The location of pacemakers in the uteri of pregnant guinea pigs and rats

Abstract

The pregnant uterus is a smooth muscle organ whose pattern of contraction is dictated by the propagation of electrical impulses. Such electrical activity may originate from one or more pacemakers, but the location of these sites has not yet been determined. To detect the location of the pacemaker in the gravid uterus, two approaches were used: 1) determine the site from where the contraction started using isolated uteri from the pregnant guinea pig, and videotape their contractions; and 2) record, in isolated uteri from pregnant term rats, with 240 extracellular electrodes simultaneously, and determine where the electrical bursts started. In both the contractile and electrophysiological experiments, there was not a single, specific pacemaker area. However, most contractions (guinea pig 87%) and bursts (rat 76%) started close to the mesometrial border (mean 2.7 ± 4.0 mm SD in guinea pigs and 1.3 ± 1.4 mm in rats). In addition, in the rat, most sites of initiations were located closer to the ovarial end of the horn (mean distance from the ovarial end 6.0 ± 6.2 mm SD), whereas such an orientation was not seen in the guinea pig. In both guinea pig and rat uteri at term, there is not one specific pacemaker area. Rather, contractile and electrical activity may arise from any site, with the majority starting close to the mesometrial border. Furthermore, in the rat, most activities started at the ovarial end of the horn. This may suggest a slightly different pattern of contraction in both species.

Identification of uterine pacemaker regions at the myometrial–placental interface in the rat

“…In previous experiments on eight rats, with a total of 29 independent observations, we demonstrated that the majority of excitations recorded at the serosal surface originate from placental sites on the mesometrial border (Lammers et al . 2015).…”

“…To date, no pacemaker regions with specific anatomical features have been described in the mammalian uterus, which is perhaps surprising given that coordinated forceful contractions are essential to parturition. Multi‐electrode recordings of rat uteri demonstrated that potentials tend to initiate either at placental sites on the mesometrial border, or near the ovarian end (Lammers et al . 2015).…”

Characteristics of bioelectrical activity of oviducts and uterus during early pregnancy in sows recorded by telemetry method

“…In the available literature, there are few articles describing the contractility of the uterus and oviduct during early pregnancy (Lammers & van der Vusse, ; Lammers et al . ). This can be attributed to the methodological difficulties in achieving a successful AI with electrodes sutured on the oviducts.…”

“…; Lammers et al . ). Owing to the lack of information on the EMG activity of the oviducts in early pregnancy in sows, it is hard to relate the present results to the works of other authors (Lei et al .…”

Estimating uterine source current during contractions using magnetomyography measurements

“…However, it is still unclear whether there exists a specific pacemaker mechanism or a specific pacemaker area [11]. Except for some observations of contractile activity originating from specialized cells [12], most activities arise from any site throughout the myometrium [13, 14]. Among these sites, fundus is one possibility [15–17], hence we set the initiation of electrical activity to occur at the fundus in the simulations.…”