Muscle metaboreflex control of coronary blood flow

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Abstract
We investigated the effect of muscle metaboreflex activation on left circumflex coronary blood flow (CBF) and vascular conductance (CVC) in conscious, chronically instrumented dogs during treadmill exercise ranging from mild to severe workloads. Metaboreflex responses were also observed during mild exercise with constant heart rate (HR) of 225 beats/min and β1-adrenergic receptor blockade to attenuate the substantial reflex increases in cardiac work. The muscle metaboreflex was activated via graded partial occlusion of hindlimb blood flow. During mild exercise, with muscle metaboreflex activation, hindlimb ischemia elicited significant reflex increases in mean arterial pressure (MAP), HR, and cardiac output (CO) (+39.0 ± 5.2 mmHg, +29.9 ± 7.7 beats/min, and +2.0 ± 0.4 l/min, respectively; all changes, P < 0.05). CBF increased from 51.9 ± 4.3 to 88.5 ± 6.6 ml/min, ( P < 0.05), whereas no significant change in CVC occurred (0.56 ± 0.06 vs. 0.59 ± 0.05 ml · min−1 · mmHg−1; P > 0.05). Similar responses were observed during moderate exercise. In contrast, with metaboreflex activation during severe exercise, no further increases in CO or HR occurred, the increases in MAP and CBF were attenuated, and a significant reduction in CVC was observed (1.00 ± 0.12 vs. 0.90 ± 0.13 ml · min−1 · mmHg−1; P < 0.05). Similarly, when the metaboreflex was activated during mild exercise with the rise in cardiac work lessened (via constant HR and β1-blockade), no increase in CO occurred, the MAP and CBF responses were attenuated (+15.6 ± 4.5 mmHg, +8.3 ± 2 ml/min), and CVC significantly decreased from 0.63 ± 0.11 to 0.53 ± 0.10 ml · min−1 · mmHg−1. We conclude that the muscle metaboreflex induced increases in sympathetic nerve activity to the heart functionally vasoconstricts the coronary vasculature.