VAGAL MODULATION OF EPICARDIAL CORONARY-ARTERY SIZE IN DOGS - A 2-DIMENSIONAL INTRAVASCULAR ULTRASOUND STUDY

Background Because the role of tonic vagus nerve activity in regulatin g conduit coronary artery size remains undefined, we investigated the response of epicardial coronary artery size to changes in resting vaga l tone resulting from vagotomy and muscarinic receptor blockade. Metho ds and Results Using intravascular ultrasound to measure left circumfl ex coronary artery cross-sectional area continuously, we examined the effects of vagotomy on left circumflex cross-sectional area in nine do gs. Lumen area decreased 20% from 8.70+/-2.81 to 6.92+/-1.97 mm(2) aft er right vagotomy, 17% to 7.19+/-2.80 mm(2) after left vagotomy (both P<.05 versus base line), and 38% to 5.42+/-2.00 mm(2) after bilateral vagotomy (P<.05 versus unilateral vagotomy). Vasoconstriction occurred despite increases in heart rate and an unchanged rate-pressure produc t. In six additional dogs, after acetylcholine (100 mu g/kg IV), lumen area increased by 18%, although heart rate, blood pressure, and rate- pressure product were unchanged. Vasodilation was prevented by prior m uscarinic blockade with glycopyrrolate. With glycopyrrolate administra tion and heart rate control by pacing, lumen area decreased by 26% (P= .011). When stellate stimulation was performed in a third group of eig ht dogs with heart rate, blood pressure, and rate-pressure product con trolled by a combination of pacing and exsanguination, there was no ch ange in coronary area, thus precluding reflex sympathetic activation a s a contributor to the vasoconstriction produced by vagal withdrawal. Conclusions Vagus nerve activity maintains tonic dilation of the left circumflex coronary artery by muscarinic receptor activation. Each vag us nerve contributes approximately equally to the tonically dilated st ate. Vagotomy-induced vasoconstriction occurs independently of local m etabolic factors and coronary distending pressure and is a result of c holinergic withdrawal rather than reflex sympathetic activation.