROGENASE (PGDH) AND STEROIDOGENIC ENZYMES IN THE EQUINE PLACENTA

15-hydroxy prostaglandin dehydrogenase (PGDH) is the critical enzyme t hat determines metabolism of primary prostaglandins. Its expression is determined in part by steroid hormones, particularly progesterone, fo rmed from Delta(5) steroids through 3 beta-hydroxysteroid dehydrogenas e (3 beta-HSD) activity. To assess whether the regulation of PGDH migh t occur in a paracrine, autocrine or intracrine fashion, we used immun ohistochemistry (IHC) to determine the localisation of key steroidogen ic enzymes in the equine placenta and compared these patterns to the d istribution of immunoreactive (IR-) PGDH. Placental tissue was obtaine d from pony or Thoroughbred mares at about Days 150, 250-280 and >300 of pregnancy (term 320-360 days; n=5-8 each group). IR-PGDH, 3 beta-HS D, cholesterol side chain cleavage enzyme (P450(scc)) and 17-hydroxyla se/lyase (P450(C17)) were localised using specific antibodies and the avidin-biotin peroxidase technique and visualised using diaminobenzidi ne as substrate. IR-P450(scc) was sec present in trophoblast cells, bu t not in maternal tissues of the microcotyledons or elsewhere in the e ndometrium. Specific staining for P450(C17) was not detected in either maternal or fetal tissues at any stage of gestation studied. IR-3 bet a-HSD was present in trophoblast cells, but not in maternal tissues of the microcotyledons. In contrast, at Days 150 and 280, IR-PGDH was pr esent in maternal epithelial and interstitial cells in the microcotyle dons, but was not detected in trophoblast epithelium, chorioallantois or endometrial glands, After Day 300, IR-PGDH was present in the mater nal epithelium and interstitial cells of the placenta and it was also present in trophoblast cells in some specimens. We conclude that the e quine placenta has the enzymes necessary to produce C(21)Delta(4) ster oids from C(27)Delta(5) Substrates in trophoblast cells by Day 150 of gestation. The localisation of these enzymes is compatible with paracr ine regulation of PGDH by products of 3 beta-HSD activity for most of pregnancy, although autocrine or intracrine regulation of placental PG DH may also be possible nearer to term.