Endothelial and Adrenergic Control

Within the last 20 years, it has become clear that the endothelium plays a key role in maintaining vascular tone within all vascular beds of the human body. Since the fi rst in vitro study showing the obligatory role of the endothelium in mediating acetylcholine-induced vasodilatation in 1980 [66], the study of endothelium-derived vasoactive substances has become an important research area. Nowadays, it is assumed that the endothelium produces a large variety of vasodilators and vasoconstrictors. Only if there is a balance between the production of endotheliumderived vasodilators and endothelium-derived vasoconstrictors is a vessel under normal physiological tone (Fig. 13.1). This also holds true for the eye, where numerous in vitro animal and human studies have proven the concept of endothelial control of blood fl ow in the ocular vascular systems.