Human fetal membranes release a Ca++ channel inhibitor.

OBJECTIVE: Our purpose was to test the hypothesis that an inhibitor of uterine contractions released by human fetal membranes acts on the dihydropyridine site of the myometrial voltage-dependent Ca++ L channel. STUDY DESIGN: Initial experiments established the time course of release of the inhibitor from term, fetal membranes. Both a competitive binding assay and a uterine contraction bioassay were used to detect the inhibitor. After optimal time of release of inhibitor was determined, a dose-response experiment was performed with the competitive binding assay. To determine the source of the inhibitor, membranes are separated into component layers to generate inhibitor, and the competitive binding assay was used to measure the inhibitor. RESULTS: An inhibitor released from fetal membranes competes with 3H-isradipine at the Ca++ L channel dihydropyridine binding site. There is a time-dependent release of the inhibitor from membranes, which is maximal at 20 minutes (P < or = .05, n = 4). A dose effect of the inhibitor is present because greater amounts of inhibitor produce greater competition at the dihydropyridine site (P < or = .005, n = 3). The data are consistent with 1-site binding. Inhibition is restricted to the chorion (64% specific inhibition) and decidua (52% specific inhibition) with little competition seen in amnion alone (4% specific inhibition) (P < or = .03, n = 3). CONCLUSIONS: These studies support the hypothesis that human chorion/decidua releases an inhibitor of uterine contractions that acts specifically at the dihydropyridine site of the myometrial Ca++ L channel.

Human fetal membranes inhibit calcium L-channel activated uterine contractions.

OBJECTIVE: Paracrine signals among fetal membranes, decidua, and uterus play an important role in the initiation of parturition in women. In previous work we demonstrated that fetal membranes inhibit uterine contractions. In the current study we test the hypothesis that the fetal membranes decrease uterine contractions by inhibition of the uterine calcium L-channel. STUDY DESIGN: Our dual-chamber fetal membrane-uterine muscle in vitro model was used in this study. Rat uterine muscle strips were anchored into the maternal sides of the chambers. Fetal membranes (or Parafilm controls) were added to the chamber in a removable cassette. Uterine contractions were stimulated with the specific calcium L-channel agonist Bay K 8644. RESULTS: When uterine muscle was exposed to full-thickness fetal membranes (amnion-chorion with attached decidua) or to the intact fetal components (chorion-amnion) or to chorion alone, the Bay K 8644 dose-response curve was significantly shifted to the right. When uterine muscle was exposed to amnion alone or to the decidua alone, the Bay K 8644 dose-response curve was not shifted. Fetal membranes, did not cause a shift in the ionomycin (a calcium ionophore) dose-response curve. CONCLUSION: These results support the hypothesis and provide evidence that human fetal membranes, most likely chorion, release an endogenous calcium L-channel inhibitor.

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.

Interleukin-6 does not stimulate rat myometrial contractions in an in vitro model.

PROBLEM: Interleukin-6 (IL-6) increases in culture-positive amniotic fluid in women with preterm labor. IL-6 stimulates the production of prostaglandins leading to increased uterine activity. METHODS: We tested the hypothesis that IL-6 increases myometrial activity through release of uterotonic mediators. We studied the effect of IL-6 on uterine contractions in the absence and presence of fetal membranes to determine if the effect was on myometrium alone or was mediated through fetal membranes/decidua. IL-6 in concentrations of 100, 10, 0.1 or 0 ng/ml was added to the maternal side of the dual chamber-fetal membrane-uterine muscle in vitro model. RESULTS: We found that 10 ng/ml of IL-6 alone, without fetal membranes, caused a significant decrease in uterine contractions over time (P < or = 0.01). This decrease was not observed with the addition of term, nonlabored fetal membranes. CONCLUSIONS: IL-6 in the presence or absence of membranes, over a four log fold dose range, did not stimulate uterine contractions.

Human fetal membranes inhibit spontaneous uterine contractions.

Fetal membranes are postulated to play a role in paracrine signaling during the initiation of labor in women. We developed a dual chamber-fetal membrane-uterine muscle model to study the effect of human fetal membranes on spontaneous uterine contractions. In this model, full-thickness fetal membranes (amnion, chorion, and maternal decidua) are sealed into a Plexiglass chamber. The membranes partition the chamber into a maternal and fetal compartment. Chorion and decidua face the maternal side, and amnion faces the fetal side. An estrogenized rat uterine muscle strip is anchored into the maternal side as a bioassay to measure effects of fetal membranes on uterine contractions. Fetal membranes cause a 40% decrease in uterine contractions compared to basal condition (no membranes). Inhibition is reversible after removal of the membranes. The inhibition is specific to the chorion/decidual side because reversal of membranes with amnion toward the muscle did not show inhibition. Uterine contractions did not change over time in control chambers in which Parafilm substituted for membranes. A model for studying paracrine regulation of uterine contractions by human fetal membranes has been developed. The model provides evidence that fetal membranes inhibit uterine contractions. This inhibitory effect may contribute to uterine quiescence during pregnancy.