Electrical and mechanical uterine activity and gap junctions in peripartal sheep

In 10 chronically instrumented pregnant ewes myometrial electrical activity and intrauterine pressure were measured and plasma levels of 17 beta-estradiol and progesterone were determined in late pregnancy and during and after labor. Myometrial biopsies were obtained to determine gap junction area. Plasma progesterone levels fell 4 days before delivery, and 17 beta-estradiol levels rose sharply 12 hours before delivery. Toward delivery the myometrial electrical pattern changed from infrequent bursts of long duration to frequent bursts of short duration, and the active pressure area of the intrauterine pressure cycles, the apparent conduction velocity, and the rate of rise of intrauterine pressure cycles increased associated with the changes in hormone levels. These changes were related to an increase in gap junction area. It is concluded that these results support the hypothesis that changes in concentrations of steroid hormones lead to an increase in myometrial gap junction area, which improves the coordination of contractile activity of the uterus during labor.