HETEROGENEOUS MICROVASCULAR CORONARY VASODILATION BY ADENOSINE AND NITROGLYCERIN IN DOGS

We investigated the effects of adenosine and nitroglycerin (NTG) on coronary microvessel diameters (intravital fluorescence microscopy) and coronary perfusion (radioactive microspheres). Measurements were performed during baseline conditions (intravenous piritramid) and during controlled hypotension (mean arterial pressure similar to 60 mmHg) induced by halothane, adenosine, and NTG. Coronary vascular resistance (CVR) remained unchanged during halothane (-7%) but decreased during adenosine (-76%) and NTG (-29%). Coronary arteriolar diameters increased during all experimental steps. In the smallest vessels (20-40 mu m), diameters increased by 14, 43, and 42% during halothane-, adenosine-, and NTG-induced hypotension, respectively. Diameter increases were less pronounced in larger vessels. The uniform action of adenosine and NTG in 20- to 500-mu m arterial vessels is in contrast to the pronounced differences in reduction of CVR. Preferential dilation of arterioles <20 mu m or recruitment of coronary microvessels by adenosine might account for the more pronounced decrease of CVR during adenosine. Intracoronary application of adenosine (0.8 mg.kg(-1).h(-1)) and NTG (1, 5, and 25 mu g kg(-1) h(-1)) equally caused near-maximum dilation of coronary arterioles >100 mu m. However, NTG dilation of arterioles <100 mu m was dose dependent and exceeded large-vessel dilation only with the highest concentration of NTG.