Introduction

The work of Starling left the physiologists with the conviction that the endothelial cells play an important role in the movements of fluids and solutes from the blood to the tissue cells. Debate continues as to the relative importance of active vs. passive mechanisms in determining the exchange rate across given endothelial membranes. In 1980, Furchgott and Zawadzki reported that the relaxations evoked by acetylcholine in the aorta and other arteries of the rabbit depend on the presence of endothelial cells. Indeed, removal of the endothelium either by gentle rubbing or by enzymatic digestion prevented the relaxation evoked by the cholinergic transmitter. In 1981, De Mey and Vanhoutte reported that anoxia evoked contractions of canine arteries which were abolished, or considerably reduced, by the removal of the endothelium. Several neurohumoral mediators (including acetylcholine) and physical stimuli can evoke endothelium-dependent contractions of isolated arteries and veins.