Activity of circular muscle of rat uterus at different times in pregnancy

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Abstract
During pregnancy, circular muscle activity changes from weak irregular contractions at midterm to strong regular contractions at term. The present study was designed to determine if this change is related to contractile ability or to membrane electrical events. Myometrial segments were taken from rats on days 16–17, 18–19, 20, and 21 (term) of gestation. Contractile ability was estimated as the maximum force elicited by 121 mM KCl or 0.6 mM acetylcholine. The K- and acetylcholine-induced contractions were significantly greater than spontaneous and no different at midterm and term. Electrical activity was measured with intracellular electrodes. Between days 16 and 21, action potentials evolved from single, plateau-type to repetitive spike trains. Plateau potentials were associated with small contractions and repetitive spikes with larger contractions. On day 21, there was a transient increase in membrane potential and cessation of spontaneous activity. Before delivery, the membrane potential declined and spontaneous spike trains appeared and triggered large contractions. Conclusion: the change in circular muscle activity during pregnancy is related to electrical events and not to contractile ability.