Relationship between electrical activity of the uterus and surgically isolated myometrium in the pregnant and non-pregnant ewe

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Abstract
Rhythmic contractions occur in the sheep uterus at estrus and during pregnancy from about 65 days to term (145 days). To define factors responsible for these contractions, the degree of synchronization of electrical activity in the uterus and isolated segments of myometrium was examined and quantified in 3 types of experiments. A segment of myometrium was totally separated from the uterus. After a period of 9-16 days the isolated tissue developed a typical pattern of uterine activity which showed no significant degree of synchronization with EMG [electromyographic] bursts in the body of the uterus. During labor, the isolated tissue showed changes in activity similar to those oberved in the uterus. In the 2nd experiment, the tubal end of one of the uterine horns was severed from the uterus, but a connection was retained with the uterus via the oviduct and ovarian blood vessels. Activity in the partly isolated segment remained in synchrony with the uterus. In a 3rd experiment, impulse propagation through nerves and smooth muscle to the tip of a horn was disrupted by severing the tip from the uterus while its blood supply from the ovarian vessels was retained. The blood vessels were momentarily frozen, and denervation confirmed by monoamine histofluorescence. In 5 of 6 animals the operated tissue showed activity that was not synchronous with the rest of the uterus. Isolated uterine muscle in vivo apparently has rhythmicity resembling that of intact myometrium. Systemic or local circulating factors evidently are not responsible for synchronizing uterine activity before parturition, although circulating factors do play a major role in increasing the uterine activity which occurs at parturition and at estrus. Hydraulic continuity between different regions of the uterus apparently is not essential for maintaining coordinated activity.