[Gap junction formation in the myometrial muscle cells of ovariectomized mid-pregnant rats during premature labor induced by estradiol].

In order to obtain a better understanding of gap junction (GJ) formation in the myometrial muscle cells of rats during the time of premature delivery, the myometrial GJs and nuclear estradiol receptor (nE2R) levels, as well as serum estradiol (E2) and progesterone (P) levels were simultaneously determined in mid-pregnant ovariectomized (OVX) rats both with and without E2 administration. Artificial premature deliveries were not induced in either the rats which were OVX on day 15 of gestation with the E2 treatment, or which were OVX on day 16 with the vehicle treatment. However, there were significant decreases in serum P and significant increases in the number of myometrial GIs in these rats compared to the levels of the non-OVX pregnant rats treated with the vehicle. These results suggest that serum P withdrawal and GJ formation are not sufficient to induce premature delivery, and that other factors are involved. Artificial premature delivery was induced in the rats which were OVX on day 16 with the E2 treatment. They were given 2.5 micrograms E2 twice daily, for two days. A significant decrease in serum P, a marked increase in the number of GJ and a significant correlation between the myometrial GJs and serum E2 levels were found in both the "premature" delivery rats and the "prior to" delivery rats. A significant difference between the nE2R levels of the two groups was found, however. These profiles closely resembled those of the full-term rats during spontaneous delivery as we reported earlier.