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The possibility of a coexistence of coronary arteriolar constriction mediated by the renin-angiotensin system and myocardial ischemia was evaluated. Left anterior descending coronary artery was cannulated and perfused at normal (mean aortic), intermediate (50 mm Hg), and low (30–40 mm Hg) pressure in analogy to a progressive coronary stenosis. Lactate production was present at low coronary pressure indicating myocardial ischemia. In control animals (n = 18), mean coronary conductance was higher (p < 0.005) at intermediate than at high coronary pressure consistent with autoregulation of coronary flow. Coronary conductance was lower (p < 0.05) at low than at intermediate coronary pressure, indicating coronary constriction during myocardial ischemia. Adenosine (20 ug/kg per min i.e., n = 6) resulted in higher coronary conductance, suggesting coronary vasodilator reserve even at low coronary pressure. In-domethacin (5 mg/kg i.v., n = 12) resulted in low coronary conductance: however, the increase at intermediate (auto regulation) and the decrease (constriction) at low pressure was maintained. Plasma renin activity increased, and saralasin (0.1 μg/kg per min i.e.) and captopril (0.25 mg/kg i.v.) acted as coronary vasodilators in various models of myocardial ischemia. Captopril limited myocardial infarct size at 6 hours of coronary occlusion, diminished flow repayment and prevented lactate production after 30 s of coronary occlusion, and abolished the deterioration of myocardial function during myocardial ischemia induced by coronary hypoperfusion and atrial pacing. Thus, (a) myocardial ischemia does not generally represent a state of maximal coronary dilatation, (b) The renin-angiotensin system is activated by myocardial ischemia and (c) may exert a coronary constrictive tone. (d) Captopril was beneficial in experimental myocardial ischemia.