Development of Myometrial Electrical Activity during the First Half of Pregnancy in the Sheep

Abstract

Uterine myoelectrical activity was recorded in seven pregnant sheep covering the period between 13 and 75 days post-coitum. Activity in the myometrium was present at day 13 and took the form of intermittent spikes of low amplitude. Bursts of spikes of irregular duration became noticeable between days 25 and 40 but most were not coordinated throughout the myometrium. Coordinated bursts of myoelectrical activity, which could be recorded at several sites simultaneously, first appeared between 40 and 65 days. These bursts had similar characteristics to the myoelectrical activity associated with uterine contractions during the last third of gestation. The myoelectrical activity showed a progressive increase in amplitude during the first half of gestation. There was no relationship between plasma progesterone levels and the increase in amplitude or appearance of coordinated bursts of uterine activity.

An in-vivo model to examine the electromyographic activity of isolated myometrial tissue from pregnant sheep

“…The occurrence, throughout pregnancy in the ewe, of low-frequency, low amplitude uterine contractions has been reported by a number of investigators (Hindson & Ward, 1973;Nathanielsz et al, 1976; Van der Weyden et al, 1981;Harding et al, 1982;Lye et al, 1983;Sigger et al, 1984a;Garcia-Villar et al, 1984). They have been termed 'contractures' by Nathanielsz et al (1976) to differentiate them from the short-duration, high-frequency 'contractions' of labour.…”

“…They have been termed 'contractures' by Nathanielsz et al (1976) to differentiate them from the short-duration, high-frequency 'contractions' of labour. Contractures are first detectable between Days 28 and 60 (Van der Weyden et al, 1981;Sigger et al, 1984a), continue to term (~Day 145), and typically occur with a frequency of 0-5-3 h, a duration of 5-8 min, and an amplitude of 2-8 mmHg (Van der Weyden et al, 1981;Harding et al, 1982;. Over 95% of these contractures are associated with a burst of myometrial electromyo¬ graphic activity of a similar duration (Harding et al, 1982).…”