Spatiotemporal Mapping of the Contracting Gravid Uterus of the Rabbit Shows Contrary Changes With Increasing Gestation and Dosage With Oxytocin.

Corrin M Hulls,Joana A B Chagas,Wei-Hang Chua,Quinten M King,John P Chambers,Roger G Lentle,Lauren Stewart,Philip Suisted

doi:10.3389/fendo.2019.00802

Corrin M Hulls, Joana A B Chagas + Show 6 more

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https://doi.org/10.3389/fendo.2019.00802

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Abstract

Spontaneous and oxytocin induced contractile activity was quantified in the bicornuate uteri of pregnant rabbits maintained in situ, using data from two- and uni- dimensional video spatiotemporal maps (VSTM) of linear and area strain rate and compared statistically. Spontaneous contractions occurred over a range of frequencies between 0.1 and 10 cpm, in gravid animals at 18–21 and at 28 days of gestation, and propagated both radially and longitudinally over the uterine wall overlying each fetus. Patches of contractions were randomly distributed over the entire surface of the cornua and were pleomorphic in shape. No spatial coordination was evident between longitudinal and circular muscle layers nor temporal coordination that could indicate the activity of a localized pacemaker. The density and duration of contractions decreased, and their frequency increased with the length of gestation in the non-laboring uterus. Increasing intravenous doses of oxytocin had no effect on the mean frequencies, or the mean durations of contractions in rabbits of 18–21 days gestation, but caused frequencies to decrease and durations to increase in rabbits of 28 days gestation, from greater spatial and temporal clustering of individual contractions. This was accompanied by an increase in the distance of propagation, the mean size of the patches of contraction, the area of the largest patch of contraction and the overall density of patches. Together these results suggest that progressive smooth muscle hypertrophy and displacement with increasing gestation is accompanied by a decrease in smooth muscle connectivity causing an increase in wall compliance and that oxytocin restores connectivity and decreases compliance, promoting volumetric expulsion rather than direct propulsion of the fetus by peristalsis. The latter effects were reversed by the β2 adrenergic receptor agonist salbutamol thus reducing area of contraction, and the duration and distance of propagation.

Highlights

The principal mechanical functions of the mammalian uterus are to act as a repository in which fetal growth can take place and to provide a means by which the fetus can be expelled once fully developed
Spontaneous local uterine contractile activity was successfully recorded in four rabbits at mid gestation (18–21 days) and four at late gestation (28 days) and after increasing IV doses of oxytocin and subsequent dosage with salbutamol at various sites across the left uterine cornu
The results provide new insights into gestational changes in the mechanics of myometrial contraction as well as providing reciprocal illumination regarding the results of existing electrophysiological studies [27]

Summary

Introduction

The principal mechanical functions of the mammalian uterus are to act as a repository in which fetal growth can take place and to provide a means by which the fetus can be expelled once fully developed The former is achieved by amelioration of the tone in the walls of the uterus and their compliance i.e., the ease with which the uterine cavity may be dilated. Qualitative direct studies of human uterine contraction using two dimensional electrohysterographic mapping show that the propagating front of electrical burst activity is irregular [10] and that uterine contractions vary in frequency and strength between proximal and distal regions and between adjacent sites [6, 11]

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