Adrenergic vasoconstriction lessens transmural steal during coronary hypoperfusion.

The hypothesis that alpha-receptor-mediated coronary vasoconstriction can lessen transmural steal during hypoperfusion was tested in 14 open-chest, chloralose-anesthetized dogs. Coronary flow to two regions of the left ventricle was controlled by separately cannulating and pump perfusing the anterior descending and circumflex coronary arteries. An intracoronary infusion of the alpha-receptor blocking agent phenoxybenzamine (0.25 mg/kg) was administered to one region while coronary sinus blood flow was trapped to minimize recirculation. All animals received atropine (0.5 mg/kg) and propranolol (2 mg/kg) intravenously. alpha-Receptor activation in both regions of the left ventricle was achieved with an intracoronary infusion of norepinephrine. Radioactive microspheres were used to measure the transmural distribution of myocardial blood flow as coronary flow in the paired regions was reduced from 100 to 80, 70, 60, and 50% of normal. When total coronary flow was reduced sufficiently to cause ischemia and maldistribution of flow across the myocardial wall, the subendocardial blood flow was greater in the alpha-receptor-intact region than in the alpha-blocked region. This indicates that alpha-receptor-mediated coronary vasoconstriction has an unexpected beneficial effect that lessens transmural steal during hypoperfusion.