Effect of β-adrenergic receptor blockade on blood flow to collateral-dependent myocardium during exercise

Background: β-Adrenergic receptors have been identified in isolated coronary collateral blood vessels, but their functional significance in the intact heart has not been demonstrated. Methods and Results: We measured myocardial blood flow with radioactive microspheres in normal and collateral-dependent myocardium in eight dogs trained to run on a treadmill before and after β-adrenergic blockade with propranolol, 200 μg/kg, a dose that effectively inhibited the increase in coronary blood flow produced by selective β₁- and β₂-adrenergic agonists. Collateral vessel growth was stimulated with 2-minute intermittent occlusions of the left anterior descending artery followed by permanent occlusion. During control exercise, blood flow in the collateral zone was 38±5% less than in the normal zone. At identical levels of exercise, with heart rate maintained constant by atrial pacing, propranolol decreased mean blood flow in the collateralized myocardium from 1.93±0.17 to 1.50±0.14 mL · min^-1 · g^-1 (P<.01), while increasing the subendocardial to subepicardial blood flow ratio from 0.78±0.11 to 0.91±0.10 (P<.05). The decrease in collateral zone blood flow in response to propranolol resulted from an increase in both transcollateral resistance from 25.9±2.3 to 35.2±4.3 mm Hg · mL^-1 · min · g (P<.05) and small-vessel resistance in the collateral-dependent myocardium from 30.9±4.7 to 44.0±8.8 mm Hg · mL^-1 •min · g (P<.07). Blood flow to the normal zone was also significantly reduced from 3.14±0.21 to 2.23±0.12 mL · min^-1 · g^-1 (P<.01) after propranolol. Conclusions: β-Adrenergic blockade decreased blood flow to collateral-dependent myocardium during exercise. These results indicate that β-adrenergic receptor activation contributes to vasodilation of coronary collateral vessels during exercise.