Uterine contractility in nonpregnant mares

Abstract

Uterine contractility has been studied by intrauterine pressure transducers - balloons or catheter tip pressure transducers - electromyography (EMG), transrectal ultrasound, and recently, by scintigraphy. Maximal uterine contractility was seen in ultrasonography during the late luteal phase (days 13 to 14), at the time of the expected luteolysis. Also in EMG, frequent spikes of short duration alternating with short periods of inactivity were reported during luteolysis. Total electrical activity was higher in dioestrus than in oestrus. Oestrus was characterised by shorter spikes of higher intensity separated by longer periods of inactivity as compared to dioestrus. Intrauterine infusion of bacteria increased myometrial electrical activity for 5 hours in all mares and for 18 hours in healthy mares resistant to uterine infections. Resistant mares exhibited greater uterine activity 10 to 20 h after inoculation than mares susceptible to uterine infections. Insemination (Al) caused spikes of higher amplitude and more frequent active phases for 2 to 7 hours as recorded by EMG. Scintigraphy showed how Al induced uterine contractions which were most frequent during the first 10 to 30 min. Rectal palpation caused a transient increase in myometrial activity. Oxytocin increased myometrial activity at all stages of the cycle and prostaglandin F2α and its synthetic antagonists in cycling and steroidtreated mares. α2-agonists, detomidine, xylazine, and romifidine increased intrauterine pressure and electrical activity. Propantheline and acepromazine suppressed myometrial activity.