Simultaneous recording of pituitary oxytocin secretion and myometrial activity in oestrous mares exposed to various breeding stimuli.

The aim of this study was to investigate the effect of different breeding stimuli on uterine contractility and pituitary oxytocin release in five oestrous mares, in order to design better treatments for mares with defective physical uterine clearance mechanisms. Electrodes and strain gauges were implanted surgically on the uterine myometrium and myometrial activity was monitored with a Grass polygraph. A catheter was placed non-surgically in the intercavernous sinus of each oestrous mare to sample pituitary venous blood and a second catheter was placed in the jugular vein. Continuous sampling was performed for 2 h to determine the baseline value and during sequentially applied stimuli of: (i) stallion call; (ii) visual contact with a stallion; (iii) active teasing; and (iv) artificial insemination. No association was observed between uterine contraction and pituitary oxytocin release episodes during baseline recording. Exposure of the mares to any of the breeding stimuli was associated with rapid onset of myometrial contractions (P < 0.0001). The application of a stimulus significantly increased pituitary oxytocin release (P < 0.02) and contraction duration (P < 0.05), and the response to artificial insemination was greater than the responses to the other treatments (P < 0.05). The onset of oxytocin secretion was generally simultaneous with the increase in uterine contraction, rather than preceding it, and in some instances the responses were dissociated, making a cause and effect relationship less likely. In conclusion, interactions between stallions and mares are important components of the uterine clearance mechanism around the time of breeding. Knowledge of these physiological responses may be useful in designing therapeutic regimens for mares with endometritis due to defective uterine contractions.