Contribution of Nitric Oxide to Metabolic Coronary Vasodilation in the Human Heart

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Abstract
Background The vascular endothelium contributes to smooth muscle relaxation by tonic release of nitric oxide. To investigate the contribution of nitric oxide to human coronary epicardial and microvascular dilation during conditions of increasing myocardial oxygen requirements, we studied the effect of inhibiting nitric oxide synthesis with N G -monomethyl- l -arginine (L-NMMA) on the coronary vasodilation during cardiac pacing in patients with angiographically normal coronary arteries with and without multiple risk factors for coronary atherosclerosis. Methods and Results In 26 patients with angiographically normal or near-normal epicardial coronary arteries, metabolic vasodilation was assessed as a change in coronary vascular resistance and diameter during cardiac pacing (mean heart rate, 141 beats per minute). Endothelium-dependent vasodilation was estimated with intracoronary acetylcholine and endothelium-independent dilation with intracoronary sodium nitroprusside and adenosine. These measurements were repeated after 64 μmol/min intracoronary L-NMMA. At rest, L-NMMA produced a 16±25% (mean±SD) increase in coronary vascular resistance ( P <.05) and an 11% reduction in distal epicardial coronary artery diameter ( P <.01), indicating tonic basal release of nitric oxide from human coronary epicardial vessels and microvessels. Significant inhibition of pacing-induced metabolic coronary vascular dilation occurred with L-NMMA, coronary vascular resistance was 38±56% higher ( P <.03), and epicardial coronary dilation during control pacing (9±13%) was converted to constriction after L-NMMA and pacing (−6±9%, P <.04). L-NMMA specifically inhibited endothelium-dependent vasodilation with acetylcholine (coronary vascular resistance was 72% higher [ P <.01]) but did not alter endothelium-independent dilation with sodium nitroprusside and adenosine. Nine patients had no major risk factors for atherosclerosis, defined as serum cholesterol >240 mg/dL, hypertension, or diabetes. The remaining 17 patients with one or more of these risk factors had depressed microvascular vasodilation during cardiac pacing (coronary vascular resistance decreased by 13% versus 36% in those without risk factors, P <.05). The inhibitory effect of L-NMMA on pacing-induced coronary epicardial and microvascular vasodilation was observed only in patients without risk factors, whereas those with risk factors had an insignificant change, indicating that nitric oxide contributes significantly to pacing-induced coronary vasodilation in patients free of risk factors and without endothelial dysfunction. Patients with risk factors also had reduced vasodilation with acetylcholine (40±28% versus 68±8% decrease in coronary vascular resistance, P <.01), but the responses to sodium nitroprusside were similar in both groups. Conclusions During metabolic stimulation of the human heart, nitric oxide release contributes significantly to microvascular vasodilation and is almost entirely responsible for the epicardial vasodilation. This contribution of nitric oxide is reduced in patients exposed to risk factors for coronary atherosclerosis and leads to a net reduction in vasodilation during stress. An important implication of these findings is that reduced nitric oxide bioavailability during stress in patients with atherosclerosis or risk factors for atherosclerosis may contribute to myocardial ischemia by limiting epicardial and microvascular coronary vasodilation.