Changes in hormone levels and gap junctions in the rat uterus during pregnancy and parturition

To study the temporal relationship between changes in hormone levels with the development of myometrial gap junctions, rats were sacrificed to collect uterine vein blood and uterine tissues daily from Day 15 to Day 20 of pregnancy, every 3 h on Day 21, during delivery, and 3 h after parturition. Levels of progesterone, estradiol and prostaglandin F2 alpha (PGF2 alpha) were measured by specific radioimmunoassay methods, and myometrial gap junctions were measured by quantitative electron microscopy. Levels of progesterone declined markedly on Day 19 of pregnancy, at least 60 to 70 h before the expected time of parturition and the decrease was more rapid in uterine tissue than in plasma. Levels of estradiol started to increase after Day 19, showed a marked rise on Day 21, and peaked at the time of parturition. Levels of PGF2 alpha also increased rapidly after Day 20, when progesterone levels had already dropped by 67% and 84% of Day 15 levels in plasma and tissue samples, respectively. Gap junctions, identified as 5- or 7-lined structures with a gap of about 2 nm at 100,000X magnification were present in the pregnant uterus. However, their number, size and area were increased significantly on Day 21 and during parturition. Increased numbers of gap junctions were still present in rats killed 3 h after parturition. These studies demonstrate that a decrease in progesterone levels followed by increases in estradiol and PGF2 alpha are coincident with the formation of gap junctions which in turn may coordinate the increased uterine activity required for parturition.