Gap junctions and myometrial steroid hormone receptors in pregnant and postpartum rats: a possible cellular basis for the progesterone withdrawal hypothesis.

Myometrial gap junctions, levels of cytosol and nuclear estradiol and progesterone receptors, and serum estradiol and progesterone levels were determined simultaneously in pregnant and postpartum rats. Between days 15 to 20 after conception, levels of progesterone nuclear receptors decreased (776.8 +/- 88.5 versus 241.1 +/- 56.5 fmol/mg of DNA, p less than 0.05). The mean serum progesterone also fell by about 50% during this period, but variation in individual levels between days 15 to 20 did not allow for this group to achieve statistical significance until day 21. As the progesterone nuclear receptors decreased, estradiol remained stable, but estradiol nuclear receptors increased (day 21 versus 22: 1710.6 +/- 61.1 versus 3254.8 +/- 203.8 fmol/mg of DNA, p less than 0.05), preceding the increase in gap junctions observed at parturition (day 21 versus 22: 2.0 +/- 0.2 versus 8.0 +/- 3.2 [per 1000 micron plasma membrane], p less than 0.05). Gap junctions fell to prepartum levels by 12 hours, when progesterone nuclear receptors were markedly increasing (6 versus 12 hours post partum: 637.7 +/- 324.8 versus 1509.6 +/- 283.2 fmol/mg of DNA, p less than 0.05). Relationships between gap junctions and cellular estradiol and progesterone nuclear receptors are clearer than could be forecast by circulating hormone measurements alone, and may offer a cellular basis for the role of progesterone in controlling labor.