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Intra-uterine pressure changes have been successfully recorded in ewes in late pregnancy and during parturition using endoradiosondes (Hindson, Schofield, Turner & Wolff, 1965; Hindson, Schofield & Turner, 1968). This device is superior to intra-uterine balloons, for the sonde is not fixed in the uterine cavity and has no connexion with the exterior. Furthermore, in rabbits, recordings may be made with the aerial around the cage, not the animal, as is necessary to pick up adequate signals in the sheep. Thus the animal is unrestrained and undisturbed, being free to move in the cage while continuous recordings are made over several days. It was thought appropriate, therefore, to extend radio-telemetry recordings to the rabbit and supplement information already obtained with intra-uterine balloons (Fuchs, 1964; Porter & Schofield, 1966).
Large New Zealand White rabbits were used in order to diminish the sonde/foetus size ratio. On the 23rd day of pregnancy the
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SUMMARY
Rabbits were injected with oestradiol-17β for 3–6 consecutive days at different times during pregnancy. The course of pregnancy was followed thereafter, or, in acute experiments, the staircase effect and the oxytocin sensitivity were determined on the day following the last injection. Results showed that doses of oestrogen sufficient to interrupt pregnancy through an effect on the endometrial component of the uterus did not directly antagonize the influence of endogenous progesterone on the myometrium. Diminution of the progesterone block indicated by the oxytocin sensitivity of the myometrium is considered to be due to degeneration of the placenta and hence of its ability to maintain the corpus luteum.
The dosage of oestrogen necessary to interrupt pregnancy was higher in late pregnancy. This is in agreement with the finding that oestrogen production increases steadily from mid-pregnancy to term.
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SUMMARY
Small balloons attached to polyethylene tubing were inserted into either end of one uterine horn in rabbits on about the 22nd day of pregnancy. The tubing was threaded subcutaneously and brought out through an incision at the back of the neck; recordings of intra-uterine pressure changes were made up to and including parturition. There were no pressure changes which could be ascribed to contraction of the myometrium until an average of 31 hr. before delivery of the first foetus but there was considerable variation. Pressure waves recorded from the two balloons were often different in frequency and amplitude and were seldom synchronous. In none of the rabbits did one end of the horn develop 'parturient pressure' before the other and, in general, pressure at the ovarian end of the uterus was higher than at the cervical end. The rabbits could be divided into two groups: (I) those in which the onset of delivery was abrupt and preceding pressure waves were of low amplitude; (II) those in which the pressure waves increased gradually in intensity over several hours before delivery. Both the duration of delivery and the mortality rate of the foetuses were significantly greater in group II than in group I. The results in group I are consistent with the concept that in the normal rabbit parturition is initiated by a release of oxytocin. It is suggested that in group II oxytocin release failed to occur and that this failure was due to 'emotional stress' and possibly circulating adrenaline.
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SUMMARY
Actomyosin concentrations and tensions were measured in the rabbit uterus during pregnancy and in ovariectomized hormone-treated animals. The results showed that actomyosin concentration and tension in g./mm.2 longitudinal muscle followed similar courses during pregnancy. This was not so in the hormone-treated animals. Small quantities of oestrogen were sufficient in ovariectomized animals to produce a major difference between the untreated and treated animals. With higher doses only the concentration of the actomyosin increased linearly.
During pregnancy a significant correlation was found between the number of foetuses in each horn, the quantity of actomyosin and the weight of the horn.
The major increase in tension and actomyosin concentration occurred in the last half of pregnancy when the increase in uterine volume accelerates. The importance of this growth, as opposed to hormonal environment, was further emphasized by experiments in unilaterally pregnant rabbits in which both these quantities were significantly greater in the parous horn than in the non-parous horn.
The effects of progesterone were also investigated.
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SUMMARY
Intra-uterine pressure changes were recorded at the end of pregnancy and during parturition in eight ewes by means of endo-radiosondes implanted in one uterine horn some weeks previously. Recordings were made at frequent intervals so that the progress of parturition could be carefully monitored. When the early stages of parturition were established, a single injection of 80 mg. progesterone in oil was made in six ewes. As a result, parturition was delayed and the intra-uterine pressure waves declined or were abolished. Delivery of the lambs occurred up to 7 days later. In two ewes the injection was given somewhat later during parturition and, in these animals, the depressant effect of progesterone was much less. The increased intra-uterine pressure waves which followed digital examination of the cervical canal were abolished for more than 20 hr. in all eight ewes. Determination of threshold doses of oxytocin showed wide variations and revealed no more than a general trend of increasing myometrial sensitivity with approaching parturition, a trend which was temporarily reversed in five ewes by the injection of progesterone. The eight ewes delivered 13 viable lambs and one dead lamb.
