Evidence against significant resting sympathetic coronary vasoconstrictor tone in the conscious dog

Abstract

The primary objective of this study was to ascertain whether resting coronary blood flow is under tonic restraint due to sympathetically mediated α-adrenergic coronary vasoconstriction. To accomplish this, we first developed and verified a technique for selectively sympathectomizing the posterior region of the canine left ventricle. This technique entailed the topical application of phenol in a thin line to specific sites on the myocardium and epicardial vessels. As part of the verification, we demonstrated that left stellate nerve stimulation caused increases in the myocardial extraction ratios for oxygen and lactate in the normally innervated region (I) of the ventricle, but not in the sympathectomized region (Sx). We then measured regional myocardial blood flow with the microspheres in phenol-treated animals under conscious, resting conditions. The animals were acclimated to the laboratory environment, and their arterial plasma norepinephrine levels averaged 135 ± 37 pg/ml. Heart rate (81 ± 3 bpm) and mean aortic pressure (100 ± 2 mm Hg) were not significantly affected by β-adrenergic blockade or combined α- and β-adrenergic blockade in these animals. Blood flow in I and Sx averaged 0.87 ± 0.08 ml/min per g and 0.85 ± 0.07 ml/min per g, respectively, and the difference was not statistically significant. The endocardial-to-epicardial blood flow ratio in I and Sx averaged 1.23 ± 0.03 and 1.29 ± 0.04, respectively, and the difference was not statistically significant. The results were not significantly affected by β-adrenergic blockade or combined α- and β-adrenergic blockade. We were unable to confirm previous evidence in the literature of significant resting sympathetic coronary vasoconstrictor tone in the conscious animal.