Coronary blood flow during exercise following nonselective and selective α₁-adrenergic blockade with indoramin

Studies were performed to compare the effects of α₁-adrenergic blockade with indoramin and nonselective α-adrenergic blockade with phentolamine on coronary blood flow and myocardial oxygen consumption during exercise. Nine dogs trained to run on a motor-driven treadmill were instrumented with electromagnetic flowmeter probes on the left circumflex coronary artery, and aortic and coronary sinus catheters for determination of myocardial arteriovenous oxygen difference. During control conditions, myocardial oxygen consumption and coronary blood flow increased as a direct function of heart rate during exercise. Phentolamine caused a significant decrease in blood pressure, while heart rate, coronary blood flow, and myocardial oxygen consumption were significantly increased at rest and during all exercise stages. Although α₁-adrenergic blockade with indoramin caused a similar reduction of arterial pressure, heart rate was unaltered both at rest and during exercise. Coronary blood flow and myocardial oxygen consumption were unchanged by α¹-adrenergic blockade at rest and during light exercise; however, during the heaviest exercise stages α₁-blockade caused a significant decrease in myocardial oxygen consumption. These findings are in agreement with the concept that phentolamine, by blocking presynaptic α₂-adrenoceptors which normaliy modulate norepinephrine release, increases sympathetic activity during exercise while indoramin, by acting as a selective α₁-adrenergic blocker, does not produce this effect.