β-Adrenergic Blockade and Metabo-Chemoreflex Contributions to Exercise Capacity

Exercise-induced dyspnea in patients with cardiopulmonary diseases may be related to sympathetic nervous system activation, with increased metabo- and/or chemosensitivities. Whether this mechanism plays a role in exercising normal subjects remains unclear. Muscle sympathetic nerve activity (MSNA), HR, ventilation (V(E)), O2 saturation (SpO2), and end-tidal PCO2 (PetCO2) were measured in 14 healthy young adults after 1 wk of beta1-receptor blockade with bisoprolol 5 mg x d(-1) versus placebo after a double-blind, placebo-controlled, randomized crossover design. The MSNA and the ventilatory responses to hyperoxic hypercapnia (7% CO2 in O2), DeltaV(E)/DeltaPetCO2, and isocapnic hypoxia (10% O2 in N2), DeltaV(E)/DeltaSpO2, and to an isometric muscle contraction followed by a local circulatory arrest (metaboreflex) were determined at rest followed by an incremental cardiopulmonary exercise test. Bisoprolol did not change the V(E) and MSNA responses to hypercapnia, hyperoxia, or isometric muscle contraction or ischemia. Bisoprolol decreased maximum O2 uptake (P < 0.05), workload (P < 0.05), and HR (P < 0.0001) and both V(E)/VO2 and V(E)/VCO2 slopes (P < 0.05). These results suggest that decreased aerobic exercise capacity after intake of beta-blockers is accompanied by decreased ventilation at any metabolic rate. However, this occurs without detectable change in the sympathetic nervous system tone or in metabo- or chemosensitivity and is therefore probably of hemodynamic origin.