The purpose of this study was to determine the coronary microvascular sites of autoregulation. The epimyocardial coronary microcirculation was observed through an intravital microscope by stroboscopic epi-illumination in anesthetized open-chest dogs (n = 20). Aortic pressure and heart rate were held constant by an aortic snare and atrial pacing, respectively. Distal pressure of the left anterior descending coronary artery was controlled by a screw occluder on the proximal left anterior descending coronary artery and monitored with a 24-gauge plastic cannula inserted into the branch or distal portion of the left anterior descending coronary artery. Distal pressure of the left anterior descending coronary artery was stepwisely reduced to 59 ± 1 mm Hg (mild stenosis, n = 20) and 38 ± 1 mm Hg (severe stenosis, n = 16). In the left circumflex coronary artery area, myocardial blood flow measured with radioactive microspheres to subepicardium, midmyocardium, and subendocardium did not change with the mild and severe stenosis from control. In the left anterior descending coronary artery area, myocardial blood flow to each layer remained at nearly control level with the mild stenosis but was reduced in midmyocardium and subendocardium with the severe stenosis. With the mild stenosis, diameters of coronary arterial microvessels less than 100 μm in diameter dilated, and those larger than 100 μm in diameter did not change. The magnitude of vasodilation in small arterial microvessels was inversely related to control diameter. With the severe stenosis, small arterial microvessels dilated, and simultaneously, large arterial microvessels constricted. Again, the magnitude of vasodilation in small arterial microvessels was inversely related to control diameter. From these results, we conclude that the primary site of autoregulation in epicardial coronary arterial microvessels resides in arterial microvessels less than 100 μm in diameter. When perfusion pressure is severely reduced, constriction of large arterial microvessels may limit the autoregulatory responses.
- Intravital microscope
- Myocardial blood flow
- Radiolabeled microsphere
ASJC Scopus subject areas
- Cardiology and Cardiovascular Medicine