Effects of nitroglycerin on the coronary microcirculation in normal and ischemic myocardium

Nitroglycerin dilates conduit coronary vessels and only transiently increases flow, however the effects of nitroglycerin in the microcirculation of normal myocardium and during myocardial ischemia have not been assessed. The goal of this investigation was to determine the effects of steady-state levels of nitroglycerin on the microcirculation of normal and ischemic myocardium. Microvessels on the left ventricle were viewed using stroboscopic epi-illumination in anesthetized, open-chest dogs. Myocardial perfusion was measured with radioactive microspheres. Aortic pressure and heart rate were kept constant by an aortic snare and left atrial pacing. Microvessel diameters were measured under control conditions and during steady-state infusion of nitroglycerin (n = 11, 0.01-100 μg kg^-1 min^-1, i.v.). Nitroglycerin selectively dilated arteries from 201 to 386 μm, but had no effect on large arterioles <200 μm. Total coronary vascular resistance remained constant except at the highest dose. When mean coronary pressure was decreased to 35 mm Hg, small arterioles <100 μm dilated. Diameters of larger arterioles decreased. Nitroglycerin (10 μg kg^-1 min^-1, i.v., n = 8) selectively dilated microvessels >200 μm in the region distal to the stenosis, although myocardial perfusion was not affected. Thus, nitroglycerin altered the distribution of microvascular resistance without altering overall resistance. We conclude that steady-state infusion of nitroglycerin selectively dilates coronary arterial microvessels >200 μm. During decreased perfusion pressure, recruitable vasodilation in response to nitroglycerin is due to dilation of microvessels >200 μm.