to the Endothelin System David M. Pollock ascular endothelial cells form a monolayer lining in all of the blood vessels of V the circulation. Initially, they were thought to function as a crude filter allow ing nutrients from the blood stream to diffuse through to the underlying tissues without letting proteins or blood cells escape. We now know that endothelial cells are important regulators of circulatory function, due in large measure to their rec ognized ability to synthesize and release many factors that regulate vascular smooth muscle tone. Endothelial-derived factors, including...