Parasympathetic control of transmural coronary blood flow in dogs

The transmural distribution of coronary blood flow was studied during vagal stimulation in closed-chest, morphine- and alpha-chloralose-anesthetized dogs. The left main coronary artery was cannulated and perfused at constant pressure. Bradycardia during vagal stimulation was prevented by atrioventricular heart block and ventricular pacing. Beta-adrenergic receptors were blocked with propranolol (1 mg/kg iv), and aortic pressure was stabilized by means of a pressure reservoir. Regional myocardial blood flow was measured with 9-micron radioactive microspheres during vagal stimulation and during intracoronary acetylcholine infusion. Vagal stimulation increased coronary blood flow uniformly across the left ventricular wall. In contrast, intracoronary acetylcholine infusion, at a rate selected to increase total flow to the same degree, vasodilated the subendocardium more than the subepicardium, increasing the inner/outer blood flow ratio. It is concluded that both vagal activation and acetylcholine produce coronary vasodilation that is independent of left ventricular preload, afterload, and heart rate. Acetylcholine vasodilation preferentially vasodilates the subendocardium, increasing the inner/outer flow ratio, but vagal stimulation produces uniform vasodilation across the left ventricular wall.