Background: Endothelium-derived nitric oxide (NO) contributes to epicardial coronary artery vasodilation during exercise. However, blockade of NO production does not impair the increase in coronary blood flow (CBF) during exercise, suggesting that NO is not obligatory for exercise-induced coronary resistance vessel dilation. In contrast, the increases in CBF produced by endothelium-dependent agonists are decreased after NO blockade. Consequently, this study was performed to determine whether the increase in coronary NO production in response to agonists is greater than that which occurs during exercise.
Methods and results: We measured the oxidation products of NO (nitrate+nitrite=NO(x)) in aortic and coronary sinus plasma using chemiluminescence to assess NO(x) production across the coronary circulation in chronically instrumented dogs during a 3-stage treadmill exercise protocol and in response to intracoronary administration of the endothelium-dependent agonists acetylcholine (37.5 microg/min) and bradykinin (3.0 microg/min). No coronary NO(x) production 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 NO(x) production measured. In contrast, coronary production of NO(x) was significantly increased in response to endothelium-dependent agonists.
Conclusions: Coronary NO production in response to endothelium-dependent agonists is greater than in response to the increase in shear stress associated with exercise. These findings support previous studies suggesting that NO is not required for the coronary vasodilation that occurs in the normal heart during exercise.