Coronary flow during exercise after selective alpha 1- and alpha 2-adrenergic blockade

This study was carried out to determine the relative importance of alpha 1- and alpha 2-adrenergic vasoconstriction in opposing the increase in coronary blood flow, which occurs during exercise. The response of left circumflex coronary artery blood flow was examined during treadmill exercise in 16 chronically instrumented dogs during control conditions, after selective alpha 1-adrenergic blockade with intracoronary prazosin, and after alpha 2-blockade with intracoronary idazoxan. During control conditions, graded treadmill exercise resulted in progressive increases of coronary blood flow and decreases of coronary vascular resistance. Prazosin produced highly selective alpha 1-adrenergic blockade; coronary blood flow was significantly higher and coronary vascular resistance significantly lower during all but the heaviest exercise stage after prazosin. Idazoxan produced highly effective, but only moderately selective, alpha 2-adrenergic blockade. However, after idazoxan, coronary blood flow and coronary vascular resistance during exercise were not significantly different from control. Combined alpha 1- and alpha 2-adrenergic blockade was not more effective in increasing coronary blood flow during exercise than was alpha 1-adrenergic blockade alone. These data support a role for alpha 1-adrenergic coronary vasoconstriction in limiting the increase in coronary blood flow, which occurs during exercise, but do not support a role for alpha 2-mediated coronary vasoconstriction during exercise.