Abstract
Muscle metaboreflex activation (MMA) during dynamic exercise increases cardiac work and myocardial O2 demand via increases in heart rate, ventricular contractility, and afterload. This increase in cardiac work should lead to metabolic coronary vasodilation; however, no change in coronary vascular conductance occurs. This indicates that the MMA-induced increase in sympathetic activity to the heart, which raises heart rate, ventricular contractility, and cardiac output, also elicits coronary vasoconstriction. In heart failure, cardiac output does not increase with MMA presumably due to impaired ability to improve left ventricular contractility. In this setting actual coronary vasoconstriction is observed. We tested whether this coronary vasoconstriction could explain, in part, the reduced ability to increase cardiac performance during MMA. In conscious, chronically instrumented dogs before and after pacing-induced heart failure, MMA responses during mild exercise were observed before and after α1-adrenergic blockade (prazosin 20-50 μg/kg). During MMA, the increases in coronary vascular conductance, coronary blood flow, maximal rate of left ventricular pressure change, and cardiac output were significantly greater after α1-adrenergic blockade. We conclude that in subjects with heart failure, coronary vasoconstriction during MMA limits the ability to increase left ventricular contractility.