In vascular diseases, endothelial dysfunction is a systemic pathological state of the endothelium. The main cause of endothelial dysfunction is impaired bioavailability of nitric oxide,[1] In addition to acting as a semipermeable membrane, the endothelium is responsible for maintaining vascular tone and regulating oxidative stress by releasing mediators, such as nitric oxide, prostacyclin and endothelin, and by controlling local angiotensin-II activity.[2][3]

Reference
Wiki: Endothelial dysfunction