Muscle Metaboreflex‐Induced Vasoconstriction of Ischemic Active Skeletal Muscle

Abstract

Metabolite accumulation due to ischemia of active skeletal muscle stimulates group III/IV chemosensitive afferents eliciting reflex increases in arterial blood pressure and sympathetic activity, termed the muscle metaboreflex. We and others have previously demonstrated sympathetically‐mediated vasoconstriction of coronary, renal and forelimb vasculatures with muscle metaboreflex activation (MMA). Whether MMA elicits vasoconstriction of the ischemic muscle from which it originates is unknown. We hypothesized that the vasodilation in the active skeletal muscle with imposed ischemia becomes progressively restrained by the increasing sympathetic vasoconstriction during MMA. We activated the metaboreflex during mild, dynamic exercise in chronically instrumented canines via graded reductions in hindlimb blood flow (HLBF) before and after α1‐adrenergic blockade (prazosin; 50 µg/kg). Hindlimb resistance (Rh) was calculated as hindlimb arterial pressure / HLBF. During mild exercise HLBF must be reduced below a threshold level before MMA occurs. With initial reductions in HLBF, vasodilation occurred with the imposed ischemia. Once muscle metaboreflex was elicited, Rh increased. This increase in Rh was abolished by α1‐adrenergic blockade. We conclude that metaboreflex activation during submaximal dynamic exercise causes sympathetically‐mediated α‐adrenergic vasoconstriction of the ischemic skeletal muscle. HL‐55473