Improved propagation in myometrium associated with gap junctions during parturition.

The hypothesis that gap junction (GJ) formation between myometrial cells at term improves electrical coupling was tested. We measured the spread of electrical excitation from six extracellular electrodes aligned on uterine strips in either the longitudinal (axial) or transverse (circumferential) direction. Spontaneous bursts propagated over the entire 15-mm recording distance in the axial direction at both preterm and parturition and showed some characteristics of a system of coupled relaxation oscillators. However, individual spikes within the bursts propagated further and with higher velocity at parturition than at preterm. In the circumferential direction, both bursts and individual spikes propagated further at parturition than before. Propagation in this axis at parturition appeared to require an intact circular muscle layer. Spikes evoked by electrical stimulation also propagated further and with higher velocity in both axes at parturition. Electron microscopy showed many GJs between uterine smooth muscle cells during parturition, but few and sometimes no GJs at preterm. Thus improved propagation was associated with increased GJ contact between myometrial cells, consistent with the hypothesis that gap junction formation at term improves electrical coupling.