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-Adrenergic vasoconstriction reduces systolic retrograde
coronary blood flow
Department of Biomedical Engineering and Systems Cardiology, Kawasaki Medical School, Kurashiki City, Okayama 701-01, Japan
There is a paradoxical
-adrenoceptor-mediated coronary vasoconstriction whenever there is
adrenergic activation of the heart, as during cardiovascular reflexes
or exercise. A previous study demonstrated that this paradoxical
vasoconstriction helps maintain blood flow to the vulnerable inner
layer of the left ventricular wall during exercise, but the mechanism
for this effect was not elucidated. The purpose of the present
investigation was to test the hypothesis that -adrenoceptor-mediated
vasoconstriction lessens the to-and-fro oscillation of blood flow that
occurs in the coronary arterial tree during systole and diastole.
Septal coronary artery blood velocity was measured in anesthetized
open-chest dogs with a 20-MHz pulsed Doppler velocimeter. Systolic
retrograde velocity and diastolic forward velocity were compared during
norepinephrine infusion before and after -adrenoceptor blockade with
phenoxybenzamine. Systolic aortic pressure was held constant by aortic
banding; heart rate was controlled by pacing at 80, 140, and 200 beats/min; and maximum left ventricular
dP/dt was unchanged by -blockade. At each pacing rate, systolic retrograde velocity was significantly greater after -blockade, indicating that -vasoconstriction
reduced systolic retrograde flow by changing coronary vascular
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
-adrenoceptor blockade at a heart rate of 250 beats/min,
demonstrating a beneficial effect of -vasoconstriction. In
conclusion, adrenergic -adrenoceptor-mediated coronary
vasoconstriction reduces systolic retrograde coronary flow during
norepinephrine infusion. This lessens to-and-fro flow oscillation in
the coronary circulation and probably is the mechanism whereby
-vasoconstriction helps maintain blood flow to the inner layer of
the left ventricle during exercise.
coronary vasoconstriction; -adrenoreceptors; coronary vascular
impedance; Doppler velocimeter; phasic coronary flow
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