Endothelial nitric oxide synthase is differently expressed in human endometrial vessels during the menstrual cycle

Endogenous nitric oxide (NO) can be synthesized by endothelial cells and ca n act as a potent vasodilator. We investigated endothelial nitric oxide syn thase (eNOS), one of the three different enzymes responsible for the synthe sis of NO by immunohistochemical methods throughout the menstrual cycle on 34 endometrial samples and compared its detection with the von Willebrand F actor (vWF) as a reliable marker molecule of the endothelium on serial sect ions. Immunoreactivity for eNOS was clearly localized in various types of a rterial and venous endothelial cells as well as in capillaries. In addition , in some samples there was a positive staining in endometrial glandular ep ithelium. There was no staining in endometrial fibro;blasts or in myometria l smooth muscle cells. Whereas the endothelium was constantly stained by th e monoclonal antibody against vWF, eNOS was not always expressed in the end othelial lining of the vessels during the menstrual cycle. The number of ve ssels positively stained for eNOS increased gradually during the proliferat ion phase and most of the vessels were positive in the early secretory phas e. These results suggest that its markedly increased expression during the early secretory phase of the menstrual cycle indicates a physiological sign ificance.