© 1996 American Heart Association, Inc.
Articles |
Role of Endothelium-Derived Nitric Oxide in Coronary Vasodilatation Induced by Pacing Tachycardia in Humans
the Research Institute of Angiocardiology and the Cardiovascular Clinic, Faculty of Medicine, Kyushu University, Fukuoka, Japan.
Correspondence to Kensuke Egashira, MD, Research Institute of Angiocardiology and Cardiovascular Clinic, Kyushu University School of Medicine, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-82, Japan.
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Abstract |
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Endothelium-derived NO contributes to the control of coronary perfusion. We investigated the roles of NO in the metabolic coronary vasodilatation induced by rapid pacing in humans. We evaluated the dilatation of large epicardial and resistance coronary arteries during rapid atrial pacing before and after intracoronary infusion of NG-monomethyl-L-arginine (L-NMMA), an inhibitor of NO synthesis, in 19 patients without significant coronary artery disease. The diameter of the large epicardial coronary artery and coronary blood flow (CBF) were assessed by quantitative coronary arteriography and by a Doppler flow velocity measurement. An increase in the heart rate increased CBF (P<.01) and the coronary artery diameter (P<.05). L-NMMA at a total dose of 200 µmol reduced basal CBF but did not significantly affect basal coronary artery diameter, arterial pressure, or heart rate. L-NMMA inhibited the pacing-induced dilatation of the large coronary arteries (P<.05) but did not affect pacing-induced increases in CBF. L-NMMA inhibited the acetylcholine-induced increase in CBF (P<.01) and acetylcholine-induced dilatation of the large epicardial coronary artery (P<.05). These results show that the contribution of NO to the metabolic vasodilatation during rapid pacing may differ between large epicardial and resistance coronary arteries in patients without significant coronary artery disease.
Key Words: nitric oxide • coronary circulation • endothelium-derived relaxing factor • acetylcholine
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Introduction |
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In the coronary circulation, myocardial perfusion under physiological conditions is determined by the interaction of several mechanisms, such as the myocardial metabolic state and endothelium-derived relaxing factors.1 2 3 Recent reports have suggested that ATP-sensitive K+ channels may be involved in metabolic coronary vasodilatation in the dog.4 5 In addition, the vascular endothelium is important in the control of coronary blood flow (CBF) by releasing a variety of endothelium-derived substances.6 7 8 NO, an important endothelium-derived relaxing factor, is synthesized from L-arginine by the enzyme NO synthase.9 The in vivo administration of inhibitors of NO synthase (NG-monomethyl-L-arginine [L-NMMA] or N
-nitro-L-arginine methyl ester [L-NAME]) attenuates acetylcholine-induced vasodilatation.10 11 Therefore, impairment of NO activity in pathological conditions may lead to impaired coronary perfusion.12 13 14 15 Nabel et al12 found that metabolic dilatation of resistance coronary arteries in response to rapid atrial pacing was impaired in patients with angiographic evidence of atherosclerosis. Thus, it is possible that NO may contribute to metabolic coronary vasodilatation. Some studies have shown that an inhibitor of NO synthase significantly reduced the metabolic coronary vasodilatation induced by rapid pacing in dogs16 and humans,17 whereas others have found no significant effect in dogs in vivo.11 18 Thus, the mechanisms of metabolic coronary vasodilatation in the human coronary circulation are not well understood. The role of NO in metabolic coronary vasodilatation remains to be further clarified. In the present study, we investigated the role of NO in metabolic coronary vasodilatation induced by rapid atrial pacing in patients without significant coronary artery disease.
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Materials and Methods |
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Study Population
We investigated 19 patients with no angiographic evidence of significant coronary artery stenosis who were undergoing coronary arteriography for evaluation of chest pain (Table 1
). All patients had a negative treadmill exercise test and a negative thallium myocardial scintigraphy stress test. Patients with previous myocardial infarction, valvular heart disease, or evidence of left ventricular dysfunction were excluded.
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Quantitative Coronary Arteriography
Coronary arteriograms were recorded and analyzed using a Siemens Angiographic System (Bicor and Hicor, Siemens-Asahi Inc). An appropriate view that permitted clear visualization of the target artery was selected. The angle of the view, the distance from the x-ray focus to the object, and the distance from the object to the image intensifier were kept constant during the study. An end-diastolic frame of the arteriogram was selected, and the luminal diameter of the segment of the artery distal to the Doppler wire was determined by a validated densitometric analysis system, as previously described.13 14 15 The readily identifiable branch points were determined as reference markers to allow assessment of serial changes in the diameter of the same arterial site.
The diameters were measured three times by examiners who had no knowledge of the patients' clinical characteristics, and the averaged value was used for analysis. The size of the Judkins catheter was used for calibrating the arterial diameter. Changes in the diameter in response to drugs are expressed as the percent change from the baseline value.
Measurements of CBF Velocity and Blood Flow
A 6F Judkins catheter was placed in the left main coronary artery by the femoral approach. An 0.018-inch Doppler-tipped guide wire (FlowWire, Cardiometrics Inc) was advanced through the Judkins catheter, and the tip was placed at the proximal segment of the left anterior or circumflex coronary artery. Peak CBF velocity was continuously monitored using a fast Fourier transform (FFT)–based spectral analyzer (FlowMap, Cardiometrics Inc). Systemic arterial pressure and heart rate were continuously recorded. The steady state signals were used for analysis.
Averaged peak blood flow velocity signals were obtained, and mean CBF was calculated as follows:
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This formula is based on the assumption that the diastolic CBF (the major part of mean CBF) is half the maximum flow and that the contribution of diastolic CBF is constant during tachycardia induced by rapid pacing and drug administration. Changes in the estimated CBF in response to drugs and during pacing are expressed as the percent change from the baseline value.
To assess changes in myocardial oxygen consumption, the pressure-rate product (systolic arterial pressurexheart rate) was calculated.
Study Protocol
The study protocols were approved by the Institutional Review Committee on Human Research of the Research Institute of Angiocardiology, Kyushu University School of Medicine. Written informed consent was obtained from each patient.
Cardiac catheterization was performed with patients in the fasting state after oral premedication with 5 mg of diazepam. Antianginal and antihypertensive medications were discontinued at least 24 hours before the study.
Protocol 1
We studied the effects of L-NMMA on coronary vasodilatation induced by rapid atrial pacing in 12 patients (Table 1
). A 6F bipolar pacing catheter was positioned at the right atrium. After steady state baseline hemodynamic parameters were recorded for 3 minutes at each patient's heart rate, myocardial oxygen consumption was increased by increasing the heart rate with rapid atrial pacing to the highest rate attainable without inducing the second-degree atrioventricular block (120 to 
130 bpm). The attained heart rate was kept constant for 2 minutes. Coronary arteriography was performed before and 2 minutes after the initiation of rapid pacing. After a 5-minute recovery period, L-NMMA was infused through a Judkins catheter into the left coronary artery over 12 minutes, and coronary arteriography was performed. The rapid atrial pacing protocol was then repeated.
Protocol 2
We studied the effects of L-NMMA on acetylcholine-induced coronary vasomotion in 13 patients (Table 1). After baseline hemodynamic parameters were recorded for 3 minutes, 3 µg/min of acetylcholine was infused through the Judkins catheter into the left coronary artery for 2 minutes using an infusion pump (1 mL/min). L-NMMA (200 µmol) was then infused into the left coronary artery over 12 minutes, and coronary arteriography was performed. Acetylcholine was then infused at the same dose as in the initial infusion. Coronary arteriography was performed before and 2 minutes after initiation of acetylcholine when all hemodynamic parameters were stable.
Statistical Analysis
Data are expressed as mean±SE. The effects of L-NMMA on acetylcholine-induced and pacing-induced changes in hemodynamic parameters were compared using the Student's t test or two-way ANOVA followed by Bonferroni's multiple comparison test. The relationship between the number of risk factors and the CBF responses to acetylcholine and rapid pacing was determined by simple linear regression analysis. A value of P<.05 was considered statistically significant.
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Results |
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Effects of L-NMMA on Metabolic Coronary Vasodilatation Induced by Pacing (Protocol 1)
Rapid atrial pacing increased CBF and dilated the large coronary artery without altering arterial pressure (Table 2
). L-NMMA reduced basal CBF (P<.05) but did not alter the diameter of the large epicardial coronary artery, arterial pressure, or heart rate. L-NMMA markedly attenuated the pacing-induced dilatation of the large epicardial coronary artery (P<.01), whereas it did not affect pacing-induced changes in CBF or arterial pressure (Table 2 and Fig 1A and 1B). The peak heart rate and the pressure-rate product attained by atrial pacing were not different before and after infusion of L-NMMA (Table 2).
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We examined the effects of L-NMMA on the pacing-induced coronary vasodilatation in a subgroup of patients in whom the L-NMMA–induced decrease in basal CBF was not significant (68±20 versus 64±19 mL/min) (n=5 [patients 8 through 12] in Table 1). L-NMMA did not affect the pacing-induced increase in CBF or the ratio of 
CBF to pressure-rate product (Fig 1C).
Effects of L-NMMA on Acetylcholine-Induced Coronary Vasomotion (Protocol 2)
Acetylcholine increased CBF (P<.01) and the diameter of the large coronary artery (P<.05) (Table 3 and Fig 2) without altering arterial pressure or heart rate. L-NMMA significantly attenuated the acetylcholine-induced increases in CBF (P<.01) and the diameter of the large coronary artery (P<.05) (Fig 2).
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Influence of Coronary Risk Factors on Pacing-Induced Coronary Vasodilatation (Protocols 1 and 2)
Acetylcholine-induced increases in CBF were negatively correlated with the number of risk factors; L-NMMA abolished the correlation (data not shown). There was no significant correlation between the number of risk factors and the pacing-induced increase in CBF before or after the infusion of L-NMMA (data not shown).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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L-Arginine analogues such as L-NMMA inhibit the release of NO from the endothelium and selectively reduce the coronary vasodilatation induced by acetylcholine administration in animals.10 11 16 17 18 This study and a recent report by Quyyumi et al17 demonstrated that the intracoronary infusion of L-NMMA reduced the basal CBF and reduced the acetylcholine-induced increase in CBF in humans. These findings suggest that (1) NO may be involved in the control of basal CBF and (2) the acetylcholine-induced coronary vasodilatation may be mediated at least in part by NO in humans. In the present study, although L-NMMA significantly inhibited acetylcholine-induced increases in CBF, it had no significant effect on the pacing-induced increase in CBF or the ratio of
CBF to pressure-rate product. These findings suggest that NO may not have been contributing to the overall increase in CBF associated with rapid atrial pacing in our patients. The present results also are consistent with our previous results18 and with studies by Smith and Canty11 in experimental dogs, although not with the clinical observation by Quyyumi et al17 that L-NMMA reduced pacing-induced increases in CBF. In a study of epicardial microvessels in dogs, Jones et al16 also observed that L-NAME inhibited pacing-induced dilatation of small arteries. Microvascular segments of different diameters and in different layers of the myocardium are recruited during metabolic coronary vasodilatation.1 2 3 Therefore, it is possible that L-NMMA–induced changes in the responses of epicardial coronary microvessels to metabolic stimuli seen in the study by Jones et al may have been compensated for by a counteracting vasodilatation of microvessels of different diameters or in different layers of the myocardium.
Quyyumi et al17 demonstrated that L-NMMA reduced pacing-induced coronary dilatation only in patients without risk factors for atherosclerosis and not in patients with one or more risk factors, consistent with loss of NO activity in these patients and as reflected in the findings of the present study demonstrating an inverse correlation between risk factors and acetylcholine-induced (albeit not pacing-induced) increase in CBF. Therefore, the higher percentage of patients with risk factors may have influenced the results in the present study. If the presence of risk factors for coronary atherosclerosis accounts for the effects of L-NMMA on coronary vasodilatation during pacing-induced metabolic stress, the CBF responses to both rapid pacing and acetylcholine infusion would be correlated with risk factors, as demonstrated by Quyyumi et al. In the present study, the presence of risk factors was associated with an impaired acetylcholine-induced increase in CBF; however, the pacing-induced increase in CBF was not correlated with risk factors before or after infusion of L-NMMA. Therefore, it is unlikely that differences in risk factor profiles account for the difference in results.
In the present study, L-NMMA significantly reduced pacing-induced dilatation of the large epicardial coronary artery, suggesting that dilatation of the large epicardial coronary artery in response to pacing is mediated by pacing-induced and flow-dependent release of NO from the endothelium.
In conclusion, the present results show that blockade of NO production results in loss of epicardial coronary artery dilatation but not of the associated increase in CBF in response to rapid pacing. These findings reflect NO-mediated, flow-related dilatation of large coronary arteries but do not necessarily exclude a NO-mediated effect in resistance arteries that may be counterbalanced by an increase in the metabolic signals.7 The findings suggest, however, that acute inhibition of NO production is unlikely to account for myocardial ischemia in patients with microvascular angina.13 19 Heterogeneity of microvascular perfusion, as described experimentally when NO production is inhibited,8 is a further possibility that cannot be excluded by the present study.
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Acknowledgments |
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This study was supported by grants-in-aid for Scientific Research from the Japanese Ministry of Education, Science, and Culture, Tokyo; a research grant from the Japan Cardiovascular Foundation, Osaka; and a research grant from the Uehara Memorial Foundation, Tokyo, Japan.
Received January 12, 1996; accepted April 29, 1996.
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