Regulation of large coronary arteries by increases in myocardial metabolic demands in conscious dogs

In order to assess the effects of increasing myocardial metabolic demand on the large epicardial coronary arteries, we measured left circumflex coronary artery diameter (ultrasonic transit time technique) and blood flow in conscious dogs with chronically implanted transducers. Myocardial oxygen consumption was increased by pacing-induced tachycardia and aortic constriction, and monitored by multiplying left circumflex coronary arterial blood flow by coronary arterio-venous oxygen content difference. Increase of heart rate by 90 beats/min caused myocardial oxygen consumption to increase by 34 ± 4.3% (1 SEM); coronary blood flow at constant arterial pressure to increase by 32 ± 6.8%; and coronary diameter to increase by 0.07 ± 0.01 mm, P<0.01. Aortic constriction, producing a 53 ± 5.1% increase of left ventricular systolic pressure, caused myocardial oxygen consumption to increase by 49 ± 7.2%, coronary blood flow to increase by 50 ± 6.0%, and coronary diameter to increase 0.13 ± 0.03 mm, P<0.01. The increases in coronary artery diameter were gradual, not immediate, in onset and not altered by β-adrenergic blockade. Thus, increased myocardial metabolic demand dilates large epicardial coronary arteries, but with a slower response time than the rapid dilation of the smaller resistance vessels.