Coronary α₁-constrictor tone during renovascular hypertension

Background: A coronary α₁-adrenergic constrictor tone exists under conditions associated with increased sympathetic stimulation but not during resting conditions in the normal heart. During renovascular hypertension, elevated circulating angiotensin H may enhance sympathetic stimulation of the heart, even at rest. This study tested the hypothesis that an α₁- adrenergic constrictor tone imposes limitations on coronary blood flow in resting dogs after development of renovascular hypertension, exacerbates coronary α-constrictor tone during exercise, and increases coronary vascular adrenergic responsiveness. Methods and Results: Left circumflex blood flow velocity (CFV), aortic pressure (AoP), and heart rate (HR) were examined in five quietly resting dogs during control conditions and after selective α₁- adrenergic blockade using an intracoronary injection of 0.5 mg prazosin. In the normotensive state, AoP was 87±7 mm Hg (mean±SD), HR was 105±25 beats per minute, and CFV was 28±6 cm/s. These parameters were not affected by α₁-adrenergic blockade. During submaximal exercise, removal of an α₁- adrenergic constrictor resulted in a 14±4% increase in CFV (P<.05). Two weeks after development of renovascular hypertension induced by stenosis of the left renal artery, mean AoP was 114±7 mm Hg (P<.05 versus normotensive state), HR was 111±28 beats per minute, and CFV was 21±8 cm/s. In contrast to the normotensive state, α₁-adrenergic blockade caused a 28±6% increase in CFV at rest (P<.05) and a 27±13% increase in CFV during exercise in the hypertensive state (P<.05 versus exercise before blockade and versus normotensive state). This resting coronary constrictor tone was associated with enhanced vasoconstrictor responsiveness to norepinephrine and phenylephrine. Conclusions: It appears that renovascular hypertension results in a significant coronary α₁-adrenergic constrictor tone in the resting dog and an enhanced constrictor tone during exercise.