Gap Junction Formation in Uterine Smooth Muscle at Parturition

Abstract

<p>The development of synchrony is characteristic of uterine muscle at the end of pregnancy. The coordinated contractile activity of the smooth muscle is effective in delivering the fetuses. A structural change occurs late in gestation, and specialized sites of intercellular communication, gap junctions, appear between the uterine smooth muscle cells. The objectives of this thesis were to characterize the time course of gap junction formation, investigate some factors that govern their appearance, and examine the possible significance of gap junction formation in the myometrium. Electrophysiological methods were used to evaluate the hypothesis that gap junction formation at parturition resulted in improved electrical communication between cells. Improved coupling might allow the synchronization of uterine activity.</p> <p>Gap junctions were shown by quantitative thin section electron microscopy to be present between uterine smooth muscle cells immediately prior to, during, and for a short time following parturition in the rat. Several experiments involving ovariectomy revealed that the stimulus for gap junction formation was systemic in nature. Bilateral ovariectomy of midterm pregnant rats resulted in premature termination of pregnancy and gap junction formation, both of which were blocked by hormone administration. These and other results suggest that progesterone withdrawal may regulate gap junction formation in rat myometrium, but many factors, such as estrogen and prostaglandins, may also be involved.</p> <p>Uterine smooth muscle is a functional electrical syncytium, and properties of electric current flow through the muscle yield indirect estimates of cell-to-cell coupling between cells. Impedance analysis showed that the specific resistance of the cytoplasm of myometrial cells was constant from before term to delivery, but the junctional resistance decreased. Shortly post partum the junctional resistance increased. Cable analysis confirmed that the internal resistance of myometrium was lower at parturition. Thus, improved cell-to-cell communication was associated with a demonstrated increase in gap junction contact between cells. These results are consistent with the hypothesis that gap junction formation at the end of gestation results in improved electrical coupling of uterine smooth muscle cells.</p>