A new concept of the significance of regional distribution of prostaglandin H synthase 2 throughout the uterus during late pregnancy: Investigations in a baboon model

Objective: We sought to identify regional differences in prostaglandin H synthase 2 messenger ribonucleic acid expressions in various intrauterine tissues in the pregnant baboon as an indicator of prostaglandin production capability to explain the various interactive roles of different intrauterine tissues in the processes that precede, promote, and complete labor. Study Design: Prostaglandin H synthase 2 messenger ribonucleic acid expression was measured by reverse transcriptase–polymerase chain reaction or Northern blot analysis in the uterine fundus, lower uterine segment, cervix, amnion, chorion, and placenta during late pregnancy and spontaneous term labor in the pregnant baboon. Myometrial electromyography enabled clear relation of the findings to uterine contractile activity. Results: There were dramatic increases of prostaglandin H synthase 2 messenger ribonucleic acid expressions during late gestation and during labor in the lower uterine segment, cervix, and decidua. The amniotic prostaglandin H synthase 2 messenger ribonucleic acid expression increased during labor. In contrast, the prostaglandin H synthase 2 messenger ribonucleic acid expressions in the uterine fundus, chorion, and placenta did not change during late gestation and labor. Conclusion: Demonstrated increased lower uterine segment and cervical prostaglandin H synthase 2 abundances would promote lower uterine segment elongation and cervical effacement. Engagement of the fetal presenting part would stimulate local prostaglandin H synthase 2 expression and obstruct diffusion of high forebag prostaglandin to the rest of the uterus, as reported previously in human pregnancy. These data support a new conceptual mechanistic framework for preparatory changes in the lower uterine segment and cervix preceding labor as precisely related to myometrial contractility changes. (Am J Obstet Gynecol 2000;183:1287-95.)