OXYTOCIN AND THE ONSET OF LABOUR IN RABBITS

Abstract

SUMMARY The course of spontaneous delivery has been followed by continuous recording of uterine motility in conscious unrestrained rabbits from the 27th or 28th day post coitum until day 1 after delivery, which occurred on the 30th–31st day. The recordings were taken by means of an intra-uterine balloon. The results were similar whether the horn containing the balloon was non-pregnant, pregnant but with all the foetuses removed, or pregnant with only one foetus replaced by the balloon. Although the spontaneous motility increased somewhat within the last 48 hr. before delivery it remained irregular and of low intensity. The typical strong oestrogen-dominated activity of the 'parturient' rabbit uterus did not start until 4–6 hr. after delivery. The sensitivity to oxytocin of the uterus increased very markedly within the last 48 hr. before parturition and the delivery of a living litter was preceded by an extremely strong, oxytocin-like response in nine out of ten rabbits. This response could not be distinguished from that produced by a single i.v. injection of 250–300 m-u. of synthetic oxytocin. The young were born in rapid succession a few minutes after the beginning of this oxytocin-like response, and the delivery of the whole litter was completed in 11 min. on the average. No additional release of an oxytocin-like substance could be observed concomitantly with the delivery of the individual foetuses in these nine rabbits. One of the ten rabbits used had a prolonged labour and the foetuses were delivered dead several hours apart. In this instance the strong oxytocin-like response of the uterus was not observed.