SUBSTANTIAL CARDIAC PARASYMPATHETIC ACTIVITY EXISTS DURING HEAVY DYNAMIC EXERCISE IN DOGS

We investigated the extent of functional parasympathetic and sympathet ic activity to the heart at rest and during mild to heavy dynamic exer cise in conscious dogs. The animals were chronically instrumented to m onitor mean arterial pressure (MAP), heart rate (HR), and terminal aor tic blood how (TAQ) and trained to run on a motor-driven treadmill. MA P, HR, and TAQ were monitored at rest and during steady-state dynamic exercise ranging from mild [3.2 kilometers per hour (kph), 0% grade] t o 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 iv) or sympathetic nerves (atenolol 2.0 mg/kg iv) t o the heart. In addition, blood samples were taken at rest and at stea dy state during exercise, and plasma levels of vasopressin and renin a ctivity were assessed. At rest and during all levels of exercise, musc arinic cholinergic receptor blockade caused a marked increase in HR ov er control (saline treated) levels with Little effect on MAP or TAQ. b eta-Adrenergic receptor blockade had no significant effect on HR at re st and during mild exercise. At moderate to heavy workloads, beta-rece ptor blockade significantly reduced MAP, HR, and TAQ and increased pla sma vasopressin levels. We conclude that, even during heavy dynamic ex ercise, significant functional parasympathetic tone to the heart exist s. Thus, over a wide range of exercise workloads, HR is under the toni c control of both sympathetic and parasympathetic nerves.