Effect of uterine distension and oestrogen treatment on gap junction formation in the myometrium of the rat.

Gap junctions were present in the myometrium of 1 out of 5 rats on Day 21 of pregnancy, 4 out of 5 on the morning of Day 22, 7 out of 7 during parturition and 1 out of 3 on Day 1 post partum. Their frequency (number per mm membrane) at these time periods increased from 0.30 to 2.20 and 6.48 and then decreased to 0.31. Ten ovariectomized post-partum rats were fitted with an intrauterine balloon in one horn for recording pressure changes. Control rats maintained continuous pressure cycles. In rats given 7.5 micrograms oestradiol-17 beta the frequency of intrauterine pressure cycles decreased from about 50 per hour to 4.3 per hour 15 h later, but the maximum rate of rise of uterine pressure cycles increased significantly from 7.2 +/- 0.95 to 11.3 +/- 1.85 mmHg sec-1 (n = 5, P less than 0.05). The overall number of gap junctions per mm membrane in the oestrogen-treated rats was 7.37 in the horns equipped with balloons and 1.03 in the empty horns, compared with 0.97 and none respectively in the control rats. The increased rate of rise of pressure in the oestrogen-treated myometrium indicated improved coupling between cells and this was associated with a high frequency of gap junctions. Both oestrogen treatment and distension (from the balloon) appeared to cause some gap junctions to form, but numbers equivalent to those at parturition were only obtained in animals in which these two treatments were combined.