Some physiological and pharmacological factors affecting uterine motility as measured by electromyography in the mare.

The researchers carried out a study on mares to understand how different physiological and pharmacological factors impact their uterine activity as measured by electromyography.

Study Design and Methods

• This research involved two groups of mares; two mares were intact (with ovaries) and two other mares had their ovaries removed.
• Bipolar electrodes were implanted in the myometrium (the middle layer of the uterine wall) of the mares to measure electromyographic (EMG) activity.
• The researchers induced normal and synthetically stimulated reproductive cycles using estrogen and progesterone and measured the EMG activity during each stage of the cycle.
• A variety of medications (oxytocin, cloprostenol, propantheline bromide, and clenbuterol) were administered at each cycle stage to study how they affect uterine motility.
• In one mare, the EMG activity was monitored through natural breeding and during the first 20 days of pregnancy.
• In some instances, the intrauterine pressure (IUP) was also recorded using an open-tipped catheter system.

Findings and Observations

• The researchers observed that during the receptive phase of the reproductive cycle (oestrus), short, intense bursts of EMG activity occur, separating quiet periods—indicating uterine contractions.
• In the non-receptive phase (dioestrus), the EMG activity continued for longer but at lower intensity and without clear quiet periods, suggesting muscle tension in the uterus (uterine tonus).
• The pattern of EMG activity during breeding revealed a short-lived increase, which the researchers deemed unlikely to have been caused by hormone release.
• Upon conception and during early pregnancy, EMG activity fluctuated according to the location of the fertilized egg (oviduct, migratory, fixed).
• Oxytocin and cloprostenol caused extended EMG activity at first, followed by a pattern of short bursts that was most evident during oestrus. This suggests that uterine activity is more stimulated in this phase of the cycle.
• Propantheline reduced EMG activity while clenbuterol caused minimal changes. Coinciding with these changes, IUP increased with uterine stimulants and decreased with uterine relaxants.

Challenges and Conclusions

• Although IUP variation was expected to align with the stages of the oestrous cycle, the study showed little statistical or visual correlation to EMG activity under different conditions.
• Based on consistent EMG results showing significant differences between oestrus and dioestrus, the study calls into question the validity of the commonly used IUP recording technique both in this and wider studies on mares.
• The researchers suggest that the IUP responses could be influenced by factors outside the uterus like intestinal movement or changes in intra-abdominal pressure.