1. Exercise training reduces resting sympathetic activity, but the effects on sympathetic activation or withdrawal during baroreflex responses to blood pressure perturbations are controversial. The purpose of this study was to investigate the effects of training on both the vagal and sympathetic reflex heart rate responses to blood pressure changes.
2. Using 10 healthy males in a randomized cross-over design, we examined the effects of three 30 min cycling sessions at 70% of maximal capacity for 4 weeks on the steady-state reflex heart rate responses to perturbations in mean arterial pressure induced with injections of nitroprusside and phenylephrine. The method provides a sigmoidal relationship between changes in heart rate and blood pressure. The upper plateau (maximum tachycardia in response to blood pressure reduction) and lower plateau (maximum bradycardia in response to blood pressure elevation) are mainly mediated by the cardiac sympathetics and vagus, respectively. The slope of the relationship is a measure of reflex gain.
3. Training, which increased maximal oxygen consumption by 13 ± 2% (mean ± standard error of the difference), reduced supine and standing blood pressures by 3 ± 1 / 3 ± 1 mmHg (P < 0.05) and 4 ± 1/2 ± 2 mmHg (P < 0.05 for systolic), respectively, whereas resting heart rate was lowered by 6 ± 1 beats/min (P < 0.05). Reflex sensitivity in the presence of functioning vagus and sympathetics was not altered with training, but the vagal component of sensitivity, as assessed after sympathetic blockade with propranolol, was significantly reduced. The maximal tachycardiac responses to blood pressure decreases were consistently reduced with training (sedentary, 106 beats/ min; trained, 97 ± 1 beats/min; P < 0.05). This attenuation was not evident in the presence of propranolol, suggesting a sympathetic origin.
4. Exercise training thus reduced the sympathetic contribution to reflex tachycardia induced by blood pressure reduction and diminished the vagal contribution to reflex sensitivity. These effects of training may be of relevance to individuals with elevated sympathetic activity who may be at risk of myocardial ischaemia, serious ventricular arrhythmias or sudden cardiac death.
