EFFECTS OF GLUCOCORTICOIDS ON ESTROGEN-RECEPTOR MESSENGER-RIBONUCLEIC-ACID IN THE PREGNANT OVINE MYOMETRIUM IN-VIVO AND IN-VITRO

We recently reported that estrogen receptor (ER) mRNA is dramatically increased in the sheep myometrium during cortisol-induced premature la bor and term spontaneous labor. In this study, we compared myometrial ER mRNA and ER protein levels in tissues from pregnant sheep during in fusions of two different glucocorticoids (dexamethasone and betamethas one). Tissues from glucocorticoid-infused sheep not yet in labor were compared with tissues from sheep in labor. The population of ER mRNA-c ontaining cells in myometrium obtained from sheep in labor was determi ned by in situ hybridization and compared with that of sheep not in la bor in order to understand how the cellular distribution of ER mRNA ch anged during labor. Finally, in vitro myometrial cell culture and immu nolocalization of glucocorticoid receptor (GR) were used to determine 1) the effects of different steroids on ER mRNA content, 2) whether or not the myometrium is a functional site for glucocorticoid action, an d 3) whether the change in ER mRNA content in the myometrium might be induced by glucocorticoid acting through GR. Increased ER mRNA and pro tein were found only in the myometrium associated with labor. In situ hybridization of ER mRNA and immunolocalization of ER protein showed t hat ER mRNA and protein were mainly located in the smooth muscle cells and endothelial cells of blood vessels. In vitro treatment of culture d myometrial cells with hydrocortisone resulted in a 3.5-fold incremen t in ER mRNA. In contrast, there was no obvious change in ER mRNA when myometrial cells were treated in vitro with either estradiol (10 nM) or progesterone (100 nM) for 24 h. GR was localized exclusively in the pregnant sheep myometrium. We conclude that 1) increased ER protein a nd ER mRNA in sheep myometrium are strongly associated with labor; 2) an increase in the population of ER-positive cells is associated with the increment of ER mRNA during glucocorticoid-induced labor; 3) in th e pregnant sheep myometrium, both the smooth muscle cells and the endo thelial cells of blood vessels are positive for ER protein ER mRNA; an d 4) hydrocortisone is a potent stimulus in vitro for increasing ER mR NA content, presumably acting through the GR present in the pregnant s heep myometrium.