 There are many substances that influence the diameter of the coronary artery and a number of these are actually released by the lining of the coronary arteries referred to as the endothelium. Over the past 15 years, our laboratory has been instrumental in establishing the role of endothelium-released relaxing factors which relax open the coronary arteries. We are now focussing our attention on factors which constrict the coronary artery, in particular a substance called endothelin-1 ET-1. This potent constrictor substance has been found to accumulate at coronary artery sites which may produce unstable angina ie the culprit lesion or stenosis. Hence ET-1 may produce localized constriction of a coronary stenosis thereby further narrowing the remaining lumen and cutting off blood flow to the heart muscle and thus leading to unstable angina. 