Muscle metaboreflex activation during dynamic exercise vasoconstricts ischemic active skeletal muscle

摘要

Metabolite accumulation due to ischemia of active skeletal muscle stimulates group III/IV chemosen-sitive afferents eliciting reflex increases in arterial blood pressure and sympathetic activity, termed the muscle metaboreflex. We and others have previously demonstrated sympathetically mediated vasoconstric-tion of coronary, renal, and forelimb vasculatures with muscle metaboreflex activation (MMA). Whether MMA elicits vasoconstric-tion of the ischemic muscle from which it originates is unknown. We hypothesized that the vasodilation in 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 alpha_1-adrenergic blockade [prazosin (50 mug/kg)], beta-ad-renergic blockade [propranolol (2 mg/kg)], and alpha_1 + beta-blockade. Hindlimb resistance was calculated as femoral arterial pressure/ HLBF. During mild exercise, HLBF must be reduced below a threshold level before the reflex is activated. With initial reductions in HLBF, vasodilation occurred with the imposed ischemia. Once the muscle metaboreflex was elicited, hindlimb resistance increased. This increase in hindlimb resistance was abolished by alpha_1-adrenergic blockade and exacerbated after beta-adrenergic blockade. We conclude that metaboreflex activation during submaximal dynamic exercise causes sympathetically mediated alpha-adrenergic vasoconstriction in ischemic skeletal muscle. This limits the ability of the reflex to improve blood flow to the muscle.