 Percutaneous coronary intervention PCI for acute coronary syndromes frequently fails to restore myocardial perfusion despite establishing epicardial vessel patency. Endothelin-1 ET-1 is a potent vasoconstrictor and its expression is increased in atherosclerotic coronary arteries. Our hypothesis is that increased activity of the endogenous endothelin system contributes to microvascular dysfunction, and adjunctive therapy with an endothelin receptor antagonist will result in improved microvascular blood flow. 