Oxytocin Receptors, Gap Junctions and the Onset of Human Labor

Introduction: Oxytocin receptors (OTR) and gap junctions (GJ) represent key factors in the development and expansion of regular uterine contractions. Its concentrations in human myometrium rise with increasing gestational age which leads to higher sensitivity for peripheral oxytocin and raising contractility. The aim of this study was to compare the density of GJ formation with the OTR concentration during the course of pregnancy. Material and Methods: Tissue samples were obtained in cesarean sections for various reasons from transverse incisions in the lower uterine segment. Five representative groups of patients were defined: 1. preterm delivery w/o labor (n=3), 2. preterm delivery with labor (n = 6), 3. term delivery w/o labor (n = 7), 4. term delivery with labor (n = 7), 5. term, failed induction of labor with oxytocin (n = 5). OTR concentrations were investigated by saturation experiments using the method of Soloff and Alexandrova. The frequency and fractional percentage of membrane occupied by GJ was obtained using electronmicroscopy and photography. Results: OTR concentrations rose to peak values in early term and preterm labor. The frequency and fractional area of GJ was low and similar in all groups although a nonsignificant trend to lower areas was observed in preterm samples. There was no correlation between OTR concentrations and GJ. OTR density, but not GJ frequency or areas, were positively correlated with the frequency of uterine contractions. OTR concentrations in samples from patients at term and after failed oxytocin induction did not differ significantly from samples at term w/o labor. Conclusions: A rise in OTR, but not in GJ, was associated with the onset of labor. It can be concluded that this increase in OTR density in the lower uterine segment is of higher importance for the onset of labor both in preterm and term labor than the rise of GJ formations.