Effect of exercise on autonomic mechanisms of baroreflex control of heart rate.

We tested whether moderate dynamic exercise alters baroreflex control of heart rate (HR) in conscious dogs and whether the autonomic mechanisms mediating arterial baroreflex control of HR differ in the two settings of rest and exercise. Conscious, chronically instrumented dogs were studied during rest and moderate treadmill exercise (6.4 kpm, 10% grade). Sustained changes in mean arterial pressure (MAP) were induced by graded intravenous infusions of phenylephrine and nitroglycerine. Baroreflex HR sensitivity was assessed as the slope of the linear regression between MAP and HR. Responses to increases and decreases in MAP were analyzed separately. Studies were repeated after muscarinic (atropine, 0.2 mg/kg iv) or beta-adrenergic (propranolol, 2.0 mg/kg iv) blockade during rest and exercise. During control experiments both at rest and during exercise, baroreflex changes in HR were greater in response to decreases than to increases in MAP. Experiments performed after separate cardiac autonomic blockade revealed that at rest baroreflex-induced bradycardia occurs via parasympathetic activation but tachycardia occurs via a combined parasympathetic inhibition-sympathetic activation. During exercise, whereas the magnitude of baroreflex responses was unchanged compared with rest, the autonomic mechanisms mediating the baroreflex changes in HR were altered. In this setting, baroreflex tachycardia and bradycardia occur via modulation of both parasympathetic and sympathetic tone.