Circadian Variation in Coronary Tone in Patients With Stable Angina
Protective Role of the Endothelium
Background Coronary endothelium plays a key role in the regulation of coronary tone, platelet adhesion, and aggregation, which are important factors in triggering acute cardiovascular events. However, its role in modulating the effects of circadian variations on coronary tone is not known.
Methods and Results Responses of 72 nonstenotic coronary segments to acetylcholine and nitroglycerin were measured in 12 patients with chronic stable angina at 6 am and 1 pm. After baseline angiography, three infusions of acetylcholine (10−6, 10−5, and 10−4 mol/L) were administered selectively into the left coronary artery, followed by nitroglycerin. Diameters (in millimeters) of proximal, middle, and distal segments were measured by quantitative techniques. Forty-seven segments showed a constrictor response to acetylcholine (group 1, dysfunctional endothelium), and 25 other segments showed a dilator response (group 2, normally functioning endothelium). In group 1, the constrictor response to acetylcholine was significantly greater in the morning than in the afternoon (23±3% and 10±1%, mean±SEM, respectively; P<.001), and the dilator response to nitroglycerin was also significantly greater in the morning than in the afternoon (19±2% and 11±2%; P<.01). In group 2, the dilator response to acetylcholine did not differ significantly between the morning and afternoon (22±3% and 17±2%, respectively; P=NS), and the dilator response to nitroglycerin was also similar at both times of the day (30±3% and 28±4%, respectively; P=NS).
Conclusions Coronary segments with dysfunctional endothelium exhibit an early morning exaggeration in vasomotor activity, whereas segments with normally functioning endothelium do not show circadian variations. This suggests a potential protective role for the endothelium in modulating variations in coronary tone that may contribute to increased incidence of cardiovascular events in the early morning hours.
In patients with coronary artery disease, acute cardiovascular events such as transient myocardial ischemia, myocardial infarction, sudden death, and stroke occur more frequently during the early morning hours.1 2 3 4 5 Potential mechanisms triggering these events include early morning increases in sympathetic activation,6 plasma catecholamine levels,7 and platelet aggregability.8 9
Coronary endothelium plays a critical role in the regulation of vascular smooth muscle tone and the maintenance of vessel wall integrity. It presents an anticoagulant, antithrombotic surface and modulates vasomotor tone through the release of different relaxing and constricting agents.10 11 12 13 However, its role in modulating circadian variations is not known.
We have recently shown that in patients with atherosclerosis and risk factors for coronary artery disease,14 15 coronary endothelium exhibits marked segmental heterogeneity in response to acetylcholine, with both dysfunctional and normally functioning endothelium expressed in adjacent segments. To further elucidate the possible interaction between coronary endothelium and the circadian effects on coronary tone, we studied responses of nonstenotic coronary artery segments to intracoronary acetylcholine and nitroglycerin. Studies were done in patients with advanced coronary artery disease who underwent cardiac catheterization in both the morning and the afternoon. The aim was to investigate the influence of time of day on endothelium-derived changes in coronary size.
We studied 12 men, aged 46 to 75 years, with chronic stable angina pectoris and objective evidence of exercise-induced myocardial ischemia (>1.0 mm horizontal or downsloping ST-segment depression). Two patients had a previously documented myocardial infarction (>6 months). Five patients had one-vessel disease (>70% stenosis), 5 had two-vessel disease, and 2 had three-vessel disease. No patient had a history suggestive of variant angina. Three patients had history of hypertension (systolic blood pressure ≥150 mm Hg and diastolic blood pressure ≥90 mm Hg), 11 had hypercholesterolemia (total serum cholesterol ≥210 mg/dL), 9 were current tobacco smokers, and 7 had a family history of coronary artery disease.
The study protocol was approved by the institutional review boards of the University of Florida Health Science Center and the Gainesville Veterans Administration Medical Center, and all patients gave informed consent.
Antianginal therapy was discontinued at least 24 hours before initial elective catheterization (2 patients were taking 100 mg/day metoprolol, 7 patients were taking 240 mg/day verapamil, and 3 patients were taking 180 mg/day diltiazem; none of these medications were in a long-acting or sustained-release form). Patients were permitted to use sublingual nitroglycerin as needed for angina, but no patient was included who had used nitroglycerin within 3 hours of study. All patients were taking 325 mg/day aspirin, and none took lipid-lowering agents. All studies were performed in a fasting nonsedated state.
Each patient underwent two identical coronary angiographic studies performed within a 24- to 72-hour period (5 within 24 hours, 6 within 48 hours, and 1 within 72 hours). Studies were performed in the early morning (6 to 7 am) and in the afternoon (noon to 2 pm). The repeat angiographic study was done as a prelude to coronary angioplasty. Throughout each study, heart rate, femoral artery pressure via an arterial sheath, and surface ECG leads I, II, and V5 were monitored and recorded continuously on analog tape (Racal Store-14). After completion of diagnostic angiograms, an optimal view was chosen to best visualize the left coronary artery and minimize overlap. All study angiograms were subsequently performed with this view and no movement of table or camera. A 5F bipolar pacing catheter was advanced to the right ventricle via the femoral vein and connected to a standby pacer set at 60 beats per minute.
A stock solution of acetylcholine (Miochol) was prepared by a research pharmacist by dissolving the sterile powder in 20% manitol. The stock solution was then diluted in saline at the beginning of the study.
The study was divided into three periods: first, a control infusion of 0.9% saline; second, infusions of increasing syringe concentrations of acetylcholine 10−6, 10−5, and 10−4 mol/L; and, third, a bolus infusion of 200 μg of nitroglycerin. Infusions were administered into the left coronary artery at room temperature through a 7F left Judkins catheter at a rate of 1 mL/min for 2 minutes with a syringe pump (IVAC Corporation). Before the start of each infusion, the catheter was primed by a premeasured volume of infusate to fill in the dead space so that when the infusion starts, the infusate is delivered into the left main coronary artery. At the end of each infusion, coronary angiograms were performed with injections of 8 to 10 mL of Hypaque 76 (Winthrop). The elapsed time between the end of acetylcholine infusion and repeat coronary angiography was 12 to 15 seconds. Two minutes elapsed between the last acetylcholine infusion angiogram and injection of nitroglycerin, unless either chest pain or ST-segment changes occurred and nitroglycerin was given immediately. To prevent bolus administration of acetylcholine, before each angiogram the catheter was emptied of infusate by rapid withdrawal into a syringe until blood appeared in the manifold at the stopcock holding the withdrawal syringe; the stopcock was then turned, connecting the contrast-filled syringe. All patients received ascending concentrations of acetylcholine infusion unless chest pain or signs of myocardial ischemia occurred or the largest dose (10−4 mol/L) was reached. No changes were made in patient position or tube-to-patient distance during the three study periods, and all angiograms were done with six or seven cine image intensification film at speed of 30 frames per second. Data for heart rate and blood pressure analysis were obtained during the final 15 seconds of each infusion.
Quantitative Coronary Angiography
Quantitative coronary angiography was done with a cinevideodensitometric technique (XR-70 Vanguard Instrument Corporation).11 12 13 14 15 End-diastolic frames from each arteriogram were selected by a cardiologist and analyzed by a technician. Nonstenotic smooth coronary arterial segments identified between easily visualized branch points were selected for analysis in the anterior descending and circumflex arteries. At least six coronary segments were identified in any given patient, and the same segments were analyzed after each intervention in all study periods. To establish the reproducibility of the method, 24 segments (8 from the proximal, 8 from the middle, and 8 from the distal segment) were analyzed by a blinded observer. Repeat analysis of the same segments was performed at remote interval by the same observer. The exact location and cine frame were identified with a Polaroid photograph made at the previous measurement setting. There was no significant difference in the variances of repeated analysis (F coefficient=0.3, P=.6, 95% confidence limits for the difference ±2 SD ([±0.15 mm]). To define a segmental constrictor or dilator change in response to acetylcholine, a value of ≥10% diameter change compared with control was used; this criterion was based on the notion that almost all measurements fall outside this range.
All data were summarized by group and expressed as mean±SEM. The data were analyzed using repeated-measures ANOVA to compare the difference in absolute response of different segment diameters to acetylcholine and nitroglycerin with the diameters of the same segments in the control period. A value of P=.05 was considered to indicate statistical significance.
Changes in Coronary Diameter
Changes in coronary diameter at control and after three doses of acetylcholine and nitroglycerin are shown in the Table⇓.
Response to Saline
Intracoronary infusion of saline was not associated with any significant change in lumen diameter in any segment.
Two patterns of segmental response to maximal acetylcholine dose were observed. Coronary lumen diameter decreased in 47 segments (17 proximal, 16 middle, and 14 distal), a response considered to reflect endothelial dysfunction (group 1). In 25 other segments (7 proximal, 8 middle, and 10 distal), coronary lumen diameter increased, a response considered to reflect normally functioning endothelium (group 2).
Control Coronary Diameter
In group 1, the mean control lumen diameter increased from 2.0±0.08 mm in the morning to 2.15±0.08 mm in the afternoon (8% increase; P<.02). In group 2, no significant variation was observed; the mean control lumen diameter increased from 1.66±0.12 mm in the morning to 1.72±0.13 mm in the afternoon (4% increase; P=NS; Fig 1⇓). An example of these segmental changes from one patient appears in Fig 2⇓.
Response to Acetylcholine
In group 1, the mean control lumen diameter decreased from 2.0±0.08 to 1.57±0.10 mm after acetylcholine (23% decrease; P<.01) in the morning and from 2.15±0.08 to 1.94±0.09 mm (10% decrease; P<.04) in the afternoon (P<.001 for morning versus afternoon). Eleven of 47 segments did not show variation between morning and afternoon.
In group 2, the mean control lumen diameter increased from 1.66±0.12 to 1.97±0.13 mm after acetylcholine (22% increase; P<.01) in the morning and from 1.72±0.12 to 1.98±0.12 mm (17% increase; P<.01) in the afternoon (P=NS for morning versus afternoon); Fig 3⇓). An example of these segmental changes from one patient appears in Fig 2⇑.
Response to Nitroglycerin
In group 1, the mean control lumen diameter increased from 2.0±0.08 to 2.34±0.08 mm after nitroglycerin (19% increase; P<.01) in the morning and from 2.15±0.08 to 2.36±0.09 mm (11% decrease; P<.03) in the afternoon (P<.01 for morning versus afternoon). Seven of 47 segments did not show variation between morning and afternoon.
In group 2, the mean control lumen diameter increased from 1.66±0.12 to 2.08±0.13 mm after nitroglycerin (30% increase; P<.01) in the morning and from 1.72±0.12 to 2.13±0.12 mm (28% increase; P<.01) in the afternoon (P=NS for morning versus afternoon Fig 3⇑).
There was no significant change in either heart rate or systolic arterial blood pressure associated with either saline or incremental concentrations of acetylcholine in any patient.
The results of the present study demonstrate that in a subset of patients with coronary artery disease and chronic stable angina, coronary segments with dysfunctional endothelium exhibit an early morning exaggeration in basal tone. This was demonstrated by a greater lumen diameter response to nitroglycerin in the morning than in the afternoon. In addition, the vasoconstrictor response to acetylcholine was also greater in the morning than in the afternoon. Segments with normally functioning endothelium did not show significant circadian variations in either the dilator response to acetylcholine or the dilator response to nitroglycerin. These results suggest a potential role for the endothelium in modulating circadian influences on coronary tone.
Until recently, the endothelium has been described as diffusely dysfunctional in patients with coronary artery disease,16 and even in patients with angiographically normal coronary arteries, if they exhibit one or more risk factors for coronary atherosclerosis.17 These observations could not explain the role of the endothelium in the focal manifestations of coronary artery responses to a variety of stimuli. We14 15 have shown that in patients with advanced atherosclerosis, coronary endothelium exhibits marked segmental heterogeneity in response to acetylcholine, with both constrictor and dilator responses observed in adjacent segments. Our observations are consistent with the patchy distribution of atherosclerosis described in pathological studies18 and in studies with intravascular ultrasound.19 Coronary segments that constrict, as well as those that dilate in response to acetylcholine, appear angiographically similar and relatively free of epicardial lumen obstruction. Such segments may also differ functionally in their response to other stimuli. The results of the present study expand these findings and suggest that the endothelium and various physiological stimuli may interact to alter coronary vasomotor responses in a circadian fashion. It is conceivable that the local differences in response could be in part related to a different local smooth muscle response to acetylcholine and nitroglycerin.
Several factors that affect vascular tone have a well-established circadian variation. These factors may act alone or in combination to produce the observed changes in coronary tone. Plasma levels of neurohormones such as norepinephrine are higher in the morning than in other times of the day,7 thus suggesting an increase in sympathetic activity at this time of day that may cause increased stimulation of α-adrenergic receptors, leading to coronary vasoconstriction.20 21
The mechanism by which functional endothelium mediates coronary response to sympathetic stimulation in humans may be explained by an increased flow-related, shear stress–induced release of endothelium-derived relaxing factor (EDRF).22 23 24 The released EDRF provides a flow-related dilator feedback to oppose the constrictor myogenic response to increased intraluminal pressure. In addition, EDRF release could also be induced by stimulation of α2 receptors on endothelial cells.25 In normal segments, the intact endothelium releases several endothelial cell–derived vasodilators and is involved in the metabolism of circulating substances, such as vasoconstrictor catecholamines and platelet products, which attenuates adrenergic-mediated vasoconstriction.12 26 27 Functional endothelium also inhibits the release of norepinephrine from sympathetic nerve terminals and enhances its metabolism.28 Thus, we speculate that in segments with normally functioning endothelium, the intense α-adrenergic–mediated constriction in response to sympathetic stimulation is counteracted by endothelium-mediated vasodilation, whereas segments with dysfunctional endothelium allow for the overexpression of vasoconstrictor forces imposed by α-sympathetic activity.
It has been shown that the vasodilator response to epinephrine in normal canine arteries was reversed to a vasoconstrictor response after removal of the endothelium.29 These findings demonstrated that endothelial integrity plays an important role in protecting against α-adrenergic–mediated vasoconstrictor responses. Our results extend these experimental observations to the human coronary artery system and show that segments with functional endothelium are less susceptible to the early morning α-adrenergic overactivity, whereas segments with dysfunctional endothelium have an exaggerated resting tone and vasoconstrictor response to acetylcholine. Other studies have also shown that canine and human coronary arteries are narrower in the morning than in the afternoon.30 31
The results of the present study suggest that the endothelium plays an important role in modifying fluctuations in coronary tone in response to various physiological stimuli that occur in a circadian fashion. These results provides further insight into the dynamic pathophysiological processes that determine the impact of circadian variations on coronary tone which, in turn, may participate in triggering acute cardiovascular events.
We thank Dr Roger M. Mills, Jr, for his valuable comments.
Reprint requests to Hassan El-Tamimi, MD, UT-Houston Medical School, Department of Medicine, 6431 Fannin, MSB 1.150 Houston, TX 77030.
Presented in part at the 67th Scientific Sessions of the American Heart Association, Dallas, TX, November 14-17, 1994.
- Received January 17, 1995.
- Revision received June 7, 1995.
- Accepted July 26, 1995.
- Copyright © 1995 by American Heart Association
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