Effects of oestradiol cypionate on spontaneous and oxytocin-stimulated postpartum myometrial activity in the cow

The effects of oestradiol cypionate (ECP) on spontaneous and oxytocin-induced postpartum myometrial activity were measured in four cows using strain gauge transducers (SGT). On the first day after parturition, prior to treatment with ECP, myometrial activity consisted mainly of single-peak contractions (mean frequency 9.6/h, mean duration 141.0s, the majority of the contractions being propagated in a tubocervical direction. Injection of ECP (5 mg i.m.) 18 h after parturition led to suppression of coordinated myometrial activity and the development of sustained low amplitude contractions of reduced frequency (mean 2.9/h, P less than 0.01) and increased duration (mean 422.2 s, P less than 0.05), with multiple superimposed small peaks. In addition, all parts of the uterus tended to contract simultaneously. These changes were apparent by 4 h after treatment and persisted until day 5 after parturition. Injection of oxytocin (25 USP units i.v.) at 24 h after parturition stimulated the reappearance of single-peak coordinated contractions. However, pretreatment with ECP did not enhance the myometrial response to oxytocin.