The effect of generalized alpha-receptor stimulation on regional myocardial blood flow distal to a severe coronary artery stenosis.

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Abstract
Stimulation of coronary artery .alpha. receptors may decrease regional myocardial blood flow in patients with coronary artery disease. Closed-chest conscious pig 13 were studied to compare the role of vasoconstriction of coronary conductance vessels and that of arteriolar resistance vessels in this response. An artificial stenosis 7.5 mm long that caused an 82% reduction in vessel diameter was placed within the lumen of the left anterior descending coronary artery. Radioactive microspheres were used to determine regional myocardial blood flow in endocardium and epicardium distal to the stenosis and in endocardium and epicardium supplied by the circumflex coronary artery. In 7 pigs, hemodynamics and regional myocardial blood flow were measured during control conditions, at the 10th min of i.v. norepinephrine [NE] infusion (to increase mean arterial pressure 20 mm Hg), during a 2nd control period and at the 10th min of i.v. adenosine infusion (to lower diastolic arterial pressure to 55 mm Hg). In response to NE, regional myocardial blood flow (ml/min .cntdot. g-1, mean .+-. SD) increased in the dista zone epicardium (1.07 .+-. 0.20 to 1.29 .+-. 0.23 mean .+-. SD, P < 0.004) and endocardium (1.04 .+-. 0.24 to 1.21 .+-. 0.24, P < 0.05). Flow increased to a similar extent during NE in the circumflex zone epicardium (1.11 .+-. 0.16 to 1.48 .+-. 0.48, P < 0.01) and endocardium (1.22 .+-. 0.19-1.62 .+-. 0.54, P < 0.02). In contrast, during adenosine, regional myocardial blood flow increased in circumflex zone endocardium (1.60 .+-. 0.51 .+-. 3.07 .+-. 0.83, P < 0.005) and epicardium (1.44 .+-. 0.44 to 3.83 .+-. 1.35, P < 0.005), declined in distal zone endocardium (1.21 .+-. 0.39 to 0.62 .+-. 0.22, P < 0.005) and remained unchanged in distal zone epicardium (1.33 .+-. 0.36 to 1.23 .+-. 0.29, NS). Six additional pigs were given i.v. propranolol, 2.0 mg/kg, and studied as described above. Regional myocardial blood flow during infusion of NE did not change significantly compared with control. Calculated coronary arteriolar resistance increased significantly (P < 0.01) compared with control in both endocardium and epicardium distal to the stenosis. In the presence of a severe fixed coronary artery stenosis, .alpha.-mediated arteriolar vasoconstriction may compete with, but is unlikely to overwhelm, metabolically mediated vasodilation.