Coronary nitric oxide production in response to exercise and endothelium-dependent agonists

Background-Endothelium-derived nitric oxide (NO) contributes to epicardial coronary artery vasodilation during exercise. However, blockade of NO produ ction does not impair the increase in coronary blood Row (CBF) during exerc ise, suggesting that NO is not obligatory for exercise-induced coronary res istance vessel dilation. In contrast, the increases in CBF produced by endo thelium-dependent agonists are decreased after NO blockade, Consequently, t his study was performed to determine whether the increase in coronary NO pr oduction in response to agonists is greater than that which occurs during e xercise. Methods and Results-We measured the oxidation products of NO (nitrate+nitri te=NOx) in aortic and coronary sinus plasma using chemiluminescence to asse ss NOx production across the coronary circulation in chronically instrument ed dogs during a 3-stage treadmill exercise protocol and in response to int racoronary administration of the endothelium-dependent agonists acetylcholi ne (37.5 mu g/min) and bradykinin (3.0 mu g/min). No coronary NOx productio n could be detected at rest or during the first 2 stages of exercise; only at the highest level of exercise was a small increase in coronary NOx produ ction measured. Zn contrast, coronary production of NOx was significantly i ncreased in response to endothelium-dependent agonists. Conclusions-Coronary NO production in response to endothelium-dependent ago nists is greater than in response to the increase in shear stress associate d with exercise. These findings support previous studies suggesting that NO is not required for the coronary vasodilation that occurs in the normal he art during exercise.