 The purpose of the study is to assess the importance of a substance called endothelin. Endothelin is produced by coronary arteries. This study examines this substance to determine whether it has an effect on controlling blood flow in coronary arteries. When these arteries release too much endothelin, the blood flow to the heart muscle is reduced and this may be important in heart conditions. This protocol examines an investigational drug called BQ-123 to see if it blocks the effect of endothelin. We assess the blood flow in the coronaries and evaluate the effects of BQ-123. It is anticipated that this endothelin blocker will open up coronary arteries and increase the blood flow to the heart. 