A marked gender difference exists in the prevalence and The normal vascular endothelium is only one cell layer severity of cardiovascular disease, even after adjustment thick, separating the blood and vascular smooth muscle. for traditional vascular risk factors, which are more Theendotheliumrespondstophysicalandchemicalstimuli prevalent in males [1]. This has led to an interest in the via the synthesis and/or release of regulatory substances role of sex steroids themselves in the promotion or affecting vascular tone and growth, thrombosis and throminhibition of atherogenic events. As endothelial dysfunc- bolysis and platelet and leukocyte interactions with the tion is an early and important event in atherogenesis [2] endothelium. Substances released by the endothelium and appears to predict adverse coronary outcomes [3], include nitric oxide (NO), prostacyclin, endothelins, intergender differences in endothelial function and the effects leukins, endothelial growth factors, adhesion molecules, of hormonal therapy on vascular function have been the plasminogen inhibitors and von Willebrand factor [4, 8–focus of considerable research interest. 10].

Thenormalvascularendotheliumregulatesarterialtone, The pivotal role of the endothelium in regulating plateletandleukocyteinteractions, coagulation, fibrinolysis vascular smooth muscle tone has only recently been and vascular growth [4]. Measurement of endothelial appreciated [11]. The existence of an endothelium-derived function in the systemic arteries has become established as relaxing factor (EDRF) was first postulated by Furchgott an important method for the detection of early (pre-symp- and Zawadzki [12], when it was observed that rabbit aortic tomatic) arterial abnormalities in humans [5]. Ultrasound- rings relaxed to acetylcholine only in the presence of an detected endothelial dysfunction in peripheral arteries intact endothelium. NO, the endothelium-derived relaxing correlates significantly with coronary endothelial dysfunc- factor, was subsequently shown to maintain a low resting tion [6], as well as with the extent and severity of coronary arterial tone in both the peripheral [13] and pulmonary [14] atherosclerosis [7]. The availability of a non-invasive circulations. method for the reproducible measurement of endothelium- NO is synthesized from L-arginine by the enzyme, nitric dependent dilatation has facilitated the study of the interac- oxide synthase (NOS)[15], a reaction which can be