Abstract
During exercise, an alteration in the balance between the two divisions of the autonomic nervous system (sympathetic activation and vagal withdrawal) increases HR. However, the contribution of the sympathetic or vagal system to cardiac-arterial baroreflex sensitvity remains unclear. PURPOSE The purpose of this study was to examine the effects of sympathetic and parasympathetic activity to the heart at rest and during exercise on the cardiac-arterial baroreflex sensitivity. METHODS Eight subjects performed 20 min bouts of exercise at a steady-state heart rate (HR) of 90 (EX90), 120 (EX120), and 150 (EX150) beats/min with and without metoprolol (0.2 mg/kg) or glycopyrrolate (0.04 mg/kg). Systolic blood pressure (SBP), mean arterial pressure (MAP) and pulse interval (RRI) were continuously measured. Arterial baroreflex sensitivity (BRS) was evaluated by analyzing the slopes of spontaneously occurring sequences of three or more consecutive beats in which systolic blood pressure and pulse interval of the following beat either increased or decreased, in the same direction, in a linear fashion. RESULTS β-1 adrenagic blockade decreased (p < 0.05) and vagal cardiac blockade increased HR (p < 0.05) at rest and during the three exercise workloads. Without blockade, BRS gradually decreased with increasing exercise workload. While there was no significant difference in BRS with and without β-1 adrenagic blockade at rest and during exercise, vagal cardiac blockade decreased BRS from 26.0 ± 4.8 to 3.2 ± 0.4 ms/mmHg at rest (p < 0.05). However, during EX150 there was no difference in BRS with and without cardiac vagal blockade (1.8 ± 0.4 vs. 1.4 ± 0.3 ms/mmHg, P > 0.05). CONCLUSION This finding suggests that during exercise the reduction of cardiac-arterial baroreflex sensitivity occurs by vagal withdrawal and this reduction is not influenced by the exercise induced increase in sympathetic activity.
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