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Mechanisms of Paracrine Regulation by Fetal Membranes of Human Uterine Quiescence

Department de Obstetricia y Ginecología, Unidad de Medicina Materno Fetal, Facultad de Medicina, Pontificia Universidad Católica de Chile, Santaigo, Chile; Department of Obstetrics and Gynecology, University of Kansas School of Medicine, Kansas City, Kansas; Marcoleta 391, Centro de Investigaciones Médicas, Pontificia Universidad Católica de Chile, Santiago, Chile jcarva{at}med.puc.cl

Rossana J. Vidal, MS

Mauricio A. Cuello, MD

Jose A. Poblete, MD

Carl P. Weiner, MD

Department de Obstetricia y Ginecología, Unidad de Medicina Materno Fetal, Facultad de Medicina, Pontificia Universidad Católica de Chile, Santaigo, Chile; Department of Obstetrics and Gynecology, University of Kansas School of Medicine, Kansas City, Kansas

Objective: To test the hypothesis that fetal membranes (chorion or amnion) release one or more factors responsible for myometrial quiescence.

Methods: Myometrial samples were excised from women at elective term cesarean delivery prior to the onset of labor. Fetal membranes were obtained after cesarean delivery either before or during labor, and either term (greater than 37 weeks) or preterm (less than or equal to 36 weeks). Myometrial strips were placed in organ baths and contractions stimulated by oxytocin (10^-8 M). Contractility was measured under isometric conditions before and after exposure to fetal memranes or conditioned medium. The impact of either memrane or conditioned media on contractility was determined before and after myometrial K⁺ channel blockade.

Results: Both chorion and amnion and their respective conbditioned mediums decrease oxytocin-stimulated myometrial contraction. The inhibitory effect was greatest with membranes from preterm pregnancies (mean gestation 32 weeks, P <.05). The inhibitory effect was detectable in the presence of term labor, but was absent when the fetal membranes were obtained after prterm labor. Iberiotoxin, an inhibitor of large conductance Ca²⁺-activated K⁺ channels (BK_Ca) reduced the effect of fetal membranes by 50% (P <.05).

Conclusion: We conclude that human fetal membranes release one or more factors that inhibit oxytocin-induced myometrial contractility. We suggest this factor (or factors) acts mainly by opening myometrial BK_Ca. The findinigs further support our hypothesis that the fetal membranes release a factor (or factors) that is central to myometrial quiescence and its premature loss leads to preterm delivery.

Key Words: Myometrial quiescence • fetal membranes • chorion • amnion • BKCa

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