Substantial cardiac parasympathetic activity exists during heavy dynamic exercise in dogs

We investigated the extent of functional parasympathetic and sympathetic activity to the heart at rest and during mild to heavy dynamic exercise in conscious dogs. The animals were chronically instrumented to monitor mean arterial pressure (MAP), heart rate (HR), and terminal aortic blood flow (TAQ) and trained to run on a motor-driven treadmill. MAP, HR, and TAQ were monitored at rest and during steady-state dynamic exercise ranging from mild 3.2 kilometers per hour (kph), 0% grade to heavy exercise (8 kph, 15% grade). Experiments were performed before and after blocking the effects of either the parasympathetic nerves (atropine 0.2 mg/kg i.v.) or sympathetic nerves (atenolol 2.0 mg/kg i.v.) to the heart. In addition, blood samples were taken at rest and at steady state during exercise, and plasma levels of vasopressin and renin activity were assessed. At rest and during all levels of exercise, muscarinic cholinergic receptor blockade caused a marked increase in HR over control (saline treated) levels with little effect on MAP or TAQ. beta-Adrenergic receptor blockade had no significant effect on HR at rest and during mild exercise. At moderate to heavy workloads, beta-receptor blockade significantly reduced MAP, HR, and TAQ and increased plasma vasopressin levels. We conclude that, even during heavy dynamic exercise, significant functional parasympathetic tone to the heart exists. Thus, over a wide range of exercise workloads, HR is under the tonic control of both sympathetic and parasympathetic nerves.