Endometrial and Myometrial Expression of Nitric Oxide Synthase Isoforms in Pre- and Postmenopausal Women1

Expression of nitric oxide synthase (NOS) protein was examined by Western immunoblot analysis and immunohistochemistry in the endometrium and myometrium of 19 premenopausal and 18 postmenopausal women undergoing hysterectomy for benign gynecological reasons. The predominant isoform of NOS in the human uterus was endothelial NOS (eNOS). Using immunohistochemistry, eNOS was localized predominantly to the glandular epithelium and endometrial microvascular endothelium. eNOS was scant and inconsistently detected in endometrial stromal cells. In the myometrium, eNOS was predominantly found in smooth muscle cells (myocytes) and was also detected in the microvascular endothelium. Neuronal NOS was not detectable by immunohistochemical techniques, and inducible NOS (iNOS) was only detectable in occasional specimens, although more often in secretory specimens. iNOS, when present, was predominantly found in glandular epithelium and occasional stromal cells. Myometrial iNOS was scant and not consistently detected. By Western immunoblot analysis, neuronal NOS or iNOS was not detected. We observed a unique menstrual cycle-dependent expression of eNOS that was different in the endometrium compared to the myometrium and was independent of uterine pathology. In the endometrium, there was 62% higher expression of eNOS during the secretory phase (P = 0.00085) compared to the proliferative phase, whereas in the myometrium, there was 74% greater expression of eNOS in the proliferative phase (P = 0.03) compared to the secretory phase. Within the secretory phase, maximal endometrial eNOS expression was found in the midportion, whereas in the myometrium, highest eNOS expression occurred during the late secretory phase. In postmenopausal women not treated with hormones, a significant reduction in endometrial and myometrial expression of eNOS occurred, which was reversed by continuous hormone replacement therapy. In summary, both endogenous ovarian steroids and exogenous sex hormones influence uterine eNOS expression. Our results suggest that estrogen may regulate myometrial eNOS, whereas progesterone or a combination of estrogen and progesterone may be more important in regulating endometrial eNOS, and NO may be a critical mediator of sex steroid actions in the human uterus.