Background and Objective: Acute coronary syndrome is characterized by compromised blood flow at the epicardial and microvascular levels. We have previously shown that thrombectomy in ST-elevation myocardial infarction (STEMI) accelerates ST-segment resolution, possibly by preventing distal embolization. Therefore, we analyzed the vasoconstrictor concentration of acute coronary thrombi, and found high concentrations of endothelin (ET) which correlated with the magnitude of ST-segment resolution within one hour of percutaneous coronary intervention (PCI). Furthermore, ET-receptor blockade by tezosentan significantly repressed vasoconstriction in an in-vitro model using porcine coronary artery rings incubated with coronary thrombus homogenates extracted from STEMI patients.