Background: Experimental studies on the effects of alpha 2-adrenoceptors on regional coronary blood flow in normal and ischemic myocardium are highly controversial. A beneficial effect on regional ischemic myocardium has been demonstrated in different animal preparations with either alpha 2-adrenoceptor blockade or stimulation. Animal studies also demonstrated that postsynaptic alpha 2-adrenoceptors mediate vasoconstriction in coronary and femoral vascular beds. The aims of the study were 1) to investigate the effects of regional alpha 2-adrenoceptor stimulation on regional coronary blood flow in subjects with angiographically normal coronary arteries, 2) to assess the effect of alpha 2-adrenoceptor blockade on coronary circulation in control subjects, and 3) to examine the influence of atherosclerosis on coronary blood flow response to alpha 2-adrenoceptor blockade.
Methods and results: The effect of regional administration of BHT 933 (a selective alpha 2-adrenoceptor agonist) was studied in eight subjects with angiographically normal coronary arteries. The coronary blood flow velocity was measured using a subselective intracoronary 3F Doppler catheter and coronary diameter by quantitative coronary angiography. BHT 933 induced a reduction in coronary artery diameter from 2.5 +/- 0.6 mm to 1.8 +/- 0.4 mm (p less than 0.05) as well as in coronary blood flow velocity (from 6.4 +/- 0.9 cm/sec to 4.6 +/- 1.9 cm/sec, p less than 0.01). In some subjects, ST segment abnormalities occurred. In patients with angiographically normal coronary arteries (n = 6), the regional infusion of a selective alpha 2-adrenoceptor blocking agent after beta-blockade did not change coronary diameter or coronary blood flow velocity. In contrast, in patients with significant coronary stenoses (n = 6), regional infusion of an alpha 2-adrenoceptor blocking agent reduced regional coronary artery diameter (from 2.3 +/- 0.5 mm to 2.1 +/- 0.6 mm, p less than 0.01) as well as coronary blood flow velocity (from 5.8 +/- 0.8 cm/sec to 3.7 +/- 0.6 cm/sec, p less than 0.05); in addition, alpha 2-adrenoceptor blockade significantly increased coronary sinus plasma norepinephrine levels (from 300 +/- 144 pg/ml to 429 +/- 207 pg/ml, p less than 0.01).
Conclusions: The selective in vivo stimulation of alpha 2-adrenoceptors produces a reduction in coronary blood flow and diameter in humans with angiographically normal coronary arteries. alpha 2-Adrenergic blockade does not change coronary blood flow in subjects with angiographically normal coronary arteries (suggesting no resting alpha 2-adrenergic vasoconstrictor tone), whereas in patients with coronary artery stenosis, regional coronary blood flow decreases after alpha 2-receptor blockade. Finally, our data also suggest that alpha 2-adrenoceptors participate in the modulation of sympathetic neuronal norepinephrine release in the human heart.