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Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle, Washington 98195-7290
Recent experiments demonstrate that
feedforward sympathetic
-adrenoceptor coronary vasodilation occurs
during exercise. The present study quantitatively examined the
contributions of epinephrine and norepinephrine to exercise coronary
hyperemia and tested the hypothesis that circulating epinephrine causes
feedforward
-receptor-mediated coronary dilation. Dogs
(n = 10) were chronically instrumented with a
circumflex coronary artery flow transducer and catheters in the aorta
and coronary sinus. During strenuous treadmill exercise, myocardial
oxygen consumption increased by ~3.9-fold, coronary blood flow
increased by ~3.6-fold, and arterial plasma epinephrine concentration
increased by ~2.4-fold over resting levels. At arterial concentrations matching those during strenuous exercise, epinephrine infused at rest (n = 6) produced modest increases
(18%) in flow and myocardial oxygen consumption but no evidence of
direct
-adrenoceptor-mediated coronary vasodilation. Arterial
norepinephrine concentration increased by ~5.4-fold during exercise,
and coronary venous norepinephrine was always higher than arterial,
indicating norepinephrine release from cardiac sympathetic nerves. With
the use of a mathematical model of cardiac capillary norepinephrine
transport, these norepinephrine concentrations predict an average
interstitial norepinephrine concentration of ~12 nM during strenuous
exercise. Published dose-response data indicate that this
norepinephrine concentration increases isolated coronary arteriolar
conductance by ~67%, which can account for ~25% of the increase
in flow observed during exercise. It is concluded that a significant
portion of coronary exercise hyperemia (~25%) can be accounted for
by direct feedforward
-adrenoceptor coronary vascular effects of
norepinephrine, with little effect from circulating epinephrine.
coronary blood flow; norepinephrine; epinephrine
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