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.