Localization and developmental changes in prostaglandin H synthase (PGHS) and PGHS messenger ribonucleic acid in ovine placenta throughout gestation

In numerous animal species, increased prostaglandin output by intrauterine tissues occurs in association with parturition. In the sheep, prostaglandin H synthase (PGHS) is particularly important in regulating this process, and it has been shown recently that the placental content of the inducible form of the enzyme (PGHS-2) increases during the latter part of gestation whereas no change occurs in the constitutive form (PGHS-1). The purpose of the present study was to examine the distribution of cells containing immunoreactive PGHS-2 and PGHS-2 mRNA in the ovine placenta throughout the second half of gestation (80-147 days). Commercially available antisera to PGHS-2 were used for immunohistochemistry with paraffin-embedded tissues, and a 35S-labeled oligonucleotide probe specific for PGHS-2 mRNA was used for in situ hybridization on frozen sections. Immunohistochemistry indicated that there was an increase in the placental content of immunoreactive PGHS-2 after 140 days gestation. At term, the PGHS-2 was located mainly in the uninucleate trophoblast cells in the placentomes. In situ hybridization showed that the distribution of PGHS-2 mRNA was confined to the trophoblasts, with little or no hybridization signal in the maternal endometrium or the maternal syncytium. Autoradiograms of the sections after in situ hybridization were quantified with a computerized image analysis system. There was an increase in the placental content of PGHS-2 mRNA from around 140 days gestation to term, but no change was found in the level of PGHS-1 mRNA. These studies indicate that an increase in the expression of PGHS-2 occurs in the placental trophoblast tissue near parturition in the sheep. The increase in PGHS-2 mRNA is probably responsible for the increase in PGHS-2 protein and PGHS activity that occurs at this time and may contribute to the large increase in prostaglandin production by the ovine placenta at term.