Hormonal control of gap junction formation in sheep myometrium during parturition.

The ultrastructure of myometrial tissues from nonpregnant, pregnant and postpartum sheep was quantitatively examined for the presence of gap junctions between smooth musde cells. Serum concentrations of estrone, estradiol and progesterone were determined in some of the sheep prior to and at the time of sacrifice. Gap junctions were not present in any tissue from nonpregnant animals or in tissues of pregnant animals at 74-100 days gestation. Small gap junctions were present in some of the tissues from pregnant animals at 127-132 days gestation. Larger gap junctions were present in increased frequency in animals at term and immediately postpartum. The formation of gap junctions correlated with a rise in fetal estrogens, a fall in maternal progesterone and increases in fetal and maternal estrogen/progesterone ratios. These results are consistent with the hypothesis that the hormonal changes which occur near the end of pregnancy bring about the formation of gap junctions and the activation of the myometrium.