ALPHA-ADRENERGIC VASOCONSTRICTION REDUCES SYSTOLIC RETROGRADE CORONARY BLOOD-FLOW

There is a paradoxical alpha-adrenoceptor-mediated coronary vasoconstr iction whenever there is adrenergic activation of the heart, as during cardiovascular reflexes or exercise. A previous study demonstrated th at this paradoxical vasoconstriction helps maintain blood flow to the vulnerable inner layer of the left ventricular wall during exercise, b ut the mechanism for this effect was not elucidated. The purpose of th e present investigation was to test the hypothesis that alpha-adrenoce ptor-mediated vasoconstriction lessens the to-and-fro oscillation of b lood flow that occurs in the coronary arterial tree during systole and diastole. Septal coronary artery blood velocity was measured in anest hetized open-chest dogs with a 20-MHz pulsed Doppler velocimeter. Syst olic retrograde velocity and diastolic forward velocity were compared during norepinephrine infusion before and after alpha-adrenoceptor blo ckade with phenoxybenzamine. Systolic aortic pressure was held constan t by aortic banding; heart rate was controlled by pacing at 80, 140, a nd 200 beats/min; and maximum left ventricular dP/dt was unchanged by alpha-blockade. At each pacing rate, systolic retrograde velocity was significantly greater after alpha-blockade, indicating that alpha-vaso constriction reduced systolic retrograde flow by changing coronary vas cular impedance. Transmural blood flow was measured with microspheres in a second group of dogs during the same experimental conditions, and flow to the inner layer of the left ventricle was diminished by alpha -adrenoceptor blockade at a heart rate of 250 beats/min, demonstrating a beneficial effect of alpha-vasoconstriction. In conclusion, adrener gic alpha-adrenoceptor-mediated coronary vasoconstriction reduces syst olic retrograde coronary flow during norepinephrine infusion. This les sens to-and-fro flow oscillation in the coronary circulation and proba bly is the mechanism whereby alpha-vasoconstriction helps maintain blo od flow to the inner layer of the left ventricle during exercise.