Endothelium-mediated regulation of coronary tone

Combination of ¹²³I-metaiodobenzylguanidine scintigraphy and flow-mediated dilation for the detection of patients with coronary spastic angina

BACKGROUND

This study evaluated the usefulness of cardiac sympathetic nerve activity, estimated by (123)I-MIBG scintigraphy, and endothelial function, estimated by flow-mediated dilation (FMD), in the detection of coronary spastic angina (CSA).

METHODS AND RESULTS

We compared 78 consecutive patients suspected of CSA with ten age-matched controls. On the basis of a spasm provocation test with acetylcholine, 53 patients were diagnosed as CSA and 25 patients were considered to have chest-pain syndrome (CPS). The total defect score (TDS) by delayed (123)I-MIBG scintigraphy was significantly higher in both patient groups than in controls (P < 0.05), and was significantly higher in CSA than in CPS patients (P = 0.02). The heart/mediastinum activity (H/M) ratio by delayed (123)I-MIBG scintigraphy and FMD were significantly lower in both patient groups than in controls (P < 0.05), and were lower in CSA than in CPS patients (P = 0.04). In receiver-operating curve analysis, the areas under the curve for TDS, H/M, and FMD were 0.78, 0.72, and 0.70, respectively. The combination of delayed (123)I-MIBG scintigraphy and FMD showed a higher diagnostic value than either method alone.

CONCLUSIONS

(123)I-MIBG scintigraphy and FMD can distinguish CSA patients among patients complaining of chest pain at rest, with good sensitivity and specificity.

Recent insights into the mechanisms of vasospastic angina

Coronary artery spasm plays an important role in the pathogenesis of many types of ischemic heart disease, not only in vasospastic angina but also in myocardial infarction and sudden death, particularly in the asian population. Patients with vasospastic angina are known to have defective endothelial function due to reduced nitric oxide bioavailability. Moreover, markers of oxidative stress and plasma levels of C-reactive protein are elevated. Smoking, polymorphysms of endothelial nitric oxide synthetase (eNOS), and low-grade inflammation have been regarded as the most important risk factors for vasospastic angina. The recent body of evidence indicates that RhoA and its down stream effector, ROCK/Rho-kinase, are associated with hypercontraction of vascular smooth muscle of the coronary artery and regulation of eNOS activity. Thus, endothelial dysfunction through abnormalities of eNOS and enhanced contractility of vascular smooth muscle in coronary artery segments are considered major mechanisms in vasospastic angina. However, the precise mechanisms for coronary vasospasm are not well understood. This article will review current understanding of the mechanism of coronary artery spasm.

Endothelial function and coronary spastic angina

Coronary spasm plays an important role in the pathogenesis of not only variant angina but also coronary heart disease in general including acute coronary syndromes, especially in the Japanese population. The vascular endothelium has been reported to be a multifunctional organ whose integrity is essential for normal vascular physiology. Vascular endothelial dysfunction can be a critical factor in the pathogenesis of ischemic heart disease. Acetylcholine and methacholine cause vasodilation by endothelium-derived relaxing factor when the endothelium is functioning normally, whereas they cause vasoconstriction when the endothelium is removed or damaged. Coronary spasm can be induced by a variety of stimuli with different mechanisms of action, including acetylcholine and methacholine. Patients with coronary spasm may have a disturbance in endothelial function as well as local hyperreactivity of the coronary arteries.

In vivo role of heme oxygenase in ischemic coronary vasodilation

The heart constitutively expresses heme oxygenase (HO)-2, which catabolizes heme-containing proteins to produce biliverdin and carbon monoxide (CO). The heart also contains many possible substrates for HO-2 such as heme groups of myoglobin and cytochrome P-450s, which potentially could be metabolized into CO. As a result of observations that CO activates guanylyl cyclase and induces vascular relaxation and that HO appears to confer protection from ischemic injury, we hypothesized that the HO-CO pathway is involved in ischemic vasodilation in the coronary microcirculation. Responses of epicardial coronary arterioles to ischemia (perfusion pressure approximately 40 mmHg; flow velocity decreased by approximately 50%; dL/dt reduced by approximately 60%) were measured using stroboscopic fluorescence microangiography in 34 open-chest anesthetized dogs. Ischemia caused vasodilation of coronary arterioles by 36 +/- 6%. Administration of N(G)-monomethyl-L-arginine (L-NMMA, 3 micromol.kg(-1).min(-1) intracoronary), indomethacin (10 mg/kg iv), and K(+) (60 mM, epicardial suffusion) to prevent the actions of nitric oxide, prostaglandins, and hyperpolarizing factors, respectively, partially inhibited dilation during ischemia (36 +/- 6 vs. 15 +/- 4%; P < 0.05). The residual vasodilation during ischemia after antagonist administration was inhibited by tin mesoporphyrin IX (SnMP, 10 mg/kg iv), which is an inhibitor of HO (15 +/- 4 vs. 7 +/- 2%; P < 0.05 vs. before SnMP). The guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (10(-5) M, epicardial suffusion) also inhibited vasodilation during ischemia in the presence of L-NMMA with indomethacin and KCl. Moreover, administration of heme-L-arginate, which is a substrate for HO, produced dilation after ischemia but not after control conditions. We conclude that during myocardial ischemia, HO-2 activation can produce cGMP-mediated vasodilation presumably via the production of CO. This vasodilatory pathway appears to play a backup role and is activated only when other mechanisms of vasodilation during ischemia are exhausted.

Role of endogenous endothelin in the regulation of basal coronary tone in the rat

1. Coronary vascular tone is a vital factor that regulates the delivery of oxygen to cardiac muscle. We tested the hypothesis that basal coronary tone may depend on the release of an endogenous vasoconstrictor peptide, endothelin (ET). 2. Using an isolated, Krebs solution-perfused rat heart we measured the changes in coronary flow following the administration over a 30 min period of the ET antagonists Ro61-0612 (mixed ETA/ETB), PD155080 (ETA) and BQ788 (ETB). 3. In a second series of experiments, hearts were randomly assigned to perfusion with plain Krebs solution, or with Krebs solution to which L-NAME and/or indomethacin had been added. The effect on coronary flow following the addition of Ro61-0612 was then measured. 4. Perfusion with Ro61-0612 (10-4 M) alone increased coronary flow by 57.8 % vs. control (P = 0.00001). PD155080 (10-4 M) increased coronary flow by 28.9 % (P = 0.009), whereas BQ788 had no effect on coronary flow. 5. In the second series of experiments, Ro61-0612 increased coronary flow by 6.6 +/- 0.8 ml min-1 in hearts perfused with plain Krebs solution, by 3.8 +/- 0.8 ml min-1 in hearts to which both L-NAME and indomethacin had been added, by 3.3 +/- 0.7 ml min-1 in hearts to which L-NAME had been added, and by 6. 9 +/- 0.5 ml min-1 in hearts to which indomethacin had been added to the Krebs buffer. 6. In hearts perfused with Krebs solution alone, nitric oxide (NO) release into the coronary sinus increased from 219. 8 to 544.9 pmol min-1 g-1 following the addition of Ro61-0612 (P = 0. 06). There was no detectable release of NO from hearts perfused with L-NAME alone or in combination with indomethacin either before or after the addition of Ro61-0612. 7. We conclude that endogenous ET plays a role in coronary tone mediated via ETA receptors. This vasodilatation is partially due to an increase in endogenous NO release. However, a significant vasodilatation is still seen following the inhibition of NO synthesis. We propose that basal coronary tone depends on a balance between the endogenous release of vasodilators such as NO and vasoconstrictors such as ET.

Nitric oxide-independent dilation of conductance coronary arteries to acetylcholine in conscious dogs

NO and prostacyclin formation cannot entirely account for receptor-operated endothelium-dependent dilation of coronary vessels, since vasodilator responses are not completely suppressed by inhibitors of these agents. Therefore, we considered that another factor, such as an endothelium-derived hyperpolarizing factor described in vitro, may participate in NO- and prostacyclin-independent coronary dilator responses. In conscious instrumented dogs, intracoronary acetylcholine (ACh, 30.0 ng.kg-1.min-1) increased the external epicardial coronary diameter (CD) by 0.18 +/- 0.03 mm (from 3.44 +/- 0.11 mm) when increases in coronary blood flow (CBF) were prevented and increased the CD by 0.20 +/- 0.05 when CBF was allowed to increase. After the administration of intracoronary N omega-nitro-L-arginine methyl ester (L-NAME), CBF responses to ACh were abolished, but CD responses (0.23 +/- 0.05 from 3.22 +/- 0.09 mm) were maintained. Blockade of NO formation was confirmed by reduced CD baselines and blunted flow-dependent CD responses caused by adenosine and transient coronary artery occlusions after L-NAME administration. ACh-induced CD increases resistant to L-NAME and indomethacin were reduced after the administration of intracoronary quinacrine, an inhibitor of phospholipase A2, or proadifen, an inhibitor of cytochrome P-450. Quinacrine or proadifen alone (without L-NAME) did not alter CD responses to ACh, but L-NAME given after proadifen blunted ACh-induced increases in CD. The increases in CD caused by arachidonic acid given after L-NAME + indomethacin were antagonized by proadifen but not altered by quinacrine. Thus, a cytochrome P-450 metabolite of arachidonic acid accounts for L-NAME-resistant and indomethacin-resistant dilation of large epicardial coronary arteries to ACh. Conversely, NO formation is the dominant mechanism of ACh-induced dilation after blockade of the cytochrome P-450 pathway.

Function and production of nitric oxide in the coronary circulation of the conscious dog during exercise

This study determined the changes in NO production from the coronary circulation of the conscious dog during exercise. The role of endogenous NO as it relates to coronary flow, myocardial work, and metabolism was also studied. Mongrel dogs were chronically instrumented for measurements of coronary blood flow (CBF), ventricular and aortic pressure, and ventricular diameter, with catheters in the aorta and coronary sinus. Acute exercise (5 minutes at 3.6, 5.9, and 9.1 mph) was performed, and hemodynamic measurements and blood samples were taken at each exercise level. Nitro-L-arginine (NLA, 35 mg/kg IV) was given to block NO synthesis, and the exercise was repeated. Blood samples were analyzed for oxygen, plasma nitrate/nitrite (an index of NO), lactate, glucose, and free fatty acid (FFA) levels. Acute exercise caused significant elevations in NO production by the coronary circulation (46 +/- 23, 129 +/- 44, and 63 +/- 32 nmol/min at each speed respectively, P < .05). After NLA, there was no measurable NO production at rest or during exercise. Blockade of NO synthesis resulted in elevations in myocardial oxygen consumption and reductions in myocardial FFA consumption for comparable levels of CBF and cardiac work. The metabolic changes after NLA occurred in the absence of alterations in myocardial lactate or glucose consumptions. NO production by the coronary circulation is increased with exercise and blocked by NLA. The absence of NO in the coronary circulation during exercise does not affect levels of CBF, because it shifts the relationship between cardiac work and myocardial oxygen consumption, suggesting that endogenous NO modulates myocardial metabolism.

Contribution of nitric oxide to metabolic coronary vasodilation in the human heart

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 NG-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 mumol/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.

Endothelial dysfunction in patients with chest pain and normal coronary arteries

BACKGROUND

A subgroup of patients with chest pain and angiographically normal epicardial coronary arteries have reduced dilator response to metabolic or pharmacological stimuli, but the mechanisms responsible for this reduced dilator response are unknown. In this study, we have investigated whether microvascular endothelial dysfunction is a cause of the observed reduced vasodilator reserve.

METHODS AND RESULTS

The functional response of the microvasculature was studied with rapid atrial pacing at 150 beats per minute. Fifty-one patients, 20 hypertensive and 31 normotensive, with chest pain and normal epicardial coronary arteries (< 10% stenosis) were studied. Endothelial function was tested with incremental infusions of acetylcholine to achieve estimated intracoronary concentrations ranging from 10(-7) M to 10(-5) M. Endothelium-independent smooth muscle vasomotion was measured using intracoronary sodium nitroprusside. Endothelial dysfunction of epicardial coronary arteries, demonstrated as severe (> 50%) constriction with < 10(-5) M acetylcholine concentration, was evident in five patients (10%). In the remaining 46 patients, coronary blood flow increased with acetylcholine (mean, 78 +/- 43%) and atrial pacing (mean, 51 +/- 37%), and coronary vascular resistance decreased by 35 +/- 16% and 29 +/- 14%, respectively, but the responses were heterogeneous. There was a correlation between the coronary resistance change with acetylcholine and the change with atrial pacing: r = 0.68, p < 0.001 in these 46 patients. Thus, patients with depressed dilation with atrial pacing had reduced endothelium-dependent dilation with acetylcholine, and vice versa. However, the microvascular dilation caused by sodium nitroprusside was not significantly different between patients with and those without reduced dilation with atrial pacing, indicating that the vasodilator defect was not caused by smooth muscle dysfunction. There were no differences in the vasodilator responses with atrial pacing, acetylcholine, or nitroprusside between normotensive and hypertensive patients. Multivariate regression analysis was performed to determine whether age, sex, serum cholesterol level, hypertension, presence of mild epicardial vessel atherosclerosis, resting left ventricular function, change in left ventricular ejection fraction with exercise, vasodilation with acetylcholine, and vasodilation with sodium nitroprusside were independently related to the vasodilator response to atrial pacing. Only the change in coronary vascular resistance with acetylcholine was independently correlated with the change in resistance with atrial pacing: R2 = 0.46, p < 0.0001.

CONCLUSIONS

Patients with chest pain, normal epicardial coronary arteries, and reduced vasodilation in response to atrial pacing appear to have associated endothelial dysfunction of the coronary microvasculature. Thus, microvascular endothelial dysfunction may contribute to the reduced vasodilator reserve with atrial pacing and anginal chest pain in these patients.

Clinical relevance of endothelium-derived relaxing factor (EDRF)

1. In addition to metabolic and neurohumoral factors endothelium-derived autacoids like the nitric oxide radical NO and prostacyclin are effective regulators of vascular tone and thus tissue perfusion. NO is produced in endothelial cells from L-arginine by a Ca2+/calmodulin-dependent enzyme NO synthase. In addition, the NO radical is ultimately cleaved from all nitrovasodilators and resembles their vasoactive and antiaggregatory principle, which is used under pathological conditions as substitution therapy for impaired endothelial function and autacoid production. Impaired endothelium-dependent vasomotor control has been documented in hypercholesterolaemia, atheromatosis, diabetes, hypertension, and in reperfusion damage. L-arginine supplementation is effective in a few instances.