Steroid hormone regulation of rat myometrial gap junction formation: Effects on cx43 levels and trafficking

The formation of myometrial gap junctions coincides with onset of labor in many mammalian species, including humans. The assembly of gap junction protein into functional gap junction plaques is a final step in a cascade that begins with estrogen-dependent expression of the connexin 43 (cx43) gene and continues with synthesis of cx43 in the rough endoplasmic reticulum (RER) and transport to the Golgi, followed by its tricking to the plasma membrane and its assembly into functional gap junctions. Moreover, in several models of preterm labor in rats, precocious synthesis, ticking and assembly of cx43 follow an increase in the estrogen:progesterone ratio. The actions of these steroids on cx43 expression, gap junction formation, and labor led us to consider whether or not the cascade of cx43 expression and gap junction assembly typical of preterm and term labor would be disrupted by manipulations that inhibit labor through experimental reduction of the estrogen:progesterone ratio. Ovariectomized and non-ovariectomized pregnant rats were treated with minimal doses of progesterone or the anti-estrogenic compound ICI 182780 over a time course sufficient to inhibit labor. We found that cx43-positive gap junction formation was prevented in all animals treated with ICI 182780 or progesterone but that the mechanism by which this disruption occurred was different in anti-estrogen- and progesterone-treated animals. We found that ICI 182780 significantly inhibited the typical rise in myometrial cx43 concentrations normally observed just before labor. In contrast, it was surprising to find that significant cx43 was synthesized in myometrium of progesterone-treated intact and ovariectomized animals even though labor was inhibited. However, we found that the ticking of myometrial cx43 from the Golgi and assembly into gap junctions at the plasma membrane were suppressed in these progesterone-treated animals, providing further support for the hypothesis that it is not synthesis of cx43 per se but ticking of cx43 to the plasma membrane and its assembly into gap junctions that are required for effective synchronized myometrial contractions typical of labor.