The Journal of clinical investigation
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The effects of methoxamine and nitroglycerin on measurements of large vessel (left circumflex) coronary dimensions were examined in eight conscious dogs using an ultrasonic dimension gauge, and total coronary resistance was calculated from measurements of arterial pressure and coronary blood flow. Methoxamine (50 mug/kg per min), after transiently increasing left circumflex coronary dimensions, induced sustained reductions in left circumflex diameter (9+/-2%) and external (18+/-4%) and internal (27+/-5%) cross-sectional areas, at a time when mean arterial pressure rose by 65+/-5%, left ventricular dP/dt had decreased only slightly, and heart rate and mean coronary blood flow remained at control levels. Calculated large vessel and total coronary resistances rose similarly, i.e., by 108+/-29 and 92+/-14%, respectively. ⋯ At this time, significantly different effects were observed on large vessel coronary resistance, which fell by 18+/-2%, and on total coronary resistance, which rose by 11+/-4%. Thus, in the conscious dog, large coronary vessels not only react passively to changes in aortic pressure but also undergo substantial active changes. Alpha adrenergic stimulation is sufficiently powerful to reduce cross-sectional area, despite the opposing elevation of distending pressure.