PLATELETS AND VASCULAR OCCLUSION Platelets, endothelium, and vasospasm

prostacyclin, a potent antiaggregatory substance with vasodilator properties. It also activates (e.g., angioten-sin I) or inactivates (e.g., bradykinin, serotonin) a number of vasoactive substances present in the blood (figure 1). Thus, it is far from being a passive diffusion barrier, but it can actively alter the amounts of vasoac-tive substances reaching the deeper layers of the blood vessel wall. In addition, endothelial cells can modulate powerfully the responsiveness of the vascular smooth muscle of the tunica media. Indeed, Furchgott and Zawadzkil first noted that rings of rabbit aorta, con-tracted with norepinephrine, would relax upon expo-sure to acetylcholine if they were handled carefully to preserve the endothelial layer; if the endothelium was simply mechanically rubbed off, no relaxation wa