Inhibitory effects of dispersed human amnion cells on production rates of prostaglandin E and F by endometrial cells

Eicosanoids and preterm labour

Products of arachidonic acid metabolism (eicosanoids, e.g. some prostaglandins and leukotrienes) have important roles in the maintenance of pregnancy, and certain diseases of pregnancy such as pregnancy-induced hypertension and the machanism(s) of parturition both at term and before term. The volume of literature describing these last relationships dictates that the present review has to be focused rather than global in nature. Only studies of preterm labour in women and the use of human tissues will be discussed in detail despite the limitations in experimental designs that are imposed on such studies. Emphasis will be placed on studies of cyclo-oxygenase and to a lesser extant lipoxygenase pathways of arachidonic acid metabolism mentioning only briefly the pathways for catabolism of these eicosanoids. It should be noted also that eicosanoids from the epoxygenase pathways and those not derived from arachidonic acid will not be discussed here. The review of literature will not be comprehensive but rather selective in order to focus on specific issues of importance or controversy. In developing concepts of regulation I have emphasized studies with amnion since this tissue has received most attention experimentally. The significance of the eicosanoids in the mechanisms of labour at term and before will be described primarily in a section in which I have attempted to deleneate those regulatory mechanisms that are considered most significant in, or specific to, pregnancy and parturition. Studies of labour at term will be freely interspersed with studies specific to preterm labour since the latter are limited, and we need a ‘strandard’ for comparison; moreover, results from one series of studies helps the development of concepts for the other. Finally, the reader is directed to several excellent reviews that concentrate on areas not emphasized in this review.1–7

Production of prostaglandin F2 alpha and E2 in explants of intrauterine tissues of guinea pigs during late pregnancy and labor

Prostaglandin production in amnion and decidua is considered important for human parturition. We investigated in pregnant guinea pigs, a species similar to women in regard to the endocrinology of pregnancy, whether the production rates of PGE2 and PGF2 alpha in various intrauterine tissues are compatible with a role in parturition. Net production rates were measured at 45, 55 and 65 days of gestation and during labor in amnion, chorion, myo-endometrium, the outer layer of the myometrium, the site of placental implantation, and placenta. Net production rates in amnion increased between 45 days and labor (30-fold for PGE2 and 8-fold for PGF2 alpha, P < 0.0001). During labor, the production rates in amnion of PGE2 (P = 0.006) and PGF2 alpha (P = 0.019) were higher than at 45, 55, and 65 days of gestation. In myo-endometrium, the production rates of PGF2 alpha were higher at 65 days of gestation than at 55 days and during labor (P = 0.046). Addition of arachidonic acid (10(-5) M) increased production of PGE2 and/or PGF2 alpha in all tissues (P < 0.05) except placenta. In amnion, the response to arachidonic acid increased with advancing gestation. This suggests that 1) PGE2 and PGF2 alpha produced by amnion have a potential role in the initiation and maintenance of labor, 2) PGF2 alpha produced by myo-endometrium has a potential role in the initiation of labor, 3) cyclooxygenase(s) are not rate-limiting except in placenta, and 4) the expression of cyclooxygenase in amnion increases with advancing gestation.

Fetal membranes inhibit prostaglandin but not oxytocin-induced uterine contractions

OBJECTIVE

Paracrine signaling in fetal membranes and uterus is hypothesized to play a role in the initiation of labor in women. Numerous fetal membrane signals that stimulate uterine contractions have been described. Recently, by means of the dual-chamber, fetal membrane, uterine muscle model we showed that fetal membranes inhibit spontaneous uterine contractions. This study was designed to test whether human fetal membranes can also inhibit agonist-induced uterine contractions.

STUDY DESIGN

A rat uterine muscle strip was mounted into the maternal side of a Plexiglas acrylic plastic chamber. Uterine contractions were recorded for 3 hours after the addition of either 50 nmol/L prostaglandin E2 or 0.1 nmol/L oxytocin to the maternal side of the chamber. During the first and third hours no fetal membranes were present (basal condition). At the beginning of the second hour fetal membranes were inserted into the chamber so that they divided the chamber into maternal and fetal compartments. The membranes were removed before the beginning of the third hour. The integrated force of uterine contractions during the second hour, when the muscle was exposed to fetal membranes, was compared with the basal condition (first and third hours) by repeated-measure analysis of variance.

RESULTS

Fetal membranes reversibly inhibited prostaglandin E2-induced uterine contractions by 22%. Fetal membranes did not inhibit oxytocin-induced uterine contractions.

CONCLUSION

Fetal membranes inhibit agonist-induced uterine contractions. The fetal membrane inhibitory system is agonist selective.

Dipalmitoylphosphatidylcholine (L-alpha-lecithin) stimulates prostaglandin E production in human amnion

The effect of dipalmitoylphosphatidylcholine (DPPC, L-alpha-lecithin), a substance not containing arachidonic acid, on prostaglandin E (PGE) production of amniotic membrane was studied. DPPC (3.125-800 micrograms/ml) stimulated PGE production of amniotic membrane (150-200%), suggesting that lecithin, the major surfactant component, is directly involved in the mechanism of human parturition as a stimulator of PGE production.

Dipalmitoylphosphatidylcholine (L-alpha-lecithin) stimulates phospholipase A2 activity in human amnion

In order to investigate the mechanism of dipalmitoylphosphatidylcholine (DPPC, L-alpha-lecithin) stimulation of the prostaglandin E (PGE) production of the amniotic membrane, effects of DPPC (50-800 micrograms/ml) on phospholipase A2 (PLA2), phospholipase C (PLC), PG endoperoxide synthase, and PGE synthase activities of human amniotic membrane were studied. Only PLA2 activity was increased by DPPC, suggesting that lecithin, the major surfactant component, increases the PGE production of the amniotic membrane by activating PLA2.

Effect of relaxin on prostaglandin E production by human amnion: changes in relation to the onset of labour

The effect of relaxin on prostaglandin E (PGE) production by human amnion in vitro was investigated. When amniotic discs were incubated in the presence of increasing concentrations of relaxin, two distinct effects were observed. Discs prepared from women delivered by caesarean section before the onset of labour showed a significant decrease in PGE output at relaxin concentrations of 0.5-2 micrograms/ml; the effect was abolished at higher relaxin concentrations. Discs obtained from women delivered after labour of spontaneous onset responded to the addition of relaxin (4-8 micrograms/ml) with a significant increase in PGE output, although this increase was only evident in patients in whom labour had started with intact membranes. These results suggest that relaxin, which is present in decidua and chorion laeve at term, may have a paracrine effect on the amnion, inhibiting PGE production during continuing pregnancy but favouring its production during spontaneous labour.

A product from human decidua inhibits prostaglandin production by human amnion

Human decidua was obtained from nonlaboring women after elective cesarean section. Decidua was incubated with media alone for 20 hours and this media (decidual conditioned media) was then incubated with amnion cells in monolayer culture and amnion rings. A 90% decrease in PGE2 production by amnion cells in monolayer culture was demonstrated in the presence of decidual conditioned media when compared to controls. In short term incubations with fresh amnion, decidual conditioned media decreased the production of PGE2 in amnion by greater than 25% of the control rate of production in 17 of 21 experiments. These observations suggest that human decidua produces a factor capable of inhibiting prostaglandin production by amnion.

Differential regulation of prostaglandin production by inhibitors and stimulators in amniotic fluids during normal and dysfunctional labor

The regulatory effect of amniotic fluid factors on prostaglandin production by sheep seminal vesicle prostaglandin synthetase was determined using samples obtained before and after the onset of labor. Variations in the enzyme incubation conditions permitted the effects on both prostaglandin E (PGE) and prostaglandin F (PGF) production to be assessed. Amniotic fluid obtained before the onset of labor and during early labor resulted in a net stimulation of PGE production and no difference was observed between these two groups. Samples obtained before and during early labor had no net effect on PGF production. However, when samples obtained late in labor were tested, there was a greater stimulation of PGF and less of PGE compared to early labor suggesting a preference for PGF production rather than PGE in late labor. When samples obtained from patients in dysfunctional labor were compared to normal labor, no difference on the effect of either PGE or PGF production was observed. This implies that the decreased PGF previously described in dysfunctional labor is due to an intrinsic abnormality of the fetal membranes rather than inhibition of prostaglandin production by factors mediated via the amniotic fluid.

Prostaglandin E production by amniotic cells in relation to term and preterm labour

The rate of prostaglandin E (PGE) production was measured in collagenase-dispersed amniotic cells obtained from 14 women after spontaneous labour at term--seven after spontaneous preterm labour, nine after delivery by elective caesarean section at term and six after induction of labour at term. Cells were incubated with and without arachidonic acid and PGE was estimated by specific radioimmunoassay. Basal PGE output (pmol/10(6) cells per 3 h) was highest in the spontaneous labour group, 27.5 (SEM 5.5) and lowest in the preterm labour group, 4 (SEM 1.2) (P less than 0.001). Values in the elective section and induction groups were 13.6 (SEM 2.7) and 10 (SEM 3.1), respectively; these values were significantly higher than in the preterm labour group and the values after induction were significantly lower than after spontaneous labour. Addition of arachidonic acid resulted in a significant increase in PGE output in all groups, but the values after preterm labour remained significantly lower than those of any group at term. These data indicate that towards term there is a maturation in the PG synthetase activity of the amnion and that PGE output in this tissue is increased in spontaneous labour.