Coronary artery spasm has been shown to play an important role in the pathogenesis of not only variant angina, but also of ischemic heart disease in general, including other forms of angina pectoris, acute myocardial infarction, and sudden death (1,2). However, the precise mechanism(s) by which coronary spasm occurs still remain unknown. It was previously shown that smooth muscle of spastic coronary arteries may have the enhanced constrictor response to most vasoconstrictors, and the enhanced constrictive reactivity of the spastic coronary arteries may play an important role in the genesis of coronary spastic angina (3-8).
It was shown that subcutaneous administration of epinephrine caused coronary artery spasm and that exercise-induced coronary spasm was suppressed by phenoxybenzamine and phentolamine (9,10). These results suggested that α-adrenoceptor-mediated smooth-muscle contraction of coronary arteries may have a role in the pathogenesis of coronary artery spasm in patients with coronary spastic angina. However, α-adrenergic stimulation is now known to dilate coronary arteries in endothelium-dependent manner through α2-receptor on endothelium (11,12), and a clinical trial showed that oral administration of α1-adrenergic blockade did not suppress episodes of spasm in patients with coronary spastic angina (13). Intravenous injection of phenylephrine, a specific α1-adrenergic stimulator, failed to induce coronary spasm in patients with coronary spastic angina (14). However, systemic administration of the agonists and the blockers for α-adrenergic stimulation induces substantial changes of systemic hemodynamics and neurohumoral balances, which could modify the direct effect of α-adrenergic stimulation on coronary artery tone. Thus it remains inconclusive whether smooth-muscle contractility of spastic coronary arteries is enhanced in response to direct stimulation of α-adrenergic receptor. This study examined the constrictor response of spastic coronary artery to the intracoronary infusion of phenylephrine in a manner that could avoid changes of the systemic hemodynamics in patients with coronary spastic angina and compare the response with that in control subjects.
The study included 10 patients with coronary spastic angina (mean age, 57 years; range, 36-74 years; eight men and two women) in whom episodes of spontaneous angina occurred at rest. Four patients showed transient ST-segment elevation, and the remaining six patients showed ST-segment depression during the spontaneous attacks. All patients with coronary spastic angina showed angiographically documented coronary spasm associated with ischemic ST-segment changes with or without chest pain after the intracoronary injection of acetylcholine (ACh), as reported previously (3,15,16). All patients with coronary spastic angina had no organic stenosis angiographically.
This study also included 10 control patients with atypical chest pain (mean age, 54 years; range, 30-73; seven men and three women). These control patients were selected to match the risk factors for coronary artery disease to those in patients with coronary spastic angina. The control patients underwent diagnostic cardiac catheterization for evaluation of chest pain. They had angiographically normal coronary arteries and did not show coronary spasm after the intracoronary injection of ACh.
Clinical characteristics of the study patients are shown in Table 1. All medications except sublingual nitroglycerin were withdrawn ≥3 days before the study. No study patient had taken nitroglycerin within 6 h of the study. No patient had previous myocardial infarction, congestive heart failure, or other serious diseases. Written informed consent was obtained from all patients before the study. The study was in agreement with the guidelines approved by the ethics committee at our institution.
Quantitative coronary angiography
A quantitative coronary angiographic study was performed with the Judkins technique by using contrast material (Ioxaglate; Guerbet S.A., Firance) in the morning, as reported previously (15-17). In brief, measurement of the luminal diameter of the coronary artery was performed quantitatively with the use of the computer-assisted coronary angiographic analysis system (Cardio 500; Kontron Instruments, Germany) by two observers blinded to the study protocol. The size of a Judkins catheter was used for calibrating the arterial diameter in millimeters. The length of the coronary segment analyzed was an ∼10 mm, and measurement was done at three points, and the measured diameters were averaged within a segment. In the control patients, each trunk of the three major coronary arteries was divided into proximal and distal segments equal in length. The luminal diameter at the center of each segment was measured, and special care was taken to measure the diameter at the same site under each condition with use of anatomic references. In the patients with coronary spastic angina, the luminal diameter was measured at the site of coronary spasm induced by the intracoronary injection of ACh. Coronary spasm was defined as total or subtotal occlusion of the epicardial coronary arteries associated with signs of myocardial ischemia, such as chest pain and ischemic ST-segment changes. When subtotal occlusion of the spasm occurred diffusely from the proximal to the distal segments of a coronary artery, the diameters were measured at both the proximal and distal segments of the spastic artery. Twelve electrocardiographic (ECG) leads and arterial pressure were continuously monitored during the study period.
The study protocol is shown in Fig. 1. After the baseline measurements of heart rates and blood pressure, the baseline angiography of the left and right coronary arteries was done. Thereafter, incremental doses of ACh were injected into the left coronary artery (50, 100 μg/min) and subsequently into the right coronary artery (RCA; 20, 50 μg/min) until coronary spasm was induced or the maximal doses were reached in all of patients and subjects in both groups. Coronary spasm induced by this method usually resolves spontaneously within 2-3 min without use of nitroglycerin and allowed further studies in all of the patients with coronary spastic angina in this study. The details of the method of injecting ACh were reported previously (3,15,16). Twenty minutes after the completion of the intracoronary injection of ACh, phenylephrine (20 μg/min, yielding ∼1 μM in the left coronary circulation) was infused through the Judkins catheter for 5 min into all of the left coronary artery with the spasm provoked by ACh (nine left coronary arteries, see later in Tables 1 and 3 of the Results section), and 10 min thereafter, phenylephrine (10 μg/min, yielding ∼1 μM in the right coronary circulation) was also infused into all of RCA with the spasm (three RCAs, see later in Tables 1 and 3 of the Results section) in patients with coronary spastic angina. In control subjects, phenylephrine was infused into nine left coronary arteries and into four RCAs in the same manner as in the patients with coronary spastic angina. Number and variation of control coronary arteries infused by phenylephrine were selected to match with those of spastic coronary arteries. Two patients with coronary spastic angina and three control subjects had phenylephrine infusion in both left and right coronary arteries. The infused doses of phenylephrine were selected on the basis of our preliminary study that examined the constrictor response of the coronary artery to the intracoronary infusion of increasing doses of phenylephrine (0.01, 0.1, 1, and 5 μM in the coronary circulation) in control subjects. Phenylephrine, at the concentration of 1 μM in the coronary circulation, was found to induce maximal contraction of coronary artery without significant changes of systemic hemodynamics, results in agreement with a previous report (18). Coronary angiography and measurements of hemodynamic parameters were repeatedly performed before and at the end of the infusion of each dose of the drugs. After an additional 10 min, intravenous bolus injection of nitroglycerin (250 μg) was done, and 2 min thereafter, the coronary angiography was repeatedly performed in all study patients in multiple projections. The responses of coronary artery diameter to ACh, phenylephrine, and nitroglycerin were expressed as a percentage change of the coronary diameter on the angiogram just before each infusion. All drug solutions were kept at 37°C.
Phenylephrine and ACh were obtained from Kyowa Co. (Nagoya, Japan) and Daiichi Pharmaceutical Co. (Tokyo, Japan), respectively, and were dissolved in physiological saline.
Data are expressed as mean ± SD unless otherwise indicated. Mean values were compared by unpaired Student's t test, and frequencies of the risk factors were compared by χ2 test between patients with coronary spastic angina and control subjects. Hemodynamic parameters were compared by one-way analysis of variance (ANOVA). A value of p < 0.05 was considered statistically significant.
As shown in Table 2, baseline values of heart rates and mean blood pressure in the patients with coronary spastic angina were not significantly different from those in the control patients. The baseline diameters of the spastic arteries in the patients with coronary spastic angina were significantly smaller as compared with the respective diameters of the control arteries in the control patients [2.6 ± 0.3 mm (n = 7) vs. 3.2 ± 0.4 mm (n = 10), p < 0.01 at the proximal segment; and 1.4 ± 0.3 mm (n = 8) vs. 1.9 ± 0.5 mm (n = 10), p < 0.01 at the distal segment, respectively, in the left anterior descending coronary artery (LAD)].
Response to ACh and provocation of coronary spasm
In all patients with coronary spastic angina, spasm occurred in the coronary artery into which ACh was injected, in association with ischemic ST-segment changes on the ECG leads corresponding to the area of myocardium perfused by the artery. Coronary spasm was documented in 15 coronary arteries, including nine LAD, three left circumflex coronary arteries (LCx), and three RCAs, as shown in Tables 1 and 3. Spasm occurred in both LAD and LCx in three patients, and it occurred in all of the three major coronary arteries in two patients, as shown in Table 1. Total occlusion occurred at the proximal segment of two coronary arteries (cases 1 and 2 in Table 1). Subtotal occlusion occurred diffusely from the proximal to the distal segments in eight coronary arteries, and it occurred at the distal segments in five coronary arteries. Spasm was induced by the ACh injection at the dose of 50 μg in two LADs and three RCAs, and it was induced at the dose of 100 μg in the remaining seven LADs and three LCxs, as shown in Table 3. On the other hand, intracoronary injection of ACh (≤100 μg) did not induce coronary spasm associated with myocardial ischemia in any of the control subjects. Response of coronary diameter to ACh infusion was evaluated at the dose of 50 μg in all of the coronary segments with spasm in patients with coronary spastic angina. Two LADs and three RCAs, in which spasm was provoked by the 50 μg of ACh as presented in Table 3, were excluded from the analyses because the diameter of the coronary arteries during total or subtotal occlusion due to coronary spasm can not be accurately measured. In control subjects, response of coronary diameter to 50 μg of ACh was evaluated in both proximal and distal segments in all of the LADs and LCxs as referenced control coronary arteries. The response of the RCAs was not analyzed also in control subjects to match the analyzed coronary arteries to those in patients with coronary spastic angina because the RCA was not included for assessment of coronary vasomotor response to ACh in patients with coronary spastic angina. The intracoronary injection of ACh at the dose of 50 μg constricted all of the 10 spastic coronary arteries. On the other hand, ACh, at the dose of 50 μg, dilated 10 coronary arteries and constricted the remaining 10 coronary arteries in control subjects. Thus the control coronary arteries as a whole showed a slight but significant constriction in response to ACh. As shown in Fig. 2, the constrictor response to ACh at the dose of 50 μg was markedly augmented in the spastic arteries as compared with the control coronary arteries.
Response to phenylephrine
The intracoronary infusion of phenylephrine did not induce coronary spasm associated with ischemic ST-segment changes or chest pain or both in any of the patients with coronary spastic angina as well as the control subjects. Response of arterial diameter to phenylephrine was measured in all of the coronary sites with the spasm provoked by ACh infusion (seven proximal and eight distal segments in nine LADs, two proximal and three distal segments in three LCxs, and two proximal and two distal segments in three RCAs, as shown in Table 3) in patients with coronary spastic angina. In control subjects, response to phenylephrine also was measured in all of the coronary segments infused by phenylephrine (both proximal and distal segments in nine LADs, nine LCxs, and four RCAs, which were selected to match the analyzed coronary arteries to those in patients with coronary spastic angina). The constrictor response to phenylephrine was comparable between the spastic coronary arteries and the control arteries, as shown in Fig. 2.
Response to nitroglycerin
Nitroglycerin increased the coronary diameter at all segments of the coronary arteries in both groups of the patients. The coronary diameters of the spastic arteries after nitroglycerin administration were not different from those of the control arteries at either the proximal segment or the distal segment [3.6 ± 0.5 mm (n = 7) vs. 3.8 ± 0.8 mm (n = 10) at the proximal segment, and 2.0 ± 0.6 mm (n = 8) vs. 2.3 ± 0.6 mm (n = 10) at the distal segment; p = NS, respectively, in LADs).
This study was the first to assess constrictor effect of intracoronary infusion of phenylephrine in spastic coronary arteries in patients with coronary spastic angina. This study showed that the constrictor response to the intracoronary infusion of phenylephrine in the spastic coronary arteries was not enhanced but was comparable with that in the control coronary arteries and that the infusion of phenylephrine did not induce coronary spasm in any of the patients with coronary spastic angina. On the other hand, the constrictor response to the intracoronary infusion of ACh was enhanced in the spastic coronary arteries as compared with that in the control coronary arteries, and the ACh infusion induced coronary spasm in all of the patients with coronary spastic angina but not in any of the control subjects. The results indicate that smooth muscle of spastic coronary arteries exhibited enhancement of constrictor response to ACh but not to phenylephrine. It was previously shown that epinephrine induced coronary artery spasm in patients with coronary spastic angina (1,9,10), but epinephrine stimulates both α1- and α2-adrenergic receptors (11). It was shown that α2-adrenoreceptor stimulation on vascular smooth muscle causes the arterial constriction (19). Therefore there is a possibility that epinephrine may induce coronary artery spasm because of synergistic effects of α1- and α2-adrenergic receptor-mediated contraction of coronary smooth muscle in patients with coronary spastic angina. However, it still remains to be determined whether the constrictor response of smooth muscle to α2-adrenergic stimulation may be enhanced in spastic coronary arteries.
A number of agonists, including ACh, ergonovine, serotonin, and histamine, can induce coronary artery spasm in patients with coronary spastic angina (3-6). These agonists cause direct smooth-muscle contraction as well as endothelium-dependent vasorelaxation, which is largely mediated by endothelium-derived nitric oxide (20,21). Therefore the response of the arteries to these agonists is determined by the balance between endothelium-dependent relaxation and direct smooth-muscle contraction (20,21). Recently we showed that the constriction by ACh after nitric oxide inhibition with NG-monomethyl-L-arginine was still greater in spastic coronary arteries than in control arteries (15), indicating that smooth-muscle contractility to ACh was enhanced in the spastic coronary arteries. Thus the deficiency in the endothelial nitric oxide activity and the enhanced contractility of smooth muscle may synergistically cause coronary spasm in response to ACh in patients with coronary spastic angina. On the basis of previous data (1-8), the hyperconstrictive response of smooth muscle in spastic coronary arteries plays a major role in the pathogenesis of coronary spastic angina. However, the supersensitive constrictor response of smooth muscle in spastic coronary arteries is not common to all vasoconstrictors but is probably specific to some agonists, because this study showed that contractile response of spastic coronary arteries was enhanced in response to ACh but not to phenylephrine in patients with coronary spastic angina. The mechanism for the supersensitive contractility of the smooth muscle in response to ACh still remains unknown, but there may be functional alteration(s) of the postreceptor regulatory pathway, distinct from that of α1-receptor stimulation, for smooth-muscle contraction.
On the basis of our finding that coronary diameters at baseline condition were smaller in spastic coronary arteries than in control arteries, the basal tone was increased in the spastic arteries as compared with that in the control arteries, a phenomenon in agreement with the previous studies (1,15,16). Thus it cannot be completely excluded that the increased responsiveness to ACh may be related to the increased basal tone.
In conclusion, this study indicates that smooth muscle of spastic coronary arteries does not exhibit the enhanced constrictor response to direct α1-adrenergic stimulation. There may be receptor-specific enhancement of constrictor response to agonists in smooth muscle of spastic coronary arteries in patients with coronary spastic angina.
Acknowledgment: This study was supported in part by a Grant-in-aid for Scientific Research on C05670622 from the Ministry of Education, Science and Culture in Japan and Smoking Research Foundation Grant for Biochemical Research, Tokyo, Japan.
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