Regulation of large coronary vessels by adrenergic mechanisms in conscious dogs

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Abstract
The effects of α and β adrenergic stimulation were examined in conscious dogs on measurements of left circumflex coronary blood flow and coronary arterial diameter and on calculations of mean coronary resistance (MCR) and left circumflex coronary internal cross-sectional area (CSA). Methoxamine (50 μg/kg/min), after transiently increasing left circumflex coronary dimensions, induced sustained reductions in left circumflex coronary diameter (9±2%) and CSA (27±5%) at a time when mean arterial pressure rose by 65±5%, left ventricular (LV) dP/dt had decreased only slightly, and heart rate and mean coronary blood flow were at control levels. Isoproterenol, (0.1 μg/kg/min), increased heart rate by 66±8%, LV dP/dt by 58±5%, and CSA by 17±3%, while it decreased mean arterial pressure by 12±1% and MCR by 44±5%. After β1 adrenergic receptor blockade and with heart rate held constant, isoproterenol did not increase LV dP/dt but decreased mean arterial pressure similarly (13±2%) and induced attenuated increases in CSA (6±1%), and decreases in MCR (17±3%). After combined β1 and β2 adrenergic receptor blockades isoproterenol induced no significant effects. 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 CSA, despite the opposing elevation of distending pressure. Moreover, large vessels appear to be regulated by α1 adrenergic mediated increase in myocardial metabolic demands, as well as by α2 adrenergic mediated vasodilation.