MYOMETRIAL ELECTRICAL-ACTIVITY AND PLASMA-CONCENTRATIONS OF PROGESTERONE, ESTROGENS AND OXYTOCIN DURING LATE PREGNANCY AND PARTURITION IN THE MINIATURE PIG

A multifactorial approach was attempted to establish the sequence of events that lead to parturition in the miniature pig. Uterine electrical activity recorded with chronic electrodes implanted in the uterine muscle was correlated with concurrent maternal plasma levels of progesterone, estrone, estradiol-17β and oxytocin. In 7 sows that farrowed at normal term the period of observation covered the last 3 weeks of pregnancy up to the end of parturition. During late pregnancy the uterine electromyographs (EMGs) consisted of irregular episodes (< 3.5/h) of prolonged phases of activity (> 3.5 min duration) which occurred close to electrodes that had been implanted on uterine segments containing a fetus. Empty parts of the uterus remained relatively inactive. There were no significant changes in the mean plasma steroid concentrations and myometrial activity over the last 3 weeks of gestation. Plasma oxytocin concentrations remained below 1.2 μU/ml in 6 animals that were sampled between 12 and 2 days prior to parturition. Between -24 and -10 h before expulsion of the first piglet, mean concentrations of progesterone had significantly fallen and those of estrogens had increased. This coincided with behavioral changes of the sow (nest-building) but myometrial activity at this stage was similar to that recorded on the previous days. It was only between -9 and -4 h before the first piglet was born that the pattern of the EMGs changed and regular phases of electrical discharges appeared with increasing frequency (7-12/h) in all parts of the uterus. At the same time oxytocin concentrations in peripheral plasma were elevated. During delivery of the piglets the mean frequency of the phases of electrical discharges increased further (18-20/h), their duration being significantly shorter than in the previous few hours. Oxytocin concentrations reached highest values at this stage, estrogen concentrations were already falling and progesterone had reached values below 4 ng/ml in all sows.