Function of myometrial graft at time of parturition in the rabbit

Thirteen rabbits equipped with ear chambers containing myometrial grafts were studied during late pregnancy, spontaneous and induced parturition and early puerperium. Spontaneous electrical activity of the graft was recorded by means of F.-M. telemetry transmission in the unrestrained animals. Furthermore the reaction to intravenously injected oxytocin (threshold determination) was recorded daily. During the last 5 days before spontaneous delivery increasing spontaneous activity of the tissue was observed. During the same period the response to oxytocin increased. Concomitantly with the delivery of the litter a period of rhythmic activity was observed. This period had a sudden onset and lasted 12-55 min, while the delivery itself only lasted 6-16 min. Similar activity patterns could be evoked in the early post-partum period by 100 mU oxytocin given iv. In 3 rabbits, doses from 80-300 mU oxytocin induced complete delivery. During the 48 hours' period after delivery spontaneous activity (single spikes and series of bursts) was observed. The response to oxytocin was further increased. It is concluded that as the graft is not influenced by nervous regulation, mechanical influence or local humoral factors the spontaneous parturition and the change in excitability must therefore be due to one (or more) systemic, humoral factor(s).