Tissue-level bioelectrical signals as the trigger for uterine contractions in human pregnancy.

OBJECTIVE: To investigate the relationship between tissue-level bioelectrical signals and tension development during oxytocin-stimulated contractions of human myometrium.
METHODS: We performed in vitro muscle bath experiments on human myometrial tissue strips while simultaneously monitoring bioelectrical activity with two loose-contact electrodes. Tissue was obtained by myometrial biopsy from term pregnant women at the time of cesarean delivery. Tissue strips (1 x 1 x 10 mm) were hung vertically and maintained in culture in media while suspending a 400-mg weight. The tissue exhibited strong isometric contractions in response to 5-nM oxytocin even after 10 to 14 days in culture. The electrodes were separated by 4 mm, and allowed us to distinguish between local and tissue-level bioelectrical signals. Electrical activity was monitored using two, independent AC-coupled amplifiers.
RESULTS: Following exposure to oxytocin, the tissue contracted periodically every 3.5 to 6 minutes, with each contraction lasting 50 to 60 seconds. Near the beginning of each contraction, synchronized spike-like bioelectrical signals were observed in both channels. These bioelectrical signals from each electrode lasted approximately 2 seconds and demonstrated unique fingerprints that were repetitive and remarkably similar over 18 contractions. In each of the contractions, the onset of rapid force increases was synchronized with the bioelectrical signals. Cell recruitment continued during the plateau phase of each contraction even though other tissue-level bioelectrical signals were not observed.
CONCLUSION: These findings suggest that the trigger for the initiation of each contraction is a tissue-level bioelectrical event, and some cells are initially recruited to participate in each contraction by excitation-contraction coupling. After the initial phase of the contraction, cells are recruited by a nonelectrical mechanism.