β-Adrenoreceptors do not oppose sympathetic vasoconstriction in resting and contracting skeletal muscle of male rats

Abstract

Sympathetic nervous system (SNS) vasoconstriction is primarily achieved through the binding of norepinephrine (NE) to α-adrenoreceptors. However, NE may also bind to β-adrenoreceptors and cause vasodilation that may oppose/blunt SNS-mediated vasoconstriction. Therefore, this study investigated the hypothesis that β-adrenoreceptor–mediated vasodilation opposes evoked vasoconstriction in resting and contracting skeletal muscle. Male (n = 9) Sprague–Dawley rats were anesthetized and surgically instrumented for stimulation of the lumbar sympathetic chain and measurement of arterial blood pressure and femoral artery blood flow. The percentage change of femoral vascular conductance in response to sympathetic chain stimulation delivered at 2 and 5 Hz was determined at rest and during triceps surae skeletal muscle contraction before (control) and after β-adrenoreceptor blockade (propranolol; 0.075 mg·kg−1, intravenously). β-Adrenoreceptor blockade did not alter (P > 0.05) baseline hemodynamics or the hyperemic response to exercise. At the 2 Hz stimulation frequency, sympathetic vasoconstriction was similar (P > 0.05) in control and β-blockade conditions in resting (control, −34% ± 6%; β-blockade, −33% ± 8%) and contracting (control, −16% ± 6%; β-blockade, −14% ± 7%) muscle. At the 5 Hz stimulation frequency, sympathetic vasoconstrictor responsiveness was reduced (main effect of drug, P < 0.05) following β-blockade (rest: control, −52% ± 7%; β-blockade, −51% ± 9%; contraction: control, −32% ± 11%; β-blockade, −29% ± 13%). Novelty These data indicate that β-adrenoreceptor blockade did not augment sympathetic vasoconstriction at rest or during exercise. The study demonstrates that β-adrenoreceptors do not oppose evoked sympathetic vasoconstriction in resting or contracting skeletal muscle or contribute to functional sympatholysis.