Effects of α and β adrenergic blockade on coronary arterial microvessels in the beating canine heart

Objective: The aim was to clarify the effects of α and β adrenergic blockade on coronary arterial microvessels and to assess the role of α and β adrenergic tone in normally beating hearts. Methods: 47 anaesthetised open chest dogs were studied. The diameters of epicardial arterial microvessels were measured in beating hearts using an incident light fluorescence microscope equipped with a floating objective. Drugs were infused into the left anterior descending coronary artery keeping the heart rate and aortic pressure at control levels. To examine the effect of α adrenergic blockade, phentolamine (100 μg·kg^-1) was given in the absence or presence of β adrenergic blockade (propranolol 50 μg·kg^-1). To examine the effect of β adrenergic blockade, propranolol (50 μg·kg^-1) or three doses of ICI 118,551 (a selective β₂ antagonist, 0.1, 0.5, and 1.0 μg·kg^-1·min^-1) was given. Results: Coronary arterial microvessels were divided into three groups according to the control diameters (D) of small (D<100 μm), medium (100≤D<200 μm) and large (D≥200 μm) groups. In the absence of β adrenergic blockade, phentolamine significantly dilated all vessel groups: small +19.6 (SEM 5.6)%, medium +5.8(2.3)%, large +5.3(0.9)%. In the presence of β adrenergic blockade, the vasodilator effect of phentolamine was completely abolished. Propranolol constricted all vessel groups: small -3.6(1.1)%, medium -4.8(1.0)%, large -3.5(1.0)%. ICI 118,551 significantly constricted the large vessel group [-2.5(0.6)%] at the mid dose, and the medium and large vessel groups [medium -3.1 (0.8)%, large -3.5(1.3)%] at the highest dose. Conclusions: These data indicate that (1) the vasodilator effect of phentolamine is induced by β adrenergic stimulation; (2) resting α adrenergic tone of coronary arterial microvessels is minimal in normally beating hearts, and (3) resting β adrenergic tone may play a physiological role in coronary arterial microvessels, and β₂ adrenergic tone predominates in arterial microvessels greater than 100 μm in diameter.