Effect of oxytocin on the pattern of electromyographic activity in the oviduct and uterus of the ewe around oestrus.

This study tested the hypothesis that the administration of oxytocin in doses equivalent to endogenous concentrations at and around oestrus could affect uterine and oviductal muscular activity at the time of gamete transport. Four ewes were fitted with recording electrodes in the left ampulla, ampullary-isthmic junction, utero-tubal junction and uterine horn. After surgical recovery, recordings from conscious free-standing animals were made for up to 10 h per day from Day -3 to Day +3 relative to oestrus in each ewe. Daily blood samples were taken for progesterone radioimmunoassay, and a vasectomized ram used to assess oestrus. A range of physiological doses of oxytocin (10-100 mU), or control saline injections were given intravenously. Electromyographic (EMG) activity was measured before and after injections. Spontaneous activity throughout the reproductive tract was low on Day -3 but increased to peak at oestrus (P < 0.05), when the pattern of activity consisted of short (2-10 s) co-ordinated high amplitude bursts (2-5 min-1). After oestrus, as overall activity declined, longer episodes of activity appeared but these occurred with a much slower frequency (1-4 h-1). Responsiveness to oxytocin was greatest on the day of oestrus at all electrode sites. Elevated responsiveness (relative to Day -3, the late luteal phase) was seen from Day -1 to Day +2 at the ampullary-isthmic junction and uterus, but on the day of oestrus only at the ampulla and utero-tubal junction. Duration of increased EMG activity after oxytocin injection ranged from 5 to 20 min. These results show for the first time that the uterine and oviductal musculatures of the ewe in vivo reached a peak in sensitivity to physiological concentrations of oxytocin at oestrus. When combined with information on oxytocin receptor populations and endogenous circulating concentrations, this suggest that endogenous oxytocin could influence oviduct and myometrial activity at this time.