Adrenergic vasoconstriction limits coronary blood flow during exercise in hypertrophied left ventricle

This study was carried out to test the hypothesis that α-adrenergic vasoconstriction limits coronary blood flow (CBF) during exercise in the chronically pressure overloaded, hypertrophied left ventricle. Studies were performed in dogs in which left ventricular hypertrophy had been produced by banding the ascending aorta at 9 wk of age. Left circumflex coronary artery blood flow and myocardial O₂ consumption (MV̇O₂) were examined at rest and during treadmill exercise during control conditions, after selective α₁-adrenergic blockade with prazosin, and after nonselective α-adrenergic blockade with phentolamine. All studies were performed after β-adrenergic blockade with propranolol. During control conditions CBF and MV̇O₂ increased progressively during exercise, while coronary sinus O₂ tension decreased. Neither prazosin nor phentolamine altered CBF at rest but, in comparison with control measurements, both agents significantly increased CBF during exercise and abolished the decrease in coronary sinus O₂ tension that normally occurred during exercise. Both prazosin and phentolamine caused similar significant increases of MV̇O₂ relative to the heart rate times systolic left ventricular pressure during exercise, indicating that the increased CBF produced by these agents enhanced MV̇O₂. Similar findings after prazosin and phentolamine indicate that adrenergic restraint of CBF during exercise resulted principally from α₁-adrenergic vasoconstrictions with little additional contribution from postjunctional α₂-adrenergic mechanisms.