A 59-year-old man experiences acute chest pain and is rushed to the emergency room. Laboratory studies and ECG demonstrate an acute myocardial infarction; however, coronary aery angiography performed 2 hours later does not show evidence of thrombosis. Which of the following mediators of inflammation causes relaxation of vascular smooth muscle cells and vasodilation of aerioles at the site of myocardial infarction in this patient ?