Identification of myoelectric signals of pregnant rat uterus: new method to detect myometrial contraction

Abstract

Aim: To develop an electromyography method for pregnant rat uterus in vivo and to separate myometrial signals from the gastrointestinal tract signals.

Methods: Pregnant Sprague-Dawley rats (n=8) were anaesthetized and their stomach, small intestine, and large intestine were removed from the abdomen. A pair of thread electrodes was inserted into the uterus, while a pair of disk electrodes was placed subcutaneously above the myometrium. Additionally, a strain gauge sensor was fixed on the surface of the myometrium and cecum for the parallel detection of mechanical contractions in rats (n=18) with intact gastrointestinal tract. The filtered electric signals were amplified and recorded by an online computer system and analyzed by fast Fourier transformation. The frequency of the electric activity was characterized by cycle per minute (cpm), the magnitude of the activity was described as power spectrum density maximum (PsDmax).

Results: The frequency of the pregnant uterine activity was 1-3 cpm, which falls within the same range as that of cecum. Measuring by both electrodes, oxytocin (1 µg/kg) increased and terbutaline (50 µg/kg) decreased the PsDmax by 25%-50% (P<0.001) and 25%-40% (P<0.01), respectively. We found a strong positive correlation between the alterations of PsDmax values and the strain gauge sensor-detected mechanical contractions (area under curve). The GI specific compounds (neostigmine, atropine) mainly affected the cecal activity, while myometrium specific drugs (oxytocin, terbutaline) influenced the myometrial signals only. Conclusion Our method proved to be able to detect the myoelectric activity that reflects the mechanical contraction. The overlapping myometrial and cecal signals are not separable, but they can be distinguished based on the much higher activity and different pharmacological reactivity of the pregnant uterus. Thus, the early signs of contractions can be detected and labor may be predicted in a fast and sensitive way.

Figure 1

Representative picture of the positioning of the electrodes and strain gauge (SG) for recording the myometrial myoelectric and mechanical signals in a rat with a resected gastrointestinal (GI) tract. The thread electrode pair and the SG were positioned on the uterus, while the disk electrode pair was positioned on the abdomen under deep anesthesia.

Figure 2

The positioning of the disk electrode and strain gauges (uterus, cecum) for recording of the uterine and gastrointestinal (GI) myoelectric and mechanical signals in a non-GI tract-resected rat under deep anesthesia.

Figure 3

Myoelectric and mechanical signals of the pregnant uterus (A), detected with different electrodes and with the strain gauge (SG). Fast Fourier transformation (FFT) analysis reveals the tissue specific spectra of the myometrium (B). Each spectrum has the characteristic frequency expressed in cycles per minute (cpm), determined by the highest peak in the spectrum.

Figure 4

Cycles per minute (cpm) values were measured with thread and disk electrodes (A). The intensity of the signals is expressed as the maximum of the power spectrum density (PsD_max), which corresponds to the highest peak in the Fast Fourier transformation spectrum. The signals of the thread electrodes were usually higher than those of the disk electrodes (B).

Figure 5

Changes in uterine activity after oxytocin and terbutaline treatments in rats with resected gastrointestinal track. The intensity of the electric signals was expressed as the maximum of the power spectrum density (PsD_max), while the mechanical contraction was evaluated by area under curve (AUC) analysis. Oxytocin increased, while terbutaline reduced the electric and mechanical activities of the smooth muscles relative to the basic activity (100%) (P < 0.05*; P < 0.01**; P < 0.001***).

Figure 6

Correlations between myoelectric and mechanical responses induced by oxytocin and terbutaline treatments in GI tract-resected animals. The myoelectric response recorded by thread (A) or disk electrode (B) is expressed as the maximum of the power spectrum density (PsD_max), while the mechanical response is expressed as the area under the curve (AUC) of the recorded smooth muscle contractions compared to the basic activity.

Figure 7

Changes in gastrointestinal (GI) and myometrial smooth muscle activities after neostigmine and atropine (A), or oxytocin and terbutaline (B) treatments in rats. with intact GI tract. The intensity of the electric signals was expressed as the maximum of the power spectrum density (PsD_max), while the mechanical contraction was evaluated by area under curve (AUC) analysis (P < 0.05*; P < 0.01**; P < 0.001***).