Interactions of xylazine and acepromazine with oxytocin and the effects of these interactions on intrauterine pressure in normal mares and mares with delayed uterine clearance.

The effects of the sedatives xylazine (predominantly alpha2 agonist) and acepromazine (predominantly alpha1 antagonist) and their influence on intrauterine pressure were measured in four reproductively normal mares and four mares with delayed uterine clearance. Intrauterine pressure was measured in each mare during oestrus for three 100 min periods using an 8-F catheter with three discrete pressure sensors. After 10 min of baseline recording, xylazine, acepromazine or saline was administered i.v. Oxytocin was administered i.v. after a further 30 min, and pressure recordings were continued for 60 min. The pressure changes analysed were: number of uterine contractions; time of onset and duration of the first contraction; and the time at which intrauterine pressure changed from baseline. Data were analysed using the mixed procedure of the Statistical Analysis System. ANOVA detected an effect of treatment (P < 0.0001) and a treatment by group interaction (P < 0.001). Xylazine administration caused a tetanic contraction in all mares. When mares with delayed uterine clearance were sedated with xylazine before administration of oxytocin, the duration of increased intrauterine pressure was longer than that of normal mares (24.87 +/- 2.52 and 15.81 +/- 2.46, respectively; P < 0.0001). Acepromazine administration had no effect on the oxytocin-induced contraction pattern in normal mares, but there was a decrease in the number of contractions in mares with delayed uterine clearance. The enhanced response to alpha-agonists and -antagonists observed in mares with delayed uterine clearance when the drugs were administered before oxytocin may be due to denervation supersensitivity. After mating, sedation of mares with xylazine is preferred to sedation with acepromazine, as xylazine increases uterine contraction time.