[Pregnancy and labor in the mare: uterine activity and endocrinology]

Electromyographic (EMG) recordings were made during the last two weeks of pregnancy from two mares. Four bipolar EMG electrodes were implanted in the uteri of the mares; near the tubo-uterine junction and bifurcation of the pregnant horn, in the body of the uterus and near the cervix. Plasma samples were collected every 4 hours during the same period and more intensely during parturition. Estradiol 17 beta, progesterone, PGF2 alpha metabolite and oxytocin were measured by radioimmunoassay. During the last week preceding delivery, EMG activity was elevated and was greatest at night. EMG activity was further increased during the last 24 hours before delivery of the foal and reached its highest intensity for 7 to 13 hours immediately prepartum. This period of intense activity is described as stage I of parturition. EMG activity decreased to very low levels 2 to 4 hours before delivery but abruptly increased again at rupture of the choriollantois and continued through delivery when activity decreased again until delivery of the placenta. The ratio of estradiol 17 beta to progesterone (E 17 beta/P) increased through the last week prepartum due to an increase in the level of estradiol 17 beta concentrations and during the last 24 hours the change in the E 17 beta/P ratio was due to a significant decrease in progesterone. Oxytocin and PGF2 alpha metabolite increased abruptly just before rupture of the fetal membranes and there is some evidence that oxytocin increased prior to PGF2 alpha metabolite. We hypothesize that the increasing E 17 beta/P ratio allows the evolution of labor to occur during the daylight hours preceding parturition.(ABSTRACT TRUNCATED AT 250 WORDS)