Cardiac autonomic blockade in exercising dogs.

Pharmacological blockade of autonomic control of the heart was studied in dogs performing mild, moderate, and severe running exercise on a level treadmill. The dogs were studied without drugs, after atropine, after propranolol, and after both atropine and propranolol. As compared with results without drugs, cardiac denervation resulted in elevated resting heart rate (+45 beats/min) but reduced heart rate during moderate (-17 beats/min) and severe exercise (-47 beats/min); no change in cardiac output at rest or during mild exercise but decreases (-23% and -25%) during moderate and severe exercise; and reduced first derivatives of left ventricular pressure at rest (-24%) and during exercise (-35, -41, and -52% for mild, moderate, and severe loads, respectively). Cardiac denervation did not alter end-diastolic left ventricular diameter but significantly increased end-systolic diameter during exercise. It is concluded that blockade of autonomic control of the heart diminishes cardiac output during exercise by reducing heart rate and myocardial contractile force but does not alter cardiac output at rest.