Mechanisms of parturition: The transfer of prostaglandin E₂ and 5-hydroxyeicosatetraenoic acid across fetal membranes

Prostaglandin E₂ production by amnion is thought to be an important event in the onset of human labor. It has been suggested that 5-hydroxyeicosatetraenoic acid, also produced in the amnion, may mediate prelabor contractions. For either of these compounds to play a paracrine role they need to cross the chorion, which has a high capacity to metabolize prostaglandins. With the use of an in vitro system we have shown that both prostaglandin E₂ and 5-hydroxyeicosatetraenoic acid cross either amnion or intact amnion-chorion-decidua at a rate similar to that of an extracellular marker sucrose. Analysis by high performance liquid chromatography revealed that at physiologic concentrations neither prostaglandin E₂ nor 5-hydroxyeicosatetraenoic acid were metabolized by amnion alone. Moreover, 100% of the 5-hydroxyeicosatetraenoic acid and 72% of the prostaglandin E₂ remained in the active form after passage across intact amnion-chorion-decidua. There did not appear to be any difference in the rate of transfer or permiability of the membranes before or after spontaneous labor. We conclude that both 5-hydroxyeicosatetraenoic acid and prostaglandin E₂ synthesised in the amnion can cross the membranes by diffusion through the extracellular space, remaining largely unmetabolized, and may play a role in the onset of human labor.