Chorion releases a factor that inhibits oxytocin-stimulated myometrial contractility in the pregnant guinea pig

It was postulated that chorion releases a substance necessary for the maint enance of uterine quiescence during pregnancy. A decrease in the release of this substance at the end of the pregnancy would be necessary for normal m yometrial activation. This hypothesis was tested by demonstrating the abili ty of chorion to inhibit oxytocin-stimulated myometrial contractility in vi tro. Tissues were obtained from timed pregnant Duncan-Hartley guinea pigs e ither at pre-term or near-term gestation. Myometrial strips were placed in organ baths for isometric tension measurement and contractions stimulated b y oxytocin (10(-8) mol/l). Petal membranes or conditioned medium from chori on were added directly to the organ bath. Near-term chorion and chorion con ditioned-medium decreased oxytocin-stimulated contractile activity to 39% a nd 49% respectively. Neither pre-term nor near-term amnion reduced oxytocin -stimulated myometrial contractile activity. Relaxation induced by pre-term chorion was greater than near-term chorion (23% and 41% of the oxytocin-in duced basal level respectively; P < 0.05). Further, chorion-induced relaxat ion was independent of the gestational age of the myometrium. Human chorion from a term, not-in-labour woman also inhibited oxytocin-stimulated guinea pig myometrial contractility. It was concluded that the chorion releases a substance or substances that reduce oxytocin-stimulated myometrial contrac tility and may be involved in the maintenance of uterine quiescence during pregnancy.