Effects of α- and β-Adrenergic Blockade on Coronary Microcirculation in the Beating Canine Left Ventricle
This sudy was carried out to clarify the effects of α- and β-adrenergic blockade on coronary arterial microvessels in beating hearts. For this purpose, we used anesthetized open chest dogs (n = 47). Coronary microcirculation of the epimyocardium was observed with a microscope equipped with a floating objective and a high-speed video system. Diameters of arterial microvessels were measured by fluorescence microangiography. Heart rate and aortic pressure were kept nearly constant throughout the experimental period. In the following experiments, drugs were administered into the left anterior descending coronary artery (LAD). To assess the effect of α-adrenergic blockade, phentolamine (100 μg/kg) was administered in the absence or presence of β-adrenergic blockade (propranolol 50 μg/kg). To assess the effect of β-adrenergic blockade, propranolol (50 μg/kg) or three doses of ICI-118551 a selective β2-antagonist (0.1, 0.5, and 1.0 μg/kg per min) were administered. Coronary arterial microvessels were divided into three groups according to control internal diameters (ID) (small, ID < 100 μm; medium, 100 ≤ ID < 200 μm; and large, ID ≥ 200 μm). In the absence of β-adrenergic blockade, phentolamine significantly dilated vessels in all groups (small, 19.6 ± 5.6%; medium, 5.8 ± 2.3%; and large, 5.3 ± 0.9%; P < 0.05 vs control). In the presence of β- adrenergic blockade, the vasodilator effect of phentolamine was completely abolished.