Cardiologia/Coração/CirurgCardíaca

Many types of cells, including lymphocytes, monocytes, fibroblasts, vascular smooth muscle cells, and endothelial cells, produce interleukin-6 (IL-6),1 which is a major determinant of C-reactive protein (CRP) production in the liver.2 The IL-6 gene and protein are also expressed in human atherosclerotic lesions,3,4 and this may contribute to the local plasma concentration of IL-6 in the coronary circulation.5,6 Systemic plasma concentrations of IL-6 and CRP are significantly correlated in patients who have acute myocardial infarction,7,8 in those who have unstable angina,9 and in healthy patients.10 We previously demonstrated that the CRP gene and protein are expressed in coronary plaque and that this might affect local plasma CRP levels in the coronary circulation.11,12 However, the relation between local plasma concentrations of IL-6 and CRP in the coronary circulation remains unknown. Moreover, local IL-6 production and release in the coronary circulation might modify coronary vasomotor tone in recipients of cardiac transplants.13 We therefore investigated whether IL-6 produced in the coronary circulation affects CRP production in the coronary circulation and microvascular resistance of the native coronary artery.

We examined 35 consecutive patients (17 men and 18 women; mean age 63.0 ± 2.0 years, range 38 to 86). Eleven had coronary artery disease (CAD) in the left anterior descending artery for which coronary intervention was performed (CAD group), and 24 had coronary arteries that were angiographically normal (control group). The CAD group included 6 patients who had unstable angina and 5 who had stable angina. Patients who had myocardial infarction related to the left anterior descending coronary artery were excluded from this study. Patients in the control group included those who had atypical chest pain (n = 18), those who had microvascular angina (n = 4), and those who had vasospastic angina (n = 2). Patients who had other noncardiac diseases that increased CRP levels, such as inflammatory disorders, malignancy, or infection, were excluded from this study. Plasma levels of IL-6 and CRP were measured in blood samples that were simultaneously collected from the orifice of the left coronary artery and the great cardiac vein after control coronary angiography and before coronary intervention. Plasma levels of IL-6 were measured with chemiluminescent enzyme immunoassay kits and plasma CRP levels were determined with the Dade Behring BN II N High Sensitivity CRP assay (Dade Behring Inc., Marburg, Germany). The lower range of values for IL-6 and CRP detected by the assays were 0.3 pg/ml and 0.175 mg/L, respectively. Written informed consent was obtained from all patients to participate in the study, and the ethics committee of our institution approved the study protocol.

The difference in plasma IL-6 (or CRP) levels between the great cardiac vein and left coronary artery was defined as the plasma IL-6 (or CRP) level at the great cardiac vein minus the plasma IL-6 (or CRP) level at the left coronary artery. This positive value suggested that IL-6 (or CRP) is produced in the coronary circulation. To evaluate the function of the coronary microcirculation in the control group, we injected papaverine (10 mg) into the left coronary artery over a period of 90 seconds. Coronary flow velocity at maximal hyperemia was measured with a 15-MHz Doppler wire (0.014-in FloWire, Volcano Therapeutics, Inc., Rancho Cordova, California). We defined the velocity-based index of microvascular resistance during the diastolic phase at maximal hyperemia as the ratio of coronary pressure to flow velocity during this period.14 These values were continuously acquired using a personal computer with a 12-bit analog-to-digital converter at a sampling frequency of 250 Hz (MP100 Systems and Acqknowledge, BIOPACK System Inc., Goleta, California).15 We used the mean value of microvascular resistance index during the diastolic phase in this study. Because the QT interval was excessively prolonged in 2 patients who received <10 mg of papaverine, we could not obtain a complete dataset from these patients. Therefore, we obtained values for microvascular resistance from 22 control patients.

Plasma IL-6 and CRP values are expressed as medians. Because the distribution of plasma IL-6 and CRP levels was skewed, we used nonparametric analysis for this comparison. We compared differences in plasma IL-6 levels between the control and CAD groups at the coronary artery and the great cardiac vein and transcoronary differences in plasma IL-6 levels using the Mann-Whitney test. We also examined transcoronary differences in plasma IL-6 levels after dividing the control group into subgroups that had stable angina and unstable angina. We used the Kruskal-Wallis test to compare results across the control, stable angina, and unstable angina groups and the Mann-Whitney test to compare data from pairs of groups (control vs stable angina, control vs unstable angina, and stable angina vs unstable angina). Linear regression analysis compared correlations between differences in plasma IL-6 levels in the coronary circulation and in plasma CRP levels in the coronary circulation or coronary resistance. A p value <0.05 was considered statistically significant.

Figure 1 shows a comparison of plasma IL-6 levels between the control and CAD groups at the left coronary artery and at the great cardiac vein. Figure 1 also shows a comparison of increases in plasma IL-6 levels throughout the coronary circulation between the control and CAD groups. Plasma IL-6 levels of the left coronary artery and great cardiac vein were significantly higher in the CAD group (medians 2.40 and 3.10 pg/ml) than in the control group (medians 1.35 and 2.10 pg/ml). Moreover, the difference between the great cardiac vein and the left coronary artery was higher in the CAD group (median 0.50 pg/ml) than in the control group (median 0.20 pg/ml). The transcoronary increase in plasma IL-6 level significantly differed (p = 0.0274) across the control group (25th percentile, median, and 75th percentile 0.00, 0.20, and 0.50 pg/ml, respectively), stable angina group (25th percentile, median, and 75th percentile 0.50, 0.50, and 0.88 pg/ml, respectively), and unstable angina group (25th percentile, median, and 75th percentile 0.20, 0.40, and 3.1 pg/ml, respectively). A comparison of the transcoronary increase in plasma IL-6 level between pairs of groups showed that significantly more IL-6 was produced in patients who had stable angina than in controls (p = 0.028). Levels in those who had unstable angina tended to be higher than those in controls (p = 0.062) but did not differ between patients who had stable angina and those who had unstable angina (p = 0.361). However, the numbers of patients who had stable angina and those who had unstable angina were too small to verify the result in the present study. Figure 2 shows a significant correlation between increased levels of IL-6 and CRP throughout the coronary circulation. Figure 3 shows that the increase in IL-6 throughout the coronary circulation was significantly correlated with coronary microvascular resistance in the control group.

The 3 principal findings of the present study were as follows. The amount of IL-6 produced in the coronary circulation was significantly increased in patients who had coronary atherosclerosis compared with those whose coronary arteries were angiographically normal. The production of IL-6 in the coronary circulation was significantly correlated with that of CRP in the coronary circulation, and IL-6 production was also significantly correlated with coronary microvascular resistance in patients who had angiographically normal coronary arteries. These results suggest that the localized cytokine/inflammatory pathway functions in the coronary circulation and that IL-6 is involved in modulating coronary vascular tone.

We have shown that IL-6 is produced in the coronary circulation of patients who have CAD. IL-6 is released into the coronary circulation of patients who have acute coronary syndrome,5,6 and the vascular endothelium or unstable coronary plaque is believed to be the predominant source of IL-6 release.5,6 Deliargyris et al6 found that IL-6 is produced in the coronary circulation of patients who have unstable angina but not in that of patients who have stable angina, which differs from our findings. We noted that the amount of plasma IL-6 level produced in the coronary circulation significantly differed across the control, stable angina, and unstable angina groups, and patients who had unstable angina tended to produce more IL-6 in the coronary circulation than did those who had stable angina. However, the level in the coronary circulation did not differ between patients who unstable angina and those who had stable angina, whereas it was significantly higher in patients who had stable angina than in controls. The IL-6 gene and protein are expressed in stable and unstable plaques.3,4,16 Our findings from a limited number of patients suggest that stable plaque is also produced and might release IL-6 into the coronary circulation. Further validation with a larger study cohort should confirm these findings. We identified CRP immunoreactivity and the CRP gene in coronary plaque that was obtained during directional coronary atherectomy.11,12 We also demonstrated that CRP produced in coronary plaque acts as an autocrine or paracrine factor and might contribute to local plasma CRP level in the coronary circulation.12 Although plasma levels of IL-6 and CRP are intimately related in the systemic circulation of patients who have acute myocardial infarction7,8 and those who have unstable angina9 and healthy subjects,10 the localized relation in the coronary circulation remains unknown. The present study discovered a significant positive correlation between productions of IL-6 and CRP in the localized coronary circulation. This suggests that the localized cytokine/inflammatory pathway plays a significant role in the coronary circulation.

Transcoronary IL-6 production and release are correlated with coronary endothelial vasomotor dysfunction in cardiac transplant recipients, which suggests that coronary endothelial dysfunction that occurs after cardiac transplantation is an immunologic phenomenon.13 IL-6 may induce endothelial dysfunction.17 Moreover, IL-6 production in the local pulmonary circulation might modify pulmonary vascular resistance in patients who have congestive heart failure.18 We identified a significant correlation between transcoronary IL-6 production and coronary microvascular resistance using a Doppler wire in patients who had atypical chest pain, microvascular angina, and vasospastic angina whose coronary arteries were angiographically normal. We evaluated coronary resistance during hyperemic stress induced by the endothelium-independent vasodilator, papaverine.19 Therefore, our results suggest that the mechanisms involved in the relation between increased IL-6 level in the coronary circulation and coronary resistance depends on an endothelium-independent effect. Although the present study did not clarify the mechanism, it might depend on a direct effect on smooth muscle cells and/or the smooth muscle cytosolic calcium level. Nonetheless, this suggests that IL-6 is involved in the modulation of coronary vascular tone in native and transplanted coronary arteries. In conclusion, proinflammatory cytokines and acute-phase proteins are intimately linked not only systemically but also locally in the coronary circulation, and transcoronary IL-6 production might modulate, at least in part, coronary vasomotor tone. Further studies with a larger cohort should validate these findings.